European conference on the dynamics of molecular systems
Book of abstracts
Department of Chemistry Jesus College
University of Oxford
1 2 Plenary talks
3 PLENARY TALK P1
Imaging studies of inelastic collisions
David H. Parker Dept of Molecular and Laser Physics, IMM, Radboud University, Nijmegen, the Netherlands. E‐mail address: [email protected]
State-to-state imaging studies of the interaction of small molecules including O2,H2O, OH, CO and ammonia with H2 and with He will be described. These molecules are prominent components of molecular interstellar matter. In order to extract quantitative information on these molecules from telescope spectra, models are employed that depend critically on the rates of rotational energy exchange due to collisions with molecular hydrogen and helium. Collision rates are currently determined by theory from the multidimensional Potential Energy Surface
(PES) describing the interaction of H2O and H2 [1] or He [2] and OH with H2 [3], or He. Our velocity map imaging [4] measurements of state-to-state differential and relative integral cross sections of rotational inelastic collisions, also as a function of collision energy, are used to test these PESs. For the H2O/H2, He system experiment is compared with state-of-the-art theoretical calculations by the group of L. Wiesenfeld (Grenoble) [5]. Our studies of astrochemistry relevant small molecules with the collision partners He and H2 at collision energy relevant to that of the interstellar media should place the theoretically determined PESs and the collision rates extracted from the PES on a firmer basis.
References
1. P. Valiron M. Wernli, A. Faure, L. Wiesenfeld, C. Rist, S. Kedžuch, J. Noga, J. Chem. Phys. 129 134306 (2008). 2. J. Brudermann, C. Steinbach, U. Buck, K. Patkowski , R. Moszynski, J. Chem. Phys. 117 11166 (2002). 3. A. R. Offer, M. C. van Hemert, J. Chem. Phys. 99 3836 (1993). 4. A.T.J.B. Eppink, D. H. Parker, Rev. Sci. Instr. 68 3477 (1997). 5. C.-H. Yang, G. Sarma, J.J. ter Meulen, D. H. Parker, G. C. Mc Bane, L. Wiesenfeld, A. Faure, Y. Scribano, N. Feautrier, J. Chem. Phys. 133, 131103 (2010).
4 PLENARY TALK P2
Electronic nonadiabaticity in collisional quenching of OH radicals
Millard H. Alexander and Jacek Klos University of Maryland, College Park, USA Paul J. Dagdigian The Johns Hopkins University, Baltimore, USA
The interaction of OH in its first electronic excited state (A+) with noble gases has provoked a long and fruitful interaction between experiment and theory. There exists a deep, angularly- constricted well in both (OHM and HOM) linear geometries. For interactions with Kr, this well is deep enough to render the crossing with the two potential energy surfaces which correlate with ground state [OH(X)]+Kr products accessible even at low collision energies. Evidence of this quenching is seen in recent results from the groups of Brouard and Aoiz (Oxford and Madrid) and Lester (Pennsylvania).
The nonadiabatic couplings responsible for this quenching arise from spin-orbit coupling as well as the mixing between the OH(A+) Kr state and the A’ component of the OH(X)Kr state. The dependence on geometry of these terms, as well the potential energy surfaces themselves, will determine the magnitude of the cross section for electronic quenching, and, more subtly, the distribution of the quenched products among the accessible vibrational, rotational, spin-orbit, and -doublet states.
We will report on progress in obtaining a full ab initio description of the potential energy surfaces and couplings, and on using these in fully-quantum simulations of the electronically non-adiabatic scattering.
5 PLENARY TALK P3
Attosecond time-resolved molecular dynamics
Marc Vrakking
Max-Born Institute, Max-Born Straße 2A, 12489 Berlin
Using attosecond light sources based on high-harmonic generation (HHG), pump-probe experiments can be performed where electron dynamics is studied on its natural timescale, providing insight into the fundamental role that electrons play in photo-induced processes. In my talk I will present some of the first applications of these techniques in molecular science, focusing on two-color experiments where several small molecules were exposed to a sequence of one or more attosecond pulses and an infrared field.
Among other things, I will present experiments where attosecond pulses probe the electronic re- arrangement that occurs in neutral molecules and their single-ionized counterparts, under the influence of a moderately strong IR field. These experiments pave the way to the observation of time-resolved electron dynamics in more complex molecules.
6 PLENARY TALK P4
Molecular processes in space: from interstellar clouds to planets
Ewine F. van Dishoeck
Leiden Observatory, Leiden University, the Netherlands
The space in between the stars is not empty but filled with a very dilute gas in which new stars and planets can form. In spite of the extremely low temperatures and densities, a surprisingly rich and interesting chemistry occurs in these interstellar clouds, as evidenced by the detection of more than 150 different molecules. The observed composition points to a chemistry that is far out thermodynamic equilibrium and is determined by the kinetics of the atomic and molecular processes. Early astrochemical models considered primarily gas-phase processes such as radiative association, dissociative recombination, photodissociation, and ion-molecule reactions. In recent years, it has become clear that processes on the surfaces of dust grains play a more important role than thought before in explaining the observed composition. Recent laboratory and theoretical results on selected processes will be reviewed, with a focus on photon-induced processes. Special attention will be paid to reactions involving water and to new data at THz frequencies from the Herschel Space Observatory.
7 PLENARY TALK P5
CHEM-ICE-TRY Solid state pathways towards molecular complexity in space
Harold Linnartz
Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, NL 2300 RA Leiden, the Netherlands
*E-mail: [email protected]
Icy dust grains in space act as catalytic surfaces onto which new molecules form. These molecules – both small, abundant as well as complex (‘prebiotic’) species – are synthesized in energetic and non-energetic processes: UV and cosmic ray irradiation and H-atom addition reactions. In order to study such processes quantitatively dedicated laboratory experiments are performed that simulate the conditions in the inter- and circumstellar medium [1-4]. The outcome is used to compare with astronomical observations and as input for astrochemical models. In parallel, the experiments provide information on the involved molecular processes. This talk gives answer on the questions how water forms in space, how complex species are formed, whether chemistry in space may be wavelength dependent, and which parameters are at play in chem-ice-try, the study of reactions in interstellar ices.
The derived reaction scheme for water formation in space. Details are available from [1].
References
[1] H.M. Cuppen, S. Ioppolo, H. Linnartz, PCCP, 12 (2010) 12077. [2] K.I. Öberg, R.T. Garrod, E.F. van Dishoeck, H. Linnartz, A&A, 504 (2009) 891. [3] S. Ioppolo, Y. van Boheemen, H.M. Cuppen, E.F. van Dishoeck, H. Linnartz, MNRAS, 413 (2011) 2281. [4] E. Congiu, S. Ioppolo, F. Dulieu, H. Chaabouni, S. baouche, J.L. Lemaire, C. Laffon, P. Parent, T. Lamberts, H. Cuppen, H. Linnartz, ApJL, 750 (2012) L12.
8 PLENARY TALK P6
Intersystem crossing dynamics in polyatomic multichannel reactions of oxygen atoms with unsaturated hydrocarbons
Piergiorgio Casavecchia
Dipartimento di Chimica, Università degli Studi di Perugia, 06123 Perugia, Italy
*E-mail: [email protected]
Comparisons between experimental cross sections and theoretical predictions on ab initio potential energy surfaces (PESs) for benchmark 3-atom, 4-atom, and recently non-complex forming 5-atom reactions have greatly advanced our understanding of chemical reactivity over the last decade. Nonetheless, experimental and theoretical investigations of the dynamics of more complex polyatomic reactions, with numerous competing product channels, e.g., 3 O( P)+C2H4, still represent a major challenge for both experiment and theory. Experimentally, a major challenge is to study all open channels with the same degree of accuracy and under the same experimental conditions. This is a prerequisite to identify the primary products and determine their relative importance (branching ratios, BRs). A “universal” detection method to interrogate all product channels on the same footing, such as mass-spectrometry (MS), is required and this can be best pursued in crossed molecular beam (CMB) experiments [1]. CMB- MS studies of these reactions have recently become feasible using “soft” ionization detection by tunable low energy electrons [2,3] or VUV synchrotron radiation [4]. Theoretically, the development of full-dimensional PESs is needed on which to perform dynamic (quasiclassical trajectory-QCT) calculations, and this is now becoming practical [3]. For reactions with the added complexity of nonadiabatic coupling (intersystem crossing, ISC) of PESs, the challenge to theory and experiment is perhaps at its highest level.
In this talk, I will highlight and discuss our recent efforts to unravel the dynamics of polyatomic nonadiabatic multichannel reactions. In particular, by studying in crossed beams the reaction dynamics of O(3P) with a variety of unsaturated hydrocarbons (acetylene, ethylene [3,5], allene [6] and methylacetylene), central issues such as the variation with Ec of the dynamics, BRs and extent of ISC from triplet to singlet PESs are examined, as well as the dependence of ISC on molecular complexity and structure [3,5,6]. The experimental results are rationalized in the light of the underlying PESs and compared with statistical predictions and/or QCT surface-hopping calculations on coupled, full-dimensional ab initio triplet and singlet PESs, providing an important test ground for theoretical dynamics methods that can be used widely to study ISC dynamics.
References
[1] P. Casavecchia, K. Liu, and X. Yang, in: Tutorials in Molecular Reaction Dynamics, Mark Brouard & Claire Vallance, eds. (RSC Publishing, Cambridge, UK), Ch. VI (2010), pp. 167-213. [2] P. Casavecchia, F. Leonori, N. Balucani, R. Petrucci, G. Capozza E. Segoloni, PCCP 11, 46-65 (2009). [3] B.Fu, Y.-C.Han, J.M.Bowman, L.Angelucci, N.Balucani, F. Leonori, P. Casavecchia, PNAS 109, 9733 (2012). [4] S. H. Lee, W. K. Chen, W. J. Huang, J. Chem. Phys. 130, 054301 (2009). [5] B. Fu, Y.-C. Han, J.M. Bowman, F. Leonori, N. Balucani, P. Casavecchia,et al., J. C. P. (submitted). [6] F. Leonori, A. Occhiogrosso, N. Balucani, A. Bucci, R. Petrucci, P. Casavecchia, J. P. C. Lett. 3, 75 (2012). 9 PLENARY TALK P7
Cold polar radical-radical collisions
Gerrit C. Groenenboom
Theoretical Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands E-mail: [email protected]
Since the beginning of this century tremendous progress has been made in cooling and trapping of molecules, but we are mostly still in the “alkali age”. The development of techniques for direct cooling and trapping of molecules depends to a large extent on understanding the collisional properties of the molecules involved. We study these properties with coupled channels calculations, employing ab initio potential energy surfaces.
The NH(3Σ—) radical has a large magnetic and electric dipole moment, a large rotational constant, and small spin-spin and spin-rotation couplings. These properties are favorable for buffer-gas cooling and loading experiments. In 2004 the Doyle group in Harvard first reported buffer-gas cooling of NH and this radical has been used in a number of buffer-gas experiments since then. We computed NH-NH potential energy surfaces of the singlet, triplet, and quintet state of the complex [1,2] and we computed elastic cross sections, which are essential for evaporative cooling of NH to the ultracold regime, and spin changing collisions that lead to trap loss [3]. We also included the effects of a magnetic field and concluded that the prospects for evaporative cooling are reasonable [4,5]. Recently, we developed a method that allows us to include the effect of chemical reactions that may occur on the triplet and singlet potentials of the NH dimer [6]. I will present results including reactive channels and show that they may dominate the inelastic processes. We also found that the reactivity depends on the short range potentials and couplings, and the universal limit cannot be applied [6].
In addition I will present recent results on elastic and inelastic cross sections for collisions of OH(2Π) and NO(2Π) radicals. In principle these collisions are governed by eight nonadiabatically coupled potential energy surfaces. We do not have all these potentials, but our calculations suggest that some of the processes well are described by a long-range model.
Acknowledgments: This work was supported by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (CW-NWO).
References
[1] G. S. F. Dhont, J. H. van Lenthe, G. C. Groenenboom, A. van der Avoird, J. Chem. Phys. 123, 184302 (2005) [2] L. M. C. Janssen, G. C. Groenenboom, A. van der Avoird, P. S. Zuchowski, and R. Podeswa, J. Chem. Phys., 131, 224314 (2009) [3] L. M. C. Janssen, P. S. Zuchowski, A. van der Avoird, J. M. Hutson, and G. C. Groenenboom J. Chem. Phys., 134, 124309 (2011). [4] L. M. C. Janssen, P. S. Zuchowski, A. van der Avoird, G. C. Groenenboom, and J. M. Hutson Phys. Rev. A, 83, 022713 (2011) [5] L. M. C. Janssen, A. van der Avoird, and G. C. Groenenboom Eur. Phys. J. D, 65, 177 (2011) 10 [6] L. M. C. Janssen, Ph. D. thesis (Radboud University Nijmegen, 2012) PLENARY TALK P8
Identification of the mechanism of photoprotection in eumelanin pigments
Alice Corani1, Annemarie Huijser1, Alessandro Pezzella3, Thomas Gustavsson4, Per-Åke Malmqvist2, Marco d’Ischia3 and Villy Sundström1
1Department of Chemical Physics Lund University, Box 124, 22100 Lund, Sweden 2Division of Theoretical Chemistry Lund University, Box 124, 22100 Lund, Sweden 3Department of Organic Chemistry and Biochemistry University of Naples Frederico II Via Cintia, 80126 Naples, Italy 4 Laboratoire Francis Perrin, CEA/DSM/IRAMIS/SPAM-CNRS URA 2453, CEA/Saclay, F-91191 Gif- sur-Yvette, France *Permanent address: Optical Sciences Group, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
Insight into the functionality of epidermal eumelanin pigments is of increasing relevance in the context of photoprotection and skin cancer risk. Current knowledge of UV-induced physical and chemical processes is however still limited, due to the highly heterogeneous and complex structure of the eumelanin components. To address these issues, we followed a bottom-up approach using eumelanin building blocks and model systems with increasing molecular size and complexity. Here we describe excited state processes induced by UV-exposure in 5,6- dihydroxyindole-2-carboxylic acid (DHICA)-based systems (monomers, dimers, trimer and polymer) studied using ultrafast time-resolved fluorescence spectroscopy. Ultrafast sub-ps excited state deactivation occurs via several competing Excited State Proton Transfer (ESPT) processes.
11 PLENARY TALK P9
Dynamical outcomes of quenching: Reflections on a conical intersection
Marsha I. Lester1*
1Department of Chemistry, University of Pennsylvania, Philadelphia, PA USA 19104-6323
Hydroxyl radicals are important in combustion and atmospheric environments, where they are often detected by laser-induced fluorescence (LIF) on the A2Σ+ -X2Π band system. However, collision partners known to quench electronically excited OH A2Σ+ radicals are ubiquitous in these environments. Thus, great effort has been made to quantify the rates and/or cross sections for collisional quenching, so that its effects on LIF signals may be taken into account to allow an accurate determination of OH concentrations. Despite extensive kinetic measurements, fundamental questions remain regarding the fate of the collisionally quenched molecules and the mechanism by which these nonadiabatic processes occur. This presentation will overview fundamental chemical dynamics studies aimed at understanding the quenching of OH A2Σ+ by molecular partners (M = H2,O2, CO, Kr). Recent experimental and theoretical studies reveal efficient quenching of OH A2Σ+ arising from strong nonadiabatic coupling in the vicinity of a conical intersection, resulting in nonreactive quenching that returns OH to its ground X2Π electronic state[1] and reactive quenching that yields new products.[2,3] The branching between these outcomes and the quantum state distributions of the products reflect the unique properties of the conical intersection region.
References
[1] J. H. Lehman, L. P. Dempsey, M. I. Lester, B. Fu, E. Kamarchik, and J. M. Bowman, “Collisional quenching of OD A2Σ+ 2 by H2: Experimental and theoretical studies of the state-resolved OD X Π product distribution and branching fraction”, J. Chem. Phys. 133, 164307 (2010). 2 + [2] J. H. Lehman, J. Bertrand, T. A. Stephenson, and M. I. Lester, “Reactive quenching of OD A Σ by H2: Translational energy distributions for H- and D-atom product channels”, J. Chem. Phys. 135, 144303 (2011). 2 + [3] J. H. Lehman, M. I. Lester, and D. H. Yarkony, “Reactive quenching of OH A Σ by O2 and CO: Experimental and nonadiabatic theoretical studies of H- and O-atom product channels”, J. Chem. Phys., submitted for publication (2012).
12 PLENARY TALK P10
Vibrationally resolved chemical reaction dynamics in solution
Andrew J. Orr-Ewing
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
Polanyi’s rules for isolated reactive collisions suggest that a transition state early along the reaction coordinate promotes vibrational excitation of one of the newly formed reaction products. Such behaviour has been confirmed for exothermic reactions in the gas phase by experimental measurements using vibrational spectroscopy or velocity map imaging. Study of these types of reaction in solution in liquids provides an opportunity to examine how the solvent modifies reactive potential energy surfaces and the associated reaction dynamics, but requires measurements of product vibrational excitation to be made with picosecond time resolution. Over tens to hundreds of picoseconds, any vibrationally excited products will relax by loss of excess energy to the surrounding solvent bath. An overview will be presented of our studies of various reactions using ultrafast time-resolved broadband infra-red spectroscopy to quantify branching between vibrational levels of the nascent products and rates of subsequent vibrational relaxation [1-5]. Illustrative examples will be drawn from our recent work on exothermic reactions of CN radicals, Cl atoms and F atoms in various organic solvents.
References
[1] S.J. Greaves, R.A. Rose, T.A.A. Oliver, D.R. Glowacki, M.N.R. Ashfold, J.N. Harvey, I.P. Clark, G.M. Greetham, A.W. Parker, M. Towrie and A.J. Orr-Ewing, Science, 331, 1423 (2011). [2] A.J. Orr-Ewing, D.R. Glowacki, S.J. Greaves and R.A. Rose, J. Phys. Chem. Lett. 2, 1139 (2011). [3] R.A. Rose, S.J. Greaves, T.A.A. Oliver, I.P. Clark, G.M. Greetham, A.W. Parker, M. Towrie, and A.J. Orr-Ewing, J. Chem. Phys. 134, 244503 (2011). [4] D.R. Glowacki, R.A. Rose, S.J. Greaves, A.J. Orr-Ewing and J.N. Harvey, Nature Chem. 3, 850 (2011). [5] R.A. Rose, S.J. Greaves, F. Abou-Chahine, D.R. Glowacki, T.A.A. Oliver, M.N.R. Ashfold, I.P. Clark, G.M. Greetham, M. Towrie and A.J. Orr-Ewing, PCCP in press (2012). DOI: 10.1039/C2CP40158D
13 PLENARY TALK P11
Chemical reaction rates from ring polymer molecular dynamics
David Manolopoulos
Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK
I will begin this talk by reviewing the ring polymer molecular dynamics (RPMD) theory of chemical reaction rates [1,2], and comparing this theory with the exact quantum mechanical and classical limit theories. I will then present some example applications of the RPMD theory to chemical reactions in the gas phase [3-5] and in solution [6-8]. Since RPMD rate theory is simply a classical rate theory in an extended phase space, it is routinely applicable to genuinely complex chemical reactions in their full dimensionality. Moreover it becomes exact in the high temperature limit, and it is expected to give a rate coefficient within a factor of 2 of the exact quantum mechanical result even at temperatures in the deep tunneling regime (where the classical rate coefficient is too small by several orders of magnitude) [9].
References
[1] I.R.Craig and D.E.Manolopoulos, J. Chem. Phys. 122, 084106 (2005). [2] I.R.Craig and D.E.Manolopoulos, J. Chem. Phys. 123, 034102 (2005). [3] R.Collepardo-Guevara, Y.V.Suleimanov and D.E.Manolopoulos, J. Chem. Phys. 130, 174713 (2009). [4] Y.V.Suleimanov, R.Collepardo-Guevara and D.E.Manolopoulos, J. Chem. Phys. 134, 044131 (2011). [5] R.Perez de Tudela, F.J.Aoiz, Y.V.Suleimanov and D.E.Manolopoulos, J. Phys. Chem. Lett. 3, 493 (2012). [6] R.Collepardo-Guevara, I.R.Craig and D.E.Manolopoulos, J. Chem. Phys. 128, 144502 (2008). [7] N.Boekelheide, R.Salomon-Ferrer and T.F.Miller, Proc. Natl. Acad. Sci. USA 108, 16159 (2011). [8] A.R.Menzeleev, N.Ananth and T.F.Miller, J. Chem. Phys. 135, 074106 (2011). [9] J.O.Richardson and S.C.Althorpe, J. Chem. Phys. 131, 214106 (2009).
14 PLENARY TALK P12
Angular-resolved fs photoelectron spectroscopy of fullerenes: Giant atoms or hot metal spheres?
Eleanor E.B. Campbell
EaStCHEM, School of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, U.K.
*E-mail: [email protected]
When ionising with fs lasers, fullerenes, and other conjugated molecules, show well-resolved peak structure in their photoelectron spectra that can be attributed to single photon ionisation of a large range of excited states, populated within the same laser pulse, thus providing a “fingerprint” of the molecule [1]. For fullerenes, strong peaks are observed that are attributed to the excitation of SAMO states (super-atom molecular orbitals), these are very simple hydrogenic- type orbitals that are centred on the fullerene core rather than on the carbon atoms. They are a consequence of the hollow nature of the molecule [2]. New studies that combine fs Rydberg fingerprint spectroscopy with angular-resolved photoelectron spectra obtained using velocity map imaging and time-dependent density functional theory [3] are shedding new light on these states and the reason for their prominence in the photoelectron spectra. Angular-resolved photoelectron spectroscopy can also provide information on the timescale of electron emission [4] and provide evidence for the occurrence of thermal electron emission. In this talk an overview of fullerene photoionisation studies and their relevance for understanding the ionisation dynamics and mechanisms of large molecules will be given.
References
[1] M. Tsubouchi et al., Phys. Rev. Lett. 86, (2001) 4500; C. Schick & P. Weber, J. Phys. Chem. A 105 (2001) 3735; M. Boyle et al., Phys. Rev. Lett. 87 (2001) 273401 [2] M. Feng, J. Zhao, H. Petek, Science 320 (2008) 359 [3] J.O. Johansson et al., Phys. Rev. Lett. 108 (2012) 173401 [4] J.O. Johansson et al., J. Chem. Phys. 136 (2012) 164301
15 PLENARY TALK P13
Controlling large molecules at high repetition rates: Toward the “molecular movie”
Jochen Küpper1,2
1Center for Free-Electron Laser Science, DESY, Notkestrasse 85, D-22607 Hamburg 2UniversitŠt Hamburg, Department Physik, Luruper Chaussee 149, D-22761 Hamburg
With our new experimental setup we aim for studying ultrafast “chemical” dynamics of large and complex molecules directly in the molecular frame. In first benchmark experiments we create supersonic, cold beams of prototypical iodobenzene (C6H5I) molecules at high repeti tion rates – up to 1 kHz. These molecular beams are quantum state selected by dc electric fields and, subsequently, laser aligned and mixed-field oriented by strong picosecond laser fields and weak dc electric fields. The resulting strongly aligned and oriented molecular samples are characterized by strong-field ionization using femtosecond laser pulses and velocity -map imaging of the produced ions to derive the angular distribution of the molecules. The degrees of alignment and orientation are characterized as a function of repetition rate, state selection, and laser parameters.
In the future, the high repetition rate production of these clean, well-defined samples will strongly benefit, or simply allow, novel time-resolved experiments on the dynamics of complex gas-phase molecules, for instance, femtosecond pump-probe measurements, X-ray or electron diffraction of molecular ensembles (including molecular-frame photoelectron angular distributions and diffraction-from-within experiments), or tomographic reconstructions of molecular orbitals. These samples could also be very advantageous for metrology applications, such as, for example, matter-wave interferometry or the search for electroweak interactions in chiral molecules. Moreover, they provide an extreme level of control for stereo-dynamically controlled reaction dynamics.
We have already exploited such state-selected and oriented samples to measure photoelectron angular distributions in the molecular frame (MFPADs) from non-resonant femtosecond-laser photoionization and using the Free-Electron-Lasers FLASH and LCLS. We have investigated (coherent) X-ray-diffractive imaging and, also using ion-momentum imaging, the induced radiation damage of these samples due to the X-ray irradiation.
16 PLENARY TALK P14
Following simple chemical reactions using homodyne high harmonic spectroscopy
David M Villeneuve1*
1Joint Attosecond Science Laboratory National Research Council of Canada University of Ottawa 100 Sussex Drive Ottawa ON K1A 0R6 Canada
*E-mail: [email protected]
High Harmonic Spectroscopy employs femtosecond high harmonic generation as a method to study the structure of molecules. In many ways it is a time-reversed version of photoelectron spectroscopy, with the advantage that the energy, phase and polarization of the emitted photons can be measured for a broad range of energies simultaneously. By applying impulsive laser alignment methods, these measurements can be made in the molecular frame. A proof of concept showed that the highest occupied molecular orbital of dinitrogen could be imaged [1].
High harmonic spectroscopy employs two timescales – femtosecond and attosecond. The attosecond time scale results from the sub-optical-cycle nature of the process, and can reveal electron dynamics [2]. The femtosecond time scale enables pump-probe techniques to be employed. We have shown [3] that we can follow the photo-dissociation of bromine, in which we observe short-term dynamics by quantum interference between atomic centers, and long- term dynamics by interference of electron trajectories between separate atoms.
Applying homodyne high harmonic spectroscopy to NO2, we can see population transferring between two diabatic states as the excited state wave packet traverses near the conical intersection [4].
References
[1] J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pépin, J. C. Kieffer, P. B. Corkum and D. M. Villeneuve, Tomographic imaging of molecular orbitals, Nature (London) 432, 867-871 (2004). [2] Olga Smirnova, Yann Mairesse, Serguei Patchkovskii, Nirit Dudovich, David Villeneuve, Paul Corkum, Misha Yu. Ivanov, High harmonic interferometry of multi-electron dynamics in molecules, Nature 460, 972-977 (2009) [3] H. J. Worner, J. B. Bertrand, D. V. Kartashov, P. B. Corkum and D. M. Villeneuve, Following a chemical reaction using high-harmonic interferometry, Nature (London) 466, 604 (2010). [4] H. J. Wörner, J. B. Bertrand, B. Fabre, J. Higuet, H. Ruf, A. Dubrouil, S. Patchkovskii, M. Spanner, Y. Mairesse, V. Blanchet, E. Mével, E. Constant, P. B. Corkum, D. M. Villeneuve, Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy, Science 334, 208 (2011)
17 PLENARY TALK P15
“Making the molecular movie”: First frames……Now with REGAE music
R. J. Dwayne Miller
Max Planck Group for Atomically Resolved Dynamics, Department of Physics, University of Hamburg, The Centre for Free Electron Laser Science, DESY and Departments of Chemistry and Physics University of Toronto
One of the great dream experiments in Science is to watch atomic motions as they occur during structural changes. In the fields of chemistry and biology, this prospect provides a direct observation of the very essence of chemistry and the central unifying concept of transition states in structural transitions. From a physics perspective, this capability would enable observation of rarefied states of matter at an atomic level of inspection, with similar important consequences for understanding nonequilibrium dynamics and collective phenomena. This experiment has been referred to as "making the molecular movie". Due to the extraordinary requirements for simultaneous spatial and temporal resolution, it was thought to be an impossible quest and has been previously discussed in the context of the purest form of a gedanken experiment. With the recent development of femtosecond electron pulses with sufficient number density to execute single shot structure determinations, this experiment has been finally realized (Siwick et al. Science 2003). Previously thought intractable problems in attaining sufficient brightness and spatial resolution, with respect to the inherent electron-electron repulsion or space charge broadening, has been solved. With this new level of acuity in observing structural dynamics, there have been many surprises and this will be an underlying theme. Several movies depicting atomic motions during passage through structural transitions relevant to condensed phase dynamics will be shown (Sciaini et al. Nature, 2009, Ernstorfer et al. Science 2009, Eichberger et al. Nature 2010, Jean-Ruel, J Phys. Chem. A 2011). The primitive origin of molecular cooperativity has also been discovered in recent studies of molecular crystals. These new developments will be discussed in the context of developing the necessary technology to directly observe the structure-function correlation in biomolecules the fundamental molecular basis of biological systems.
The future is even brighter with the advent of a new concept in relativistic electron guns that will open up direct observation of atomic motions in solution phase to gas phase systems with 10 femtosecond time resolution to watch even the fastest atomic motions. Some of the important scientific problems to be addressed with ultrabright electron sources will be discussed to give an impression of the potential impact of this emerging field.
18 PLENARY TALK P16
Ion chemistry in doped He nanodroplets
M. Daxner1, S. Zöttl1, P. Bartl1, M. Goulart1, C. Leidlmair1, H. Schöbel1,L.AnderLan1, O. Echt2, A.M. Ellis3, D.K. Bohme4 A. Mauracher1, M. Probst1, S. Denifl1, and P. Scheier1
1 Institut für Ionenphysik und Angewandte Physik, University of Innsbruck, Austria 2 Department of Physics, University of New Hampshire, USA 3 Department of Chemistry, University of Leicester, UK 4 Department of Chemistry, York University Toronto, Canada
E-mail: [email protected]
Pickup of atoms or molecules by He nanodroplets leads to the formation of cold complexes with unique properties. Long-range electrostatic interaction leads so the orientation of polar molecules prior to the attachment to the dopant cluster, which results in dipolar chains [1,2]. The binding energy of a newly adsorbed particle is completely dissipated into the surrounding heat bath before the next particle arrives. Thereby these metastable dipole-oriented chains are stabilized. Ionization or electronic excitation of the surrounding He matrix leads to ionization of the dopant via charge transfer or Penning ionization [3,4]. Both processes are highly exothermic with most dopants. Competition between fragmentation and efficient cooling by the surrounding He matrix determines the resulting product ions.
Submersion of alkali clusters [5,6] and chemical reactions of alkali atoms with water and halogen containing molecules will be discussed. Decoration of fullerenes and their relevance in space [7,8] as well as novel ionization mechanisms [4] demonstrate additional potential of doped helium nano-droplets as unique nano-cryo-reactors [9].
This work was supported by the FWF, Wien (projects P19073 and L633).
References
[1] K. Nauta, D.T. Moore and R.E. Miller, Faraday Discuss. 113 (1999) 261 [2] F. Zappa et al. J. Am. Chem. Soc. 130 (2008) 5573 [3] A. Scheidemann et al., J. Phys. Chem. 97 (1993) 2128 [4] H. Schöbel et al., Phys. Rev. Lett. 105 (2010) 243402 [5] L. An der Lan et al., J. Chem. Phys. 136 (2011) 044309 [6] L. An der Lan et al., Phys. Rev. B (2012) in print [7] C. Leidlmair et al., Phys. Rev. Lett. 108 (2012) 076101 [8] C. Leidlmair et al., Astrophys. J. 738 (2011) L4 [9] M. Farnik and J.P. Toennies, J. Chem. Phys. 122 (2005) 014307
19 PLENARY TALK P17
Cold chemistry with Coulomb-crystallised atomic and molecular ions in traps
Stefan Willitsch1*
1Department of Chemistry, University of Basel, Klingelbergstrasse 80, 4056 Basel, Switzerland
*E-mail: [email protected]
The study of chemical processes at extremely low temperatures T << 1 K is a new exciting development in chemical dynamics which has recently received considerable attention [1-4]. In the talk, recent progress in the study of ion-neutral reactions down to millikelvin temperatures will be reviewed. The presentation will begin with a brief overview of the generation of translationally cold atomic and molecular ions by laser- and sympathetic cooling in traps. Recently developed methods for the preparation of sympathetically-cooled molecular ions in well-defined rotational-vibrational quantum states will be discussed [5] which now enable fully state- and energy-selected studies of ion-molecule reactions with single localised ions [6]. The recent development of combined ion and atom traps paves the way to study reactive collisions between laser-cooled ions and ultracold atoms down to millikelvin temperatures and explore the rich dynamics of ion-neutral collisions in a physical regime which has not been accessible before. Recent experiments on cold reactive collisions between Ca+ and Ba+ ions with Rb atoms will be presented [7,8] and the extension of these experiments to molecular species will be discussed. The talk will conclude with an outlook on future developments in the field.
References
[1] O. Dulieu, R.V. Krems, M. Weidemüller and S. Willitsch, Phys. Chem. Chem. Phys., 42, 18703 (2011) [2] R.V. Krems, Phys. Chem. Chem. Phys. 10, 4079 (2008) [3] M.T. Bell and T.P. Softley, Mol. Phys. 107, 99 (2009) [4] S. Willitsch, Int. Rev. Phys. Chem., 31 (2012), DOI: 10.1080/0144235X.2012.667221 [5] X. Tong, A.H. Winney and S. Willitsch, Phys. Rev. Lett. 105, 143001 (2010) [6] X. Tong, T. Nagy, J. Yosa Reyes, M. Germann, M. Meuwly and S. Willitsch, in preparation [7] F.H.J. Hall, M. Aymar, N. Bouloufa-Maafa, O. Dulieu and S. Willitsch, Phys. Rev. Lett. 107, 243202 (2011) [8] F.H.J. Hall, M. Aymar, N. Bouloufa-Maafa, M. Raoult, O. Dulieu and S. Willitsch, in preparation
20 Invited talks
21 INVITED TALK I1
Chiral cavity ring down
Lykourgos Bougas,1,2 Giorgos E. Katsoprinakis,2 T. Peter Rakitzis1,2*
1 Department of Physics, University of Crete—71003 Heraklion-Crete, Greece 2 Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas 71110 Heraklion-Crete, Greece
*E-mail: [email protected]
Polarization-dependent cavity ring-down spectroscopy, for the measurement of optical activity in gas-phase samples, has been achieved by Vacarro and coworkers [1] by introducing two quarter wave-plates into a (two-mirror) cavity, with optical axes offset by angle . This arrangement causes the linear polarization of the light within the cavity to rotate by 2per round trip, which produces a polarization beating in the ring-down trace of frequency . Introduction of a chiral gas within the cavity changes the beating frequency by , allowing the optical activity to be measured.
We discuss the extension of these ideas to ring cavities. The round-trip polarization rotation can be achieved using two additional methods: (a) geometrically (by bringing one of four mirrors out of the plane defined by the other three, which causes a geometrical rotation of the light polarization by a fixed angle for each round trip), or (b) using the Faraday effect (applying a longitudinal magnetic field to a magneto-optical crystal in the cavity, producing a round-trip polarization rotation proportional to the magnetic field). The first method requires no intracavity optics, allowing cavities of extremely high finesse. The second method can be performed using signal reversal procedures (e.g., alternating the sign of the magnetic field), which allows the absolute determination of optical activity using a robust subtraction procedure that doesn’t require the removal of the sample.
We demonstrate these chiral cavities [2], and discuss how they can be applied to sensitive polarization spectroscopies, such as the measurement of circular dichroism in molecular beams, and the measurement of optical activity of microsamples.
References
[1] T. Müller, K. B. Wiberg, P. H. Vaccaro, J. Phys. Chem. A 104, 5959 (2000). [2] L. Bougas, G. E. Katsoprinakis, W. von Klitzing, J. Sapirstein, T. P. Rakitzis, Phys. Rev. Lett. 108, 210801 (2012).
22 INVITED TALK I2
Molecular line lists for the opacity of exoplanets, cool stars and other atmospheres
Jonathan Tennyson
Department of Physics & Astronomy, University College London, London WC1E 6BT, UK
At elevated temperatures the spectra of polyatomic molecules become extremely complicated with millions, or even billions, of transitions potentially playing an important role. The atmospheres of cool stars and ”hot Jupiter” extrasolar planets are rich with molecules in the temperature range 1000 to 3000 K and their properties are strongly influenced by the infrared and visible spectra of these molecules. So far there are extensive, reliable lists of spectral lines for a number species including some stable diatomics, water, ammonia. Data is almost completely lacking for many key species such as methane.
The ExoMol project (www.exomol.com) aims to construct line lists of molecular transitions suitable for spectroscopic and atmospheric modelling of cool stars and exoplanets [1]. At elevated temperatures it is necessary to consider many millions, even billions, of lines for a single species. Line lists therefore are computed on the basis of robust theoretical models tested against available laboratory data rather than constructed experimentally. Illustrative examples will be discussed and as will progress on obtaining the full set of molecular data. The need to consider other aspects, such as pressure effects, in the radiative transport model will also be considered.
References
1. Tennyson, J. and Yurchenko, S. N.: 2012, MNRAS (in press). arXiv:1204.0124
23 INVITED TALK I3
The dynamics behind atom-diatom collisions with competing reaction pathways
T. González-Lezana
Instituto de Física Fundamental (CSIC) Madrid (Spain)
Some A+BC reactions in which different dynamics are found to play a significant role have been theoretically investigated. In this talk, a review with recent results obtained by means of an + 1 ample variety of methods for the H + H2, C + OH and O( D) + HCl collisions will be presented. The presence of deep potential wells suggests in principle that the complex-forming pathway may govern the overall dynamics of the reaction. Interestingly some features invite to consider some other alternatives depending of the energy regime under consideration, the precise product channel for the reaction or the potential energy surface.
+ The H + H2 reaction and its isotopic variants have been studied as a function of the collision energy [1]. The possibility to describe the main features of the process by statistical means [2], a clear indication of the existence of a complex-forming dynamics, is analysed in detail. Rate coefficients for Astrophysical applications are also calculated [3].
The investigation of the two possible product channels of the O(1D) + HCl reaction and the corresponding comparison with previous experimental work [4,5] revealed differences regarding the existing dynamical mechanism on each case [6].
Finally, a study of the C+OH reaction on its second excited 14A” potential energy surface shed some light about the most probable route followed by reactants in the course of the process [7].
References
[1] T. González-Lezana et al. J. Chem. Phys. 123 194309 (2005).; T. González-Lezana et al. J. Chem. Phys. 125 094314 (2006); E. Carmona-Novillo et al. J. Chem. Phys. 128 014304 (2008); T. González-Lezana et al. J. Chem. Phys. 131, 044315 (2009). [2] T. González-Lezana Int. Rev. Phys. Chem. 26, 29 (2007) and references therein. [3] P. Honvault et al. Phys. Rev. Lett. 107, 023201 (2011) ; P. Honvault et al. Phys. Chem. Chem. Phys. 13, 19089 (2011). [4] H. Kohguchi and T. Suzuki, ChemPhysChem. 7 1250 (2006); H. Kohguchi et al. J. Phys. Chem. A 112 818 (2008). [5] N. Balucani et al. Chem. Phys. Lett. 180, 34 (1991). [6] P. Bargueño et al. J. Phys. Chem. 113, 14237 (2009) ; P. Bargueño et al. Phys. Chem. Chem. Phys 13, 8502 (2011). [7] A. Zanchet et al. J. Chem. Phys. 136, 164309 (2012).
24 INVITED TALK I4
Crossed-beam scattering experiments at energies approaching the cold regime
Simon Chefdeville, Kevin M. Hickson, Astrid Bergeat, Christian Naulin, and Michel Costes*
Université de Bordeaux, CNRS UMR 5255, Institut des Sciences Moléculaires, 33400 Talence, France
*E-mail: [email protected]
In this presentation, we will describe recent progresses in the study of inelastic and reactive collisions at collision energies approaching the cold regime using a newly developed crossed- beam apparatus and will give comparisons with the results of quantum mechanical (QM) calculations.
The CO(j = 0) + H2(j = 0) CO(j = 1) + H2(j = 0) inelastic process is known to be of paramount importance in the interstellar medium. State-to-state integral cross section measurements show a very sharp rise at threshold (3.85 cm–1) followed by successive waves which are the manifestation of orbiting resonances, never previously observed in inelastic collision events [1]. Agreement with theory strongly depends on the characteristics of the potential energy surface (PES) used to perform the QM treatment. Such inelastic collision experiments show that the requirements imposed on the accuracy of the PES are more stringent than for a successful description of the infra-red spectroscopy of the CO–H2 van der Waals complex.
1 Our interest is also on reaction dynamics of prototypical insertion reactions [2,3]. The S( D2) + HD system is particularly interesting since the branching ratio between the DS + H and HS + D channels as measured in previous crossed-beam experiments [4] has been the subject of great debate in numerous theoretical works. We will show that the dynamics of this reaction are indeed very difficult to rationalize.
References
[1] S. Chefdeville, T. Stoecklin, A. Bergeat, K. M. Hickson, C. Naulin, and M. Costes, Phys. Rev. Lett., 2012, 109, 023201. [2] C. Berteloite, M. Lara, A. Bergeat, S. D. Le Picard, F. Dayou, K. M. Hickson, A. Canosa, C. Naulin, J.-M. Launay, I. R. Sims, and M. Costes, Phys. Rev. Lett., 2010, 105, 203201. [3] M. Lara, F. Dayou, J.-M. Launay, A. Bergeat, K. M. Hickson, C. Naulin, and M. Costes, Phys. Chem. Chem. Phys., 2011, 13, 8127. [4] S.-H. Lee and K. Liu, Chem. Phys. Lett., 1998, 290, 323.
25 INVITED TALK I5
The prospects of forming micro-kelvin molecules by sympathetic cooling with ultracold atoms
Jeremy M. Hutson and Maykel Leonardo González-Martínez
Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom
E-mail: [email protected]
There are several experimental methods that are capable of producing samples of molecules such as ND3, OH and NH at temperatures of 10 to 100 mK in electrostatic or magnetic traps. These include helium buffer-gas cooling and molecular beam deceleration. However, there is so far no way to transfer these cold molecules to the ultracold regime below 1 mK. Ultracold molecules would offer a wide range of opportunities for quantum simulation, quantum information processing, and the development of a controlled molecular assembly in which chemical transformations are carried out coherently on an entire sample of molecules.
Sympathetic cooling, in which cold species are cooled further by thermal contact with ultracold atoms, usually alkali metals, has been used successfully for atoms and molecular ions but has not yet been achieved for neutral molecules. The difficulty is that molecules are usually trapped in states that are not the lowest in the applied field, and can undergo inelastic (deexcitation collisions) that release kinetic energy and eject both collision partners from the trap.
We have explored sympathetic cooling theoretically in many different systems, in order to find collision partners that do not have this problem. We have found that, in general, light collision partners are favourable for sympathetic cooling because inelastic collisions are suppressed by centrifugal barriers at low fields and low collision energies. However, even atoms such as Li [1] and Mg [2] are predicted to succeed only at temperatures below about 10 mK, which is on the edge of what is possible.
In an exciting development, we have recently discovered that ultracold hydrogen atoms, which can be produced at very high densities at temperatures down to 50 1zK, have very favourable properties for sympathetic cooling. The potential energy surfaces for (spin-polarised) interactions of H with NH and OH are quite weakly anisotropic and the centrifugal barriers are very high. We predict that sympathetic cooling with ultracold H atoms can succeed from starting temperatures as high as 1 K, which are much easier to achieve.
26 FIG. 1: Contour plot of the ratio of elastic and inelastic cross sections for spin-polarised collisions of NH with H in a magnetic field. The ratio is much greater than 100, and thus favourable for sympathetic cooling, at all fields sampled by molecules at temperatures below 1 K in a quadrupole trap (below the diagonal black line).
References
[1]A. O. G. Wallis, E. J. J. Longdon, P. S. ˙Zuchowski, and J. M. Hutson, Eur. Phys. J. D 65, 151 (2011). [2]A. O. G. Wallis and J. M. Hutson, Phys. Rev. Lett. 103, 183201 (2009).
27 INVITED TALK I6
N + OH → NO + H: New theoretical results and comparison with experiment
P. Honvault
Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR CNRS 6303, Univ. de Bourgogne, 21078 Dijon Cedex, and UFR ST, Univ. de Franche-Comté, 25030 Besançon Cedex, France
*E-mail: [email protected]
We have recently studied the radical-radical N(4S) + OH(2) → NO(2) + H(2S) reaction ([1] and references therein). This reaction could be the major source of NO in dense interstellar clouds 4 2 1 + 3 and, in conjunction with the N( S) + NO( ) → N2( g ) + O( P) reaction, mediates the transformation from atomic to molecular nitrogen. Observations show that the atomic-to- molecular nitrogen abundance ratio, N/N2, in such environments is too high to be compatible with current gas-phase formation mechanisms.
We have treated the dynamics of N + OH using accurate and approximate quantum methods on a new global potential energy surface (PES) for the lowest triplet electronic state (a3A’’) of HNO [2]. Opacity functions, product state-resolved integral cross-sections, total reaction cross sections, state-specific and thermal rate constants, have been obtained by means of a time dependent wave packet method and of a time independent hyperspherical approach for several collision energies and temperatures. Quasi-classical trajectory calculations have also been performed.
The computed rate constants are in excellent agreement with the new measurements [1] at all temperatures. These results provide insight into the gas-phase formation mechanisms of molecular nitrogen in interstellar clouds. They suggest that the N + OH reaction may present a less pronounced variation with temperature, yielding substantially smaller rate constants at low temperatures than currently predicted. It may therefore present a bottleneck to N2 formation in dark clouds, thereby bringing models more into line with observations. In addition, the excellent agreement between theoretically and experimentally determined rate constants for the N + OH reaction validates these methods and should permit the measurement and calculation of rate constants for a wide range of atom-radical reactions in the near future.
References
[1] J. Daranlot, M. Jorfi, C. Xie, A. Bergeat, M. Costes, P. Caubet, D. Xie, H. Guo, P. Honvault, K.M. Hickson, Science 334, 1538 (2011). [2] A. Li, C. Xie, D. Xie, H. Guo, J. Chem. Phys. 134, 194309 (2011).
28 INVITED TALK I7
Following and controlling fundamental photochemistry: Quantum dynamics simulations using the MCTDH method.
G. A. Worth
School of Chemistry, University of Birmingham, Birmingham, B15 2TT, U.K.
The direct numerical solution of the time-dependent Schrödinger equation has become an es- sential tool for the theoretical study of fundamental molecular processes. The Multi- configuration time-dependent Hartree (MCTDH) method [1,2] provides a powerful quantum dynamics algorithm, which enables us to include more degrees of freedom than other methods. This is particularly useful in the study of photochemistry, where non-adiabatic effects can couple the motion of a number of degrees of freedom leading to the need to simulate a multi- dimensional problem [3]. A benchmark example was a study of the pyrazine molecule including explicitly all 24 vibrational modes [4].
In this talk, the method will be presented with some recent applications demonstrating the information that can be obtained. For example simulations on benzene, combined with experiments made by the Fielding group at UCL, have uncovered a channel with ultrafast inter- system crossing that plays a role in the classic channel 3 problem [5]. And the photodissociation of pyrrole is shown to be a multi-state problem in competition with a radiationless decay not seen in reduced dimensionality models. In addition to following the time-evolution of a system after excitation, laser control simulations will also be described using the local control scheme that requires less computer power than the more common optimal control [6].
References
[1] H.-D. Meyer, U. Manthe and L. S. Cederbaum, Chem. Phys. Lett. 165 (1990) 73. [2] M. H. Beck, A. J¨ackle, G. A. Worth and H.-D. Meyer, Phys. Rep. 324 (2000) 1. [3] G. A. Worth, H.-D. Meyer, H. Köppel, L. S. Cederbaum and I. Burghardt, Int. Rev. Phys. Chem. 27 (2008) 569. [4] A. Raab, G. Worth, H.-D. Meyer and L. S. Cederbaum, J. Chem. Phys. 110 (1999) 936. [5] R. S. Minns, D. S. N. Parker, T. J. Penfold, G. A. Worth and H. H. Fielding, PCCP 12 (2010) 15607. [6] G. A. Worth and C. Sanz Sanz, PCCP 12 (2010) 15570 , DOI: 10.1039/c0cp01740j.
29 INVITED TALK I8
Ultrafast dynamics and nanoclustering of water molecules at a membrane surface
Huib J. Bakker, Janneke de Heij, Lukasz Piatkowski
FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Water plays an important role in the transport and signalling functions of cell membranes [1]. Here we report on the use of femtosecond vibrational spectroscopy to study the dynamics and relative positioning of water molecules in a model membrane system. We determine the distribution of the water molecules by measuring the rate of resonant (Förster) vibrational energy transfer between the water molecules.
The femtosecond infrared laser pulses used in the experiments have a pulse duration of 100 fs, a center wavelength of 4 m (2500 cm-1), and a pulse energy of 5 J. The pulses are used in a polarization-resolved pump-probe experiment in which we measure the anisotropy of the excitation of the O-D stretch vibration as a function of time delay. The anisotropy decays as a result of molecular reorientation and Förster energy transfer between differently oriented O-D stretch vibrations of donor and acceptor HDO and D2O molecules [2,3]. The samples are 1,2- dioleoyl-sn-glycero-3-phosphocholine (DOPC) membranes that are hydrated with isotopic mixtures of HDO and D2O. We have studied stacks of lipid bilayers hydrated with x = 2.3, 3.5, 6.4 and 11.5 water molecules per lipid molecule. At each hydration level we performed measurements for four different isotopic compositions of water (10%, 25%, 50%, and 100% D2O in H2O).
From the decay of the anisotropy due to Förster energy transfer we determine the distribution of water molecules at the membrane surface. We find that water forms nanoclusters already at low hydration levels with an average distance between the O-D groups of 3.4 Å. In the case of a homogeneous distribution of water molecules over the membrane, the average distance between the O-D groups would have varied from 7 Å for x=2.3 to 4.2 Å for x=11.5. For pure D2O, the average distance between the O-D groups is 2.5 Å. Hence, the average distance of 3.4 Å in the water clusters is significantly smaller than the intermolecular distance in case the water molecules would have been homogeneously distributed over the lipid layers, but larger than the average distance in liquid water. Interestingly, the effective density and average distance hardly change when the hydration level is increased, which indicates that the size and shape of the clusters remain the same. Only the density of the nanoclusters increases when the hydration level is increased.
References
[1] A.G. Lee, Biochim. Biophys. Acta 1666, 62 (2004). [2] S. Woutersen and H.J. Bakker, Nature 402, 507 (1999). [3] L. Piatkowski, K.B. Eisenthal, and H.J. Bakker, Phys. Chem. Chem. Phys. 11, 9033 (2009).
30 INVITED TALK I9
Electronic structure and photoionization processes in clusters, fullerenes and endofullerenes
V.K. Ivanov
St.Petersburg State Polytechnic University, Politekhnicheskaya 29, 195251 St.Petersburg, Russia
*E-mail: [email protected]
The main goal of the present study is to determine the role of many-electron effects in the description of photoionization processes in metal clusters, fullerenes and endofullerenes. In this contribution the results of recent calculations of electronic structure and photoionization cross sections of these systems are discussed.
The calculations of photoprocesses are performed within the consistent many-body theory based on the jellium model. The single-electron energies and wave functions of valence-electron system are determined both within the Hartree-Fock (HF) and Local Density (LD) approximations. Then using the HF or LD wave functions of excited states the photoionisation amplitudes and cross sections are calculated within both the single-electron approach and the Random Phase Approximation with Exchange (RPAE).
For metal clusters valence electrons are considered in the field created by the uniformly charged ball. For fullerenes the ionic core is presented by an uniform distribution of positive charge (Z=240) over spherical layer of the finite thickness [1]. However the calculations show that the ground state properties of fullerenes cannot be described properly by the standard jellium model which produces, in particular, unreliable values for the total energy [1]. To improve the description of the ground state spectrum we add the pseudopotentials of two types: structureless and structured ones. The latter originates from the comparison of an accurate ab initio calculation with the jellium model one. Using this pseudopotential as a correction to the standard jellium model one can account, at least partly, for the sp2-hybridization of carbon atomic orbitals and relate parameters of the jellium model with the features of the system obtained from the more precise calculation.
The results of photoionization cross section calculations show extremely important role of many- electron correlations in metal cluster anions, fullerenes and endofullerenes in the vicinity of Plasmon resonance. Meanwhile there are some problems and limitations of the approach which are discussed in the presentation.
References
[1] Yannouleas C and Landman U 1994 Chem. Phys. Lett. 217 175
31 INVITED TALK I10
Liquid microjet synchrotron radiation photoelectron spectroscopy of chemical state change in aqueous solutions
Manfred Faubel
Max-Planck-Institute for Dynamics and Self-Organization, Goettingen, FRG
The free vacuum surface of aqueous solutions is prepared as a tiny, 15 μm diameter, sub‐ Knudsen size liquid jet in high vacuum. X-ray photoelectron spectroscopy of chemical shifts for 1s shells of N, O or C-atoms can thus be exploited for site specific binding state analysis in the liquid stqate.
Examples, illustrating this, are the pH dependent protonation/deprotonation of amino acid groups in organic molecules, or, the carbon dioxide capture in fire gas-washing aqueous solutions. In addition, near surface depth concentration profiles of individual solute groups at the liquid-gas interface have been investigated by variation of the photon/photoelectron energy.
32 INVITED TALK I11
Laser induced alignment of molecules dissolved in helium nanodroplets
Dominik Pentlehner1, Jens H Nielsen2, Klaus Mølmer2, Alkwin Slenczka3, Henrik Stapelfeldt1*
1Department of Chemistry, Aarhus University, Denmark 2Department of Physics and Astronomy, Aarhus University, Denmark 3Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Regensburg, Germany
*E-mail: [email protected]
Controlling how molecules are turned in space offers unique opportunities for studying and exploiting the ubiquitous orientational dependence of molecules’ interactions with other molecules, atoms or polarized electromagnetic interaction and for eliminating the blurring of molecular properties and processes which usually occur in observations of randomly aligned molecules.
For isolated molecules in the gas phase moderately intense, nonresonant laser pulses provide a versatile approach to creating very precise confinement of the molecular orientation along axes fixed in the laboratory frame [1]. Laser based alignment techniques, developed over the past 10- 15 years, are now used in a range of applications in physics and chemistry. By contrast, for molecules in a dissipative environment there have been no experiments and only a few theoretical works on laser based alignment, despite the tremendous potential of being able to control the spatial orientation of molecules inside, e.g., a liquid.
In this talk we present the first experimental studies of laser induced alignment of molecules embedded in superfluid helium nanodroplets [2-3]. The alignment is conducted in the nonadiabatic or impulsive limit where the pulse duration (here 0.45 ps) is much shorter than the classical rotational period(s) of the molecules. We show that methyliodide molecules inside a He nanodroplet reach maximum alignment about 20 ps after the alignment pulse and gradually loose the alignment completely in another 60 ps. This dynamics is completely different from that of isolated methyliodide molecules where alignment occurs in regularly spaced (by 33.3 ps), narrow time windows, termed revivals.
References
[1] H. Stapelfeldt and T. Seideman, Rev. Mod. Phys. 75, 543 (2003). [2] J. P. Toennies and A. F. Vilesov, Angew. Chem. Int. Ed. 43, 2622 (2004). [3] F. Stienkemeier and K. K. Lehmann, J. Phys. B: At. Mol. Opt. Phys. 39, R127 (2006).
33 INVITED TALK I12
Quantum coherence at room temperature: from single molecules to light harvesting complexes
Daan Brinks1*, Richard Hildner2, Fernando D. Stefani3, Niek F. van Hulst1,4
1ICFO – Institute of Photonic Sciences, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain 2Experimentalphysik IV, Universität Bayreuth, 95440 Bayreuth, Germany 3 Instituto de Física de Buenos Aires (IFIBA CONICET), Universidad de Buenos Aires, 1428 Buenos Aires, Argentina 4ICREA - Inst. Catalana de Recerca i Estudis Avancats, 08015, Barcelona, Spain
*E-mail: [email protected]
For experimental research into quantum effects in complex molecules, coherences are usually established by synchronizing a subset of molecules in an ensemble using ultrashort laser pulses. However, physical processes in intricate systems such as natural light- harvesting complexes are generally influenced by differences in molecular conformations and environments. This restricts the understanding that data obtained in uniform bulk experiments can provide on the functioning of natural light harvesting. Moreover, even the synchronized subset of molecules will still possess an intrinsic inhomogeneity that limits the degree of coherence that can be created for control and computation purposes. The natural and ultimate solution to overcome these limitations is the investigation of the behaviour of one molecule at a time.
Recently we developed techniques to create and investigate electronic and vibrational coherence in single molecules at room temperature. After a discussion of some proof of principle experiments on the ultrafast coherent control of single molecules in disordered environments at room temperature [1,2], our most recent work on probing the effect of coherence in energy transfer in individual Light Harvesting complexes (LH2-complexes from the purple bacterium Rhodopseudomonas acidophila ) will be treated [3]. This involves the investigation of differences in transfer behaviour between individual complexes, and resolving changes in transfer pathways in individual complexes in time.
References
[1] Brinks et al., Nature 465, 905 (2010) [2] Hildner, Brinks, van Hulst, Nature Physics 7, 172 (2011) [3] Hildner, Brinks, Cogdell, van Hulst, submitted
34 INVITED TALK I13
Proteins in the freezer: Embedding mass/charge selected biomolecular ions in liquid helium droplets
Doo-Sik Ahn, Isabel Gonzalez, Frank Filsinger, Gerard Meijer and Gert von Helden*
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
*E-mail: [email protected]
Liquid helium droplets are ideal nano-cryostats for the investigation of dopant molecules. The conditions inside a helium droplet are isothermal at 0.38 K and, as helium droplets are superfluid, the interaction between the helium matrix and the molecules are weak, causing often only small perturbations on the molecule. Furthermore, helium itself is transparent over a wide spectral range from the far IR to the deep UV. Because of these unique properties, liquid helium droplets have been used as matrices in many spectroscopic experiments [1].
We have developed an experimental approach in which mass-to-charge selected ions that are stored and accumulated in an ion trap are picked up by helium droplets traversing the trap [2]. The approach is conceptually similar to pickup experiments of neutral molecules from gas cells. A crucial difference is, however, that in the case of the ion trap, use is made of the high kinetic energy of the heavy helium droplets, which allows the ion-doped droplets to overcome the longitudinal trapping potential and to escape the trap. Further downstream, the charged droplets can be investigated and detected. The dopant molecules can be as large as the protein Cytochrome C (MW ~ 12,000 amu). The sizes of the ion-doped droplets are accurately determined via time-of-flight measurements. Depending on the experimental conditions in the droplet source, the observed doped droplet sizes range from 1 x 104 helium atoms to very large droplets, containing more than 1010 helium atoms. Progress on spectroscopic experiments on dopant species in helium droplets will be reported upon.
References
[1] J. P. Toennies, A. F. Vilesov, Angew. Chem. Int. Ed. 43, 2622 (2004). [2] F. Bierau, P. Kupser, G. Meijer, G. von Helden, Phys.Rev.Lett. 105, 133402 (2010).
35 INVITED TALK I14
Flexibility and structure of peptides from far-infrared spectroscopy using free electron lasers
Wim J van der Zande, Anouk Rijs, Michael Schmitt, Sander Jaeqx
In the Institute for Molecules and Materials within the Radboud University Nijmegen intense infrared and far-infrared lasers sources based on free electron lasers (FELs) are combined in the FELIX radiation facility. A new FEL, FLARE, covers the wavelength range from 100 m to 1500 m. Two instruments, one (FELICE) even allowing for intra-cavity experiment, cover the range from 3 m to about 200 m. The strength of the FELs is their enormous fluence, with energies over the whole wavelength range of about 100 mJ per macropulse. Weak FIR transitions can be saturated. The short duration of the macropulses of about 8 – 10 s makes it possible to employ various forms of action spectroscopy for example combining FIR with UV REMPI schemes in ion- dip spectroscopy.
We will present new data, taken at the FOM Institute Rijnhuizen, showing the potential of FIR spectra using action spectroscopy. Theory is not yet able to predict accurately the large amplitude motions excited at the long wavelengths, motions that involve many atoms and a large fraction of the backbone. Eventually, we want to understand the microscopic motion that is excited by these long wavelengths and learn to obtain detailed structural information from the FIR response. We may even discover that these motions are mirrored in the protein motion associated with their catalytic activity. We will present experimental data on small peptides containing three to twelve amino-acid residues.
36 Contributed talks
37 CONTRIBUTED TALK C1
Roaming in the dark: Deciphering the mystery of the NO3 photolysis mechanism
Michael P. Grubb1*, Michelle L. Warter1, Hongyan Xiao2, Satoshi Maeda2-3, Keiji Morokuma2-4, Simon W. North1
1Department of Chemistry, Texas A&M University, College Station, Texas 77842, USA. 2Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano Nishihiraki-cho,Sakyo, Kyoto 606-8103, Japan. 3The Hakubi Center, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan. 4Cherry L. Emerson Center for Scientific Computationand Department of Chemistry, Emory University, Atlanta, GA 30322, USA.
Although the NO3 →NO + O2 photolysis reaction is of significant interest to atmospheric chemistry, the mechanism by which this reaction occurs has remained a mystery and no energetically accessible transition state has been previously calculated. Velocity map ion imaging experiments [1] combined with ab initio calculations [2] have revealed that the reaction proceeds exclusively via the unusual “roaming mechanism”, with no evidence of a competing traditional “tight” transition state pathway. Futhermore, we find that this reaction proceeds along two electronic potential surfaces, a characteristic that has not been observed in previous roaming systems [3]. We will discuss the significance of this discovery in regards to roaming dynamics in general, for which this system has provided new insight.
References
[1] M. P. Grubb, M. L. Warter, K. M. Johnson and S. W. North, J. Phys. Chem. A, 2011, 115, 3218-3226. [2] H. Y. Xiao, S. Maeda and K. Morokuma, J Phys Chem Lett, 2011, 2, 934-938. [3] M. P. Grubb, M. L. Warter, H. Y. Xiao, S. Maeda, K. Morokuma and S. W. North, Science, 2012, 335, 1075-1078.
38 CONTRIBUTED TALK C2
Energy flow and sequestration in photoexcited ICN-
W. Carl Lineberger1*
1JILA and Department of Chemistry University of Colorado
*E-mail: [email protected]
– We report on the dynamics of photoexcited ICN (Ar)0-5. Photodetachment produces quasi- thermal electron emission that leaves ICN with up to 2.85 eV of internal energy. Photodissociation at 2.5 eV shows one-atom caging and highly solvated anion products. Quantum dynamics calculations indicate efficient energy transfer into CN rotation upon excitation to the 2 1/2 excited state. CN rotation, indicated by photodetachment and photodissociation, proves vital to explain the unique dynamics observed.[1, 2]
References
[1] A. S. Case, E. M. Miller, J. P. Martin, Y.-J. Lu, L. Sheps, A. B. McCoy, and W. C. Lineberger, "Dynamic Mapping of CN Rotation Following Photoexcitation of ICN– " Angew. Chem., Int. Ed. Eng. 51, 2651-53 (2012). [2] E. M. Miller, L. Sheps, Y.-J. Lu, A. S. Case, A. B. McCoy, and W. C. Lineberger, "New View of ICN Ã continuum using Photoelectron Spectroscopy of ICN-," J. Chem. Phys. 136, 044313 [7 pages] (2012).
39 CONTRIBUTED TALK C3
- Towards probing the dynamics of the endothermic SN2-reaction I + CH3Cl using velocity map imaging
E. Carrascosa, A. H. Kelkar, M. Stei, F. Hochheimer, J. Wildauer, T. Best and R. Wester
Institut f. Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstrasse 25/3, A-6020 Innsbruck, Austria
The bimolecular nucleophilic substitution (SN2) reaction is one of the most widely studied reactions in gas phase due to its fundamental simplicity and as a model reaction in physical organic chemistry. Earlier experimental studies were focused on the investigation of energetics and on the measurement of reaction cross sections [1]. In the past, understanding of the reaction dynamics relied strongly on chemical dynamics simulations. However, advancements in the cross beam scattering and velocity map imaging techniques in the previous decade have led to new insights into the reaction dynamics from measurements of correlated energy and angle differential cross sections.
In our group we have investigated the dynamics of the exothermic SN2 reaction of chloride ion with iodomethane [2]. In the present study we currently focus on the inverse (endothermic) - - reaction I + CH3Cl ---> CH3I + Cl , using a cross beam velocity map imaging setup [3]. The low - energy I ions are produced from a mixture of CH3I + Argon in an electron impact supersonic expansion source via dissociative attachment. Velocity spread of the pulsed ion beam can be further controlled in an rf multipole ion trap placed collinear to the ion beam direction. In the interaction region a pulsed supersonic beam of CH3Cl is crossed with the ion beam and the complete velocity vector of the product anion is measured using velocity map imaging. In this poster, we will present the experimental setup in detail as well as discuss the scope and status of the experiment. With this study, we aim to investigate experimentally the energy distribution within the ion-dipole complex and its dependence on the internal quantum states.
As an outlook of future research plans for our setup, we will present plans for the dynamical + + + studies on the reaction CO + H3 → HCO /HOC , one of the most dominant reactions in interstellar space, which has been already widely discussed theoretically [4]. As HCO+ is the precursor of many organic molecules, the understanding of this reaction, including a study of the ratio between both product isomers, could help to explain how and why complex organic molecules are formed in the interstellar space and how they develop.
References
[1] L. A. Angel, K. M. Erwin, J. Phys. Chem. A 2004, 108, 9827. [2] J. Mikosch, S. Trippel, C. Eichhorn, R. Otto, U. Lourderaj, J. X. Zhang, W. L. Hase, M. Weidemüller and R. Wester, Science 2008, 319, 183. [3] J. Mikosch, U. Frühling, S. Trippel, D. Schwalm, M. Weidemüller and R. Wester, Phys. Chem. Chem. Phys. 2006, 8, 2990. [4] H. Li,T. Hirano, T. Amano and R.J. Le Roy, J. Phys. Chem 2008, 129, 244306
40 CONTRIBUTED TALK C4
New concepts and approaches for precise quantification and exact quantum description of the dynamics of molecular collisions
Marcelo P. de Miranda
School of Chemistry, University of Leeds Leeds LS2 9JT, United Kingdom
In this presentation we will review our recent and ongoing quantum theoretical work on stereochemical, mechanistic and geometrical aspects of the dynamics of isolated molecular collisions, and reactive collisions in particular. The focus will be on the new concepts we have developed or whose use we advocate. They include:
1. Canonical mechanisms theory, in which collision mechanisms are defined as pure-state transformations [1]. This theory is rigourously quantum mechanical and involves no approximation whatsoever. It allows for identification of the number of independent (“canonical”) mechanisms contributing to a molecular collision, for quantitative determination of the contribution of each mechanism to the collision cross section, and for identification of reagent states that do not react and product states that cannot be formed.
2. Collision entropy, which is a rigourously-defined and convenient quantity that allows for precise quantification of the sensitivity of a molecular collision to steric arrangements of reagents and products [2]. The collision entropy is a real number in the 0 S 1 range.
3. The use of antieigenvalues, antieigenvectors and other operator trigonometric concepts in the analysis and geometrisation of molecular collision stereodynamics [3]. Operator trigonometry [4] facilitates rationalisation of the relation between entrance- and exit- channel stereochemistries. From a geometrical point of view, it leads to mathematically well defined angles between pure reagent states, between pure product states, and therefore between pure reagent-to-product transformations (i.e., collision mechanisms).
4. The use of Majorana’s stellar representation [5] of pure but otherwise general angular momentum states in the analysis and geometrisation of molecular collision stereodynamics. Majorana stars and Majorana constellations strikingly reveal the similarity — even the “sameness” — between the reagent and product states partaking in a pure- state transformation (i.e. a collision mechanism). From a geometrical point of view, the stellar representation allows for a natural incorporation of Fubiny-Study distances [6] into the description of molecular collision dynamics.
The examples will come from application of these concepts to the interpretation of quantum scattering or experimental data on benchmark inelastic [7] or reactive [1,2,8] collisions, with the latter taking place in the hyperthermal [1,2], ultracold [1] or statistical [8] regimes.
41 References
[1] J. Aldegunde, F. J. Aoiz, V. Sáez-Rábanos, B. K. Kendrick and M. P. de Miranda, Phys. Chem. Chem. Phys. 9, 5794 (2007). [2] M. P. de Miranda and B. K. Kendrick, J. Phys. Chem. A 113 (Vincenzo Aquilanti Festschrift), 14943 (2009). [3] M. P. de Miranda, S. D. S. Gordon and J. Aldegunde, Mol. Phys., in press (Dudley Herschbach Festschrift). DOI: 10.1080/00268976.2012.689869. [4] K. Gustafson, Antieigenvalue Analysis (World Scientific, Singapore, 2011). [5] E. Majorana, Nuovo Cimento 9, 43 (1932). [6] I. Bengtsson and K. ˙Zyczkowski, Geometry of Quantum States (Cambridge University Press, Cambridge, 2006). [7] P. G. Jambrina, J. Kłos, F. J. Aoiz and M. P. de Miranda, Phys. Chem. Chem. Phys. 14 , 9826 (2012). [8] P. G. Jambrina, J. Aldegunde, M. P. de Miranda, V. Sáez-Rábanos and F. J. Aoiz, Phys. Chem. Chem. Phys. 14 , 9977 (2012).
42 CONTRIBUTED TALK C5
Reactive collisions and interactions of ultracold dipolar molecules
Svetlana Kotochigova
Department of Physics, Temple University, Philadelphia, Pennsylvania, 19122, USA
Recent successful production of ultracold polar molecules in the ground rovibrational state has allowed an unprecedented control of molecular interactions. For example, heteronuclear molecules prepared in the lowest rovibrational ground state have a permanent electric dipole moment, which gives rise to tunable dipole-dipole interactions between them. One of the challenges that quantum physics with polar molecules faces is understanding the level of control of molecular interactions as well as their reactivity for low collision energy. First experimental and theoretical efforts to describe quantum chemical reactions of ultracold KRb molecules have been based on probing the inelastic collision loss rates. A next step is to provide information on the reaction products.
Here I describe some of our results related to the interactions and chemical reactivity of ultracold neutral and ionic two-atomic molecules. First, we explore physics of the long-range dispersion potentials of interacting neutral alkali-metal, alkaline-earth, and rare-earth molecules. We calculate ab initio van der Waals coefficients and use a quantum formalism to study scattering properties of such molecules in the presence of an external electric field and optical lattice [1]. In particular, the chemical reaction between ultracold 2 molecules are investigated, which at long-range have both electrostatic and magnetic interactions. Our group has already started to study LiCa, LiSr, and LiYb molecules [2], which are subject of several ongoing ultracold experiments.
I will also describe our work on charge-exchange chemical reactions with cold ionic molecules. Recently experimental data has become available for charge-exchange chemical reactions with heavy ionic molecules. We first calculated the electronic structure of these ionic molecules as they are typically unknown in the literature. To obtain rate constants we also needed dipole moments and other molecular properties. Then using this information we performed accurate quantum-mechanical calculations of radiative and non-radiative rates constants relevant to the particular reaction. We found that rate constants for such reactions show unique temperature dependences [3].
References
[1] G. Quemener, J. Bohn, A. Petrov, and S. Kotochigova, Phys. Rev. A 84, 062703 (2011). [2] S. Kotochigova, A. Petrov, M. Linnik, J. Klos, and P. S. Julienne, J. Chem. Phys. 135, 164108 (2011). [3] W. G. Rellergert, S. T. Sullivan, S. Kotochigova, A. Petrov, K. Chen, S. J. Schowalter, and E. R. Hudson, Phys. Rev. Lett, 107, 243201 (2011).
43 CONTRIBUTED TALK C6
Is there intermolecular Coulomb decay (ICD) in liquid water and water clusters? Dynamical perspective
Petr Slavíček1,2, Milan Ončák1,2 and Bernd Winter3
1Department of Physical Chemistry, Institute of Chemical Technology, Technická 5, 16628 Prague 6, Czech Republic 2J. Heyrovský Institute of Physical Chemistry, v.v.i., Dolejškova 3, 18223 Prague 8, Czech Republic 3Helmholtz-Zentrum Berlin für Materialien und Energie, and BESSYD-12489 Berlin, Germany
Absorption of soft X-ray radiation by water clusters, liquid water and aqueous solution leads to the formation of ionized and electronically excited water molecules and solute species. These highly unstable structures relax by different channels. Experimentally, most of the information on electronic relaxation in bulk water is obtained via X-ray fluorescence. Much more important autoionization processes started to be studied in detail only recently, leading to identification of novel electronic phenomena such as Intermolecular Coulomb Decay (ICD).1,2
Here, we focus on theoretical simulations of the processes following the valence and core electron detachment of liquid water. We combine techniques of path integral molecular dynamics, non-adiabatic quantum dynamics and ab initio simulations to gain insight into the nature of electronic relaxation following the creation of electronic hole. We identify a mechanism by which doubly ionized yet charge separated molecular complexes are formed within a local Auger process assisted by nuclear dynamics. We demonstrate that nuclear dynamics is important even for the ultrafast Auger processes following the core electron ionization. New experimental data on isotope and temperature control of the electronic relaxation processes following the X-ray photoionization in liquid water3 are interpreted. Dynamics following low-energy excitation or ionization1 with closed auto-ionization channel will also be discussed.
References
1. Mucke, M.; Braune, M.; Barth, S.; Forstel, M.; Lischke, T.; Ulrich, V.; Arion, T.; Becker, U.; Bradshaw, A.; Hergenhahn, U., A hitherto unrecognized source of low-energy electrons in water. Nat Phys 2010, 6 , 143- 146. 2. Aziz, E. F.; Ottosson, N.; Faubel, M.; Hertel, I. V.; Winter, B., Interaction between liquid water and hydroxide revealed by core-hole de-excitation. Nature 2008, 455, 89-91. 3. Thürmer, S.; Ottosson, N., Seidel, R.; Hergenhahn, U.; Bradforth, S.E., Slavicek, P. and Winter, B. Controlling energy transfer in liquid water upon X-ray exposure via isotopic substitution and temperature, in preparation. 4. Svoboda, O.; Oncak, M.; Slavicek, P., Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. II. Photoionization. J Chem Phys 2011, 135, 154302.
44 CONTRIBUTED TALK C7
Elucidating charge delocalization in aqueous polypyridyl Fe(II) compounds during photo-induced intersystem crossing via time- resolved spectroscopy in the X-ray water window
Nils Huse,1* Benjamin E. Van Kuiken,2 Hana Cho,3,4 Matthew L. Strader,3 Tae Kyu Kim,4 Munira Khalil,2 and Robert W. Schoenlein3
1Max Planck Research Department for Structural Dynamics, University of Hamburg & Center for Free Electron Laser Science, 22607 Hamburg, Germany 2Department of Chemistry, University of Washington, Seattle, Washington 98195, USA 3Ultrafast X-ray Science Lab, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 4Department of Chemistry, Pusan National University, Busan 609-735, Korea
*E-mail: [email protected]
Ultrafast core-level spectroscopy is unique in its ability to probe transient states of matter with chemical specificity of single atom species by exciting core-level transitions from highly localized initial states. In principle, this allows for multiple ‘views’ of valence charge density depending on the probed atomic species. Previously, we have employed ultrafast X-ray spectroscopy to probe the ground-state low-spin and transient high-spin state valence charge density of polypyridyl iron(II) complexes in solution via metal 2p spectroscopy [1,2]. The latter directly probes the spin- state and valence charge density of the metal and its immediate chemical environment on ultrafast time‐scales by dipole 2p→3d transitions. Here we report time‐resolved nitrogen K‐edge 2+ spectroscopy of [Fe(bpy)3] to provide a ‘ligand view’ of the valence charge density upon metal- to-ligand charge-transfer excitation and subsequent multiple ultrafast intersystem crossings. These measurements in the X-ray water window are compared with ab initio DFT calculations.
2+ Our experimental results on aqueous [Fe(bpy)3] before and after laser excitation in combination with ab initio DFT calculations suggest a very localized charge density in the low-spin (LS) ground- state featuring a contracted ligand cage while the valence charge distribution of the high-spin state with a dilated ligand cage is strongly delocalized, exhibiting strong charge-transfer character. This finding is very different from what we had initially concluded in previous work. We conclude that the strong charge‐transfer character of the high‐spin state facilitates MLCT→HS relaxation (but not necessarily bypassing the other metal-centered ligand-field states).
References
[1] N. Huse et al., J. Am. Chem. Soc. 132, 6809 (2010). [2] N. Huse et al., J. Phys. Chem. Lett. 2, 880 (2011).
45 CONTRIBUTED TALK C8
Sticky ice - The first step to heterogeneous chemistry
Lengyel J. 1, Kočišek J.1*, Fedor J. 2, Poterya V. 1, Pysanenko A. 1, Svrčková P. 1, Fárník M. 1
1J. Heyrovský Institute of Physical Chemistry v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague, Czech Republic 2Department of Chemistry, University of Fribourg, Chemin du Musee 9, CH-1700 Fribourg, Switzerland
*E-mail: [email protected]
The results of simple, but unique experiment, where absolute cross sections for pickup of several important species on ice clusters were measured are presented. Observed cross sections significantly differ from widely accepted geometrical ones, what can change the present picture of heterogeneous chemistry.
The most striking example of heterogeneous chemistry on ice nanoparticles is the ozone depletion cycle. Several mechanisms were proposed for the production of halogen radicals in stratospheres resulting in formation of ozone hole. Whether they are based on photon or electron induced chemistry, all are involving heterogeneous reactions on ice nanoparticles. Another important example of heterogeneous chemistry is taking place in interstellar molecular clouds, where icy mantles of dust grains act like reservoirs for the reactive species and important energy exchangers. While the reactions are often studied into stunning details, the most important step is omitted – the adsorption of the reactants on the icy particle. The most important characteristics of this step, its cross section, has been experimentally obtained in present study.
Clusters with mean size 260 or 430 water molecules and amorphous character were prepared in water vapor expansion into vacuum. Skimmed beam passed through pickup chamber, where the gas of interest was introduced. After pickup the cluster speed distribution was measured using pseudorandom chopper technique. The pickup cross section was then estimated on the basis of the speed variation with the pickup cell pressure. Details can be found in current publication [1]. Studied molecules include water, methane, methanol, ethanol, nitrogen monoxide and dioxide, hydrogen bromide, hydrogen chloride and freon CFC12.
Acknowledgement: This work has been supported by the Grant Agency of the Czech Republic project No.: 203-09-0422 and P208-11-0161 and by the European project FP7 MC-ITN No.: 238671 Iconic.
References
[1] a) Lengyel J. et al. J. Chem. Phys. 2012, 137 (3) In press. b) Fedor J., Poterya V., Pysanenko A., Fárník M., J Chem Phys. 2011, 135 (10) 104305.
46 CONTRIBUTED TALK C9
H2 formation on non-ideal graphene in interstellar space
Sean J. Stayte, Benjamin J. Irving, and Anthony J. H. M. Meijer*
Dept. of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK
* Electronic address: [email protected]
Molecular hydrogen (H2) is common in the interstellar medium (ISM), and plays an important role in thermal regulation. Due to the low densities and energies of interstellar space, it is accepted that molecular hydrogen formation is catalysed on the surface of graphitic dust grains.1 The key step in this mechanism is the adsorption of the first H atom onto the surface, which has previously been shown to be an activated process.
Due to the presumed graphitic nature of the dust grains, most previous work has used graphenic systems as models for the graphite surface. However, the ISM is far from an ideal environment, and it is likely that the surfaces of dust grains are not perfect, infinite graphene sheets. As such, our work focusses on deviations from ideal systems that are thought to be present in the ISM and thus offer a route to H2 formation. We (and others) have shown that hydrogen atom adsorption para to a previously adsorbed hydrogen can be a barrierless process.2–4 Our current work builds on this observation, examining the influence of heteroatom defects and ionisation on hydrogen adsorption dynamics using DFT calculations on both molecular and periodic systems.
References
1. H. C. van der Hulst, Rec. Astron. Obs., 1949, XI(II). 2. L. Jeloaica and V. Sidis, Chem. Phys. Lett., 1999, 300, 157. 3. X. Sha and B. Jackson, Surf. Sci., 2002, 496, 318. 4. B. J. Irving, A. J. H. M. Meijer, and D. Morgan, Phys. Scr., 2011, 84, 028108.
47 CONTRIBUTED TALK C10
Interfacial Collisions Dynamics of OH with organic liquid surfaces
Grant Paterson*, Kerry L. King, Giovanni Rossi, Marija Iljina, Robin Westacott, Matthew L. Costen and Kenneth G. McKendrick.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS
*E-mail: [email protected]
The dynamics of potentially reactive gas–liquid interfacial collisions remain relatively unexplored, but are of intrinsic fundamental interest. They are also of widespread applied importance, for example in heterogeneous atmospheric chemistry. We present an experimental investigation of the dynamics of gas–liquid interfacial collisions of OH radicals with organic liquids, chosen partly for their interest as mimics of the surfaces of atmospheric aerosols. Specifically, we compare the inelastic scattering of OH from potentially reactive long-chain saturated (squalane) and unsaturated (squalene) hydrocarbons with an inert organic liquid (perfluoropolyether, PFPE).
The OH radicals were produced by laser photolysis of allyl alcohol at 193 nm. The resulting OH photofragments, which have hot translational and rotational distributions, travel towards a continuously refreshed liquid surface. Those OH molecules that survive and recoil from the liquid are probed by laser-induced fluorescence (LIF) on the OH A2Σ+ –X2Π band. Using this all‐optical approach, we measure state-specific appearance profiles for a selection of OH rotational (and 2 fine-structure) quantum states (X Π3/2, v = 0, N = 1, 5 and 10) to determine the translational distribution of the scattered products. In addition, we measure excitation spectra to reveal the internal product-state distribution for velocity subgroups corresponding to chosen photolysis- probe time delays.
We find strong and unambiguous positive correlations between product rotation and translation in the scattered OH for all the liquids examined. Overall, the OH experiences a significant net rotational cooling. There are clear dynamical differences in the inelastic scattering from the liquids studied and these effects are attributed to differences in the chemical structure at the surface. We also explore differences in OH yield from the liquids and, again, correlate any reactive uptake with the chemical structure of the liquid. These observations complement our previous experimental programme using HONO as a photolytic source of OH, which yields a much colder initial rotational distribution compared to allyl alcohol. Our experimental findings are supported by molecular dynamics simulations of the liquid surface structures.
48 CONTRIBUTED TALK C11
Probing the ultrafast dynamics of highly-excited states in N2 molecules excited by attoseconds XUV pulses
F. Calegaril*, A. Trabattonil, L. Wang", S. Anumulal, M. Lucchinil, F. Kelkensberg2, W. Siu2, G. Sansone', M.J.J. Vrakking3, M. Hochlaf4, M. Nisolil
Politecnico di Milano, Dept. Physics, IFN-CNR, Piazza L. da Vinci 32, 20133 Milano, Italy 2FOM Institute AMOLF, Science Park 104, NL-1098 XG Amsterdam, The Netherlands 3Max-Born-Institut, Max Born Strasse 2A, D-12489 Berlin, Germany 4Univ. Paris-Est, MSME UMR 8208 CNRS, 5 bd Descartes,77454, Marne-la-Vallee, France
The ultrafast dynamics induced by photo-ionization in atoms and molecules plays an important role in a number of fundamental physical and chemical phenomena [1]. Understanding the coupling of electronic and nuclear degrees of freedom as well as the electron correlations immediately following, or accompanying, optical excitation is particularly important and requires the use of ultrafast methods [2,3].
In this work we photo-ionized N2 molecules with short attosecond pulse trains (APTs) and we probed the ultrafast dynamics using 800-nm 5-fs pulses having a peak intensity of about 1013 W/cm2. APTs were produced by high-order harmonic generation (HHG) in a cell filled with Argon or Xenon. In the case of APTs generated in Argon the XUV bandwidth extends beyond the second ionization threshold of N2 at 42.88 eV, thus promoting dissociation from both highly excited + 2+ + electronic states of N2 or low-excited energy states of N2 . The N angular distribution and yield are recorded by a velocity-map imaging spectrometer (VMIS).
In the kinetic energy released (KER) spectrum of the N+ fragments, acquired as a function of the delay between XUV and IR, two different processes are visible. The first one, in the energy range between 2.3 eV and 3.7 eV, is characterized by a temporal evolution of ~120 fs. The second one at ion energies below 2.5 eV exhibits a faster temporal evolution. We will show that the first process is + related to autoionization of highly excited states of N2 ; in particular a control of the autoionization process by intense ultrashort IR pulses can be achieved. The dynamics of the second process is related to the duration of the IR probe pulse and it displays a clear oscillatory behaviour with a characteristic period of the order of half the optical cycle of the probe pulse.
References
[1] F. Krausz and M. Ivanov, Rev. Mod. Phys. 81, 163 (2009). [2] G. Sansone et al., Nature (London) 465, 763 (2010). [3] A. S. Sandhu et al., Science 322, 1081 (2008).
49 CONTRIBUTED TALK C12
Electronic spectroscopy of phthalocyanines and porphyrines inside superfluid helium nanodroplets
Ricarda Riechers1,2, Alkwin Slenczka1*
1Institute for Physical und Theoretical Chemistry, University of Regensburg, 93053 Regensburg, Germany; 2Zeiss AG, Oberkochen, Germany.
*[email protected]‐regensburg.de
Electronic spectra of Phthalocyanine and Porphyrin derivatives doped into superfluid Helium nanodroplets have been recorded. The spectral resolution allows for analyzing the electronic origin for fine structure due to spolvation in a Helium droplet. Both the zero phonon line (ZPL) and the phonon wing (PW) reveals rather complicated fine structures. Despite of the structural similarity of all the compounds, the spectral fine structure is highly dopant specific some of which can only be resolved with narrow band laser systems.
The investigation of electronic spectra of molecules in Helium droplets aims to study the dopant to Helium interaction. In contranst to molecular rotation of vibration the electronic degree of freedom appears to be particularly sensitive to the helium environment which becomes visible in the fine structure splitting which is unknown from the corresponding gas phase data. The spectra obtained for Phthalocyanine and Porphyrin derivatives confirm the changing electron density distribution as the key quantity for line widths in helium droplet spectroscopy. Our data which are most detailed for Phthalocyanine [1] are benchmarks for simulations on microsolvation of molecules in superfluid helium droplets.
References
[1] J. Chem. Phys. 115 (2001) 10199; J. Chem. Phys. 115 (2001) 10206; J. Chem. Phys. 118 (2003) 8256; J. Chem. Phys. 120 (2004) 5064; J. Chem. Phys. 122 (2005) 244317; J. Chem. Phys. 131 (2009) 194307; J. Chem. Phys. 133 (2010) 114505; J.Phys.Chem. A 115 (2011) 7034; ChemPhysChem 5 (2004) 1014; ChemPhysChem 10 (2009) 761; Rev.Sci.Instrum. 80 (2009) 043302.
50 CONTRIBUTED TALK C13
Molecular superfluidity in helium studied using impulsive alignment
Gediminas Galinis1, Luis Guillermo Mendoza Luna1, Mark Watkins1, Russell Minns3, Andrew Ellis2, Edmond Turcu4, Cephise Cacho4, Emma Springate4, Klaus von Haeften1*.
1Department of Physics and Astronomy, University of Leicester, UK. 2Department of Chemistry, University of Leicester, UK. 3Department of Chemistry, University of Southampton, UK. 3CLF, STFC, Rutherford Appleton Laboratories, UK.
E-mail:*[email protected]
Superfluidity is a remarkable phenomenon commonly associated with frictionless flow. Although, this macroscopic effect is understood quite well, our understanding of how superfluid helium responds to motion on the nanoscale is much weaker. This issue was initially addressed by infrared and microwave spectroscopic studies of molecules in helium droplets; it was found that the molecules rotate almost freely. This effect was named Molecular Superfluidity [1]. More recently the minimum number of helium atoms required to see a superfluid effect, a phenomenon we can call incipient superfluidity, has been studied for small helium clusters using spectroscopy [2]. However, while intriguing results have been obtained, there are serious limitations in using spectroscopy to extract information on rotation of probe molecules in helium clusters and helium nanodroplets.
Here we apply an entirely new approach to study the response of superfluid helium to molecular rotation. A femtosecond pump-probe laser setup was used to excite a rotational wave packet and to follow its propagation in time. The periodically recurring molecular alignment was probed at a variable time delay by detecting ion fragments of Coulomb-exploded molecules with a velocity map imaging spectrometer. The revivals of the wave packet showed how the rotational periods varied for molecules inside helium clusters of different sizes. By following molecular motion in the time domain, the probe molecule does not need to have a dipole moment. We also have the further and very important advantage that the rotation of heavy molecules becomes accessible for investigation.
Our first results using CO as the probe molecule show a number of distinct peaks in the Fourier transform of the time-resolved spectra, which we tentatively assign to the rotational periods of CO attached to clusters of N = 3-8 helium atoms. We find the B constants (effective moments of inertia) of CO to be in agreement with previously measured values for N = 3-8 [2]. Larger clusters of N = 9-10, however, show a larger reduction of the B constants than predicted [2].
References
[1] S. Grebenev et. al., Science, 279, 2083, (1998). [2] L. A. Surin et. al., Phys. Rev. Lett., 101, 233401, (2008).
51 CONTRIBUTED TALK C14
Observation of quantum effects in sub-Kelvin cold reactions
A.B. Henson, S. Gersten, Y. Shagam, J. Narevicius, E. Narevicius
Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel
There has been a long-standing quest to observe chemical reactions at low temperatures where reaction rates and pathways are governed by quantum mechanical effects. So far this field of Quantum Chemistry has been dominated by theory. The difficulty has been to realize in the laboratory low enough collisional velocities between neutral reactants, so that the quantum wave nature could be observed. We report the first realization of merged neutral supersonic beams, and the observation of clear quantum effects in the resulting reactions. We observe orbiting resonances in the Penning ionization reaction of argon and molecular hydrogen with metastable helium leading to a sharp increase in the absolute reaction rate in the energy range corresponding to a few degrees kelvin down to 10 mK. Our method is widely applicable to many canonical chemical reactions, and will enable experimental studies of Quantum Chemistry.
He* + H2
3 * Reaction rate measurements for the ( S) He and H2 Penning ionization reaction are shown in black with error bars. Blue dash-dot line is the reaction rate calculated using the latest “experimental” potential. Red solid line is the calculated reaction rate using the Tang-Toennies potential with parameters that give the best fit to our measured results. Shaded area corresponds to the energy range where our results overlap with the earlier measurements. Peaks in the reaction rate correspond to the positions of orbiting resonances.
52 Posters
53 POSTER SESSION 1 P1-1
Laser induced quantum dynamics simulation of the electronic and nuclear motion in the ozone molecule on the attosecond time scale
G. J. Halász (1), A. Perveaux (2), B. Lasorne (2), M. A. Robb (3), F. Gatti (2) and Á. Vibók (4)
1Department of Information Technology, University of Debrecen, H-4010 Debrecen, PO Box 12, Hungary 2CTMM, Institut Charles Gerhardt Montpellier, F-34095 Montpellier Cedex 5, France 3Imperial College London, Department of Chemistry, London SW7 2AZ, UK 4Department of Theoretical Physics, University of Debrecen, H-4010 Debrecen, PO Box 5, Hungary
The nonadiabatically coupled dynamics of electrons and nuclei is investigated for the ozone molecule on the attosecond time scale. A coherent superposition of nuclear wave packets located on different electronic states in the Chappuis and in the Hartley bands are created by pump pulses. The MCTDH (multiconfiguration time– dependent Hartree) method is used to solve the coupled nuclear quantum dynamics in the framework of the adiabatic separation of the time– dependent Schrödinger equation. Our nuclear wave packet calculations demonstrate that the coherence between the Hartley state B and one of the Chappuis states (Chappuis 1) is significantly large while it is almost negligible for the other two cases (between Hartley B and Chappuis 2, or between Chappuis 1 and Chappuis 2) . In the present stage we limited our description of the electronic motion to the Franck-Condon (FC) region only, due to the localization of the nuclear wave packets around this point during the first 5 - 6 fs.
54 POSTER SESSION 1 P1-2
Electronic and vibrational relaxation of oxygen in the triplet Herzberg states
Matthew W. Jones1 and George C. McBane1,2*
1Department of Chemistry, Grand Valley State University, Allendale, Michigan USA 2Department of Chemistry, Durham University, Durham DH13LE
The Herzberg states of oxygen have three electrons each in the 2p π and 2p π* molecular orbitals. They are formed in the atmosphere by O atom recombination, by direct photoexcitation from the ground state, and as a minor product of ozone photodissociation. Experiments by Copeland and coworkers have shown that about two-thirds of Herzberg state molecules in intermediate vibrational levels (v ≈ 6‐10) are removed from the atmosphere by collisions with nitrogen. The detailed pathways of the removal process are unknown, though it is clear that chemical reactions are unlikely to be important. A computational study of vibrational and electronic relaxation by both N2 and Ne will be presented.
55 POSTER SESSION 1 P1-3
Anisotropy induced Feshbach resonances in a quantum dipolar gas of dysprosium atoms
Alexander Petrov,' Eite Tiesinga,2 and Svetlana Kotochigova'
1Department of Physics, Temple University, Philadelphia, Pennsylvania 19122 and National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 2Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland, Gaithersburg, Maryland 20899, USA
We explore the anisotropic nature of Feshbach resonances in the collision between ultracold magnetic submerged-shell dysprosium atoms, which can only occur due to couplings to rotating bound states. This is in contrast to well-studied alkali-metal atom collisions, where most Feshbach resonances are hyperfine induced and due to rotation-less bound states. Our first- principle coupled-channel calculation of the collisions between spin-polarized bosonic dysprosium reveals a striking correlation between the anisotropy due to the magnetic dipole- dipole interaction on the one hand and the anisotropy due to dispersion interactions on the other. In fact, the Feshbach spectrum as a function of an external magnetic field cannot be understood if one of these two interactions is omitted. Over a 20 mT magnetic field range we predict about a dozen Feshbach resonances and show that the resonance locations are exquisitely sensitive to the dysprosium isotope.
56 POSTER SESSION 1 P1-4
A scaling rule for the collision energy dependence of a rotationally inelastic differential cross section: a case study of NO(X) + He
Xia Zhang1, Craig A. Taatjes2, Dajun Ding1 and Steven Stolte1,3,4*
1Institute of Atomic and Molecular Physics, Jilin University, Changchun, 130012, China. 2Combustion Research Facility, Mail Stop 9055, Sandia National Labs, Livermore, Ca 94551. 3Laser centre and Phys. Chemistry, Faculty of Exact Science, Vrije Universiteit, Amsterdam, 1081HV, The Netherlands. 4Laboratoire Francis Perrin, Bȃtiment 522, DRECEM/SPAM/CEA Saclay, 91191 Gif sur Yvette, France
*E-mail: [email protected]
The QM coupled channel calculated state resolved DCS’s for the rotationally inelastic scattering of NO(X, j=½, ε=1) + He, carried out on the accurate CCSD(T) ab initio PES, provided remarkable structured angular distributions (induced by a quantum inference) at the high collision energy of = 147meV. At the low collision energy of = 63 meV these structures are absent [1]. In theு case of a hard shell PES [2], the scaled rotationally inelastic DCS’s from to reproduce exactlyܧ those at . This is not necessarily trueܧ for a non-hard shell PES.ு The first step of the scaling procedure is to transform the DCS’s at 147 meV from the collision frameܧ to theܧ apse frame in which the scaܧttering angle θ is replaced by with . Next,
after dividing the value of the apse DCS’s at 147 meV by , one results isߚ toؠ transform࢜ෝ ȉࢇෝ the scaledࢇؠ ࢜ apseԢ െ DCS’s࢜ back to ݏin the scaled apse DCS’s at 63 meV. The final stepܿ the collision frame.ܿݏ Theߚ scaled and regular He-NO DCS’s at 63ඥ meV͵ ݉ turnܸ݁Ȁ outͳͶ to݉ ݁ agreeܸ well, but significant differences show up especially in the near backwards directions. These differences provide a signature of the NO-He PES’s, which are expected to increase with the reduced mass of the collision partners and also with . The collision energy scaling difference between a theoretically and an experimentally obtainedு DCS’s offers a promising tool to obtain more insight in the underlying factors regardingܧ theȀܧ specific inaccuracy of the PES and/or particular experimental short comings.
References
[1] Klos, J.; Aoiz, F.J.; Verdasco, J. E.; Brouard, M.; Marinakis, S.; Stolte, S. J. Chem.Phys. 2007, 127, 031102. [2] Ballast A.; Gijsbertsen, A.; Linnartz, H.; Stolte, S. Mol phys, 2008, 106, 315.
57 POSTER SESSION 1 P1-5
Resonances in rotationally inelastic collisions
Paul J. Dagdigian1*, Qianli Ma1, Millard H. Alexander2, Sebastiaan Y. T. van de Meerakker3, and Ad van der Avoird3
1Department of Chemistry, The Johns Hopkins University, Baltimore, MD 21218 USA 2Department of Chemistry and Biochemistry and Institute for Physical Science and Technology, College Park, MD 20742 USA 3Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
*E-mail: [email protected]
Measurements of state-to-state cross sections provide important tests of the reliability of computed potential energy surfaces (PES’s) describing the interaction of atoms and molecules. Cross sections for collision-induced rotational transitions are sensitive to the anisotropy of the PES, largely of the repulsive part. By contrast, the energies of bound levels of van der Waals complexes are mainly sensitive to the attractive part of the PES. As we go higher in the manifold of these levels, their energies become greater than the dissociation energy, and the levels become quasi-bound. These quasi-bound levels can manifest themselves as shape and Feshbach resonances in the collision energy dependence of state-to-state cross sections. Due to their sensitivity to the PES, resonances can reveal important information on the PES, as illustrated by recent calculations on OH(X2)–He/Ne collisions [1].
Here, we present detailed calculations on rotationally inelastic scattering of NH3(j=1, k=1, =–1) with H2, using a state-of-the-art PES [2]. The energy-dependent integral cross section for the transition to the j=1, k=1, =+1 level in collisions with p-H2 shows sharp but complicated structure. We identify both shape and Feshbach resonances in this energy-dependent cross section. By contrast, the energy-dependent cross section for the corresponding transition in collisions with o-H2 is much less structured, but likely also has significant contributions from resonances. We also consider the scattering of other isotopomers (ND3 and D2) and the effect of resonances on the differential cross sections. Finally, we discuss the prospects for experimentally observing scattering resonances using Stark decelerated beams of NH3 or ND3 [3]. The widths of the resonant features are found to be much broader than in the OH–Rg systems; this should facilitate the experimental observation of such features.
References
[1] K. B. Gubbels, Q. Ma, M. H. Alexander, P. J. Dagdigian, D. Tanis, G. C. Gronenboom, A. van der Avoird, and S. Y. T. van de Meeraker, J. Chem. Phys. 136, 144308 (2012). [2] S. Maret, A. Faure, E. Scifoni, and L. Wiesenfeld, Mon. Not. R. Astron. Soc. 399, 425 (2009). [3] S. Y. T. van de Meerakker, H. L. Bethlem, and G. Meijer, Nat. Phys. 4, 495 (2008). [4] L. Scharfenberg, S. Y. T. van de Meerakker, and G. Meijer, Phys. Chem. Chem. Phys. 13, 8448 (2011).
58 POSTER SESSION 1 P1-6
From simulations to reality: An apparatus for performing time-resolved electron diffraction experiments
Derek A. Wann*, Paul D. Lane, Matthew S. Robinson, and Stuart Young
School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ.
*E-mail: [email protected]
Since the 1930s electron diffraction has been used as a technique for determining molecular structures, primarily in the gas phase [1]. For a long time the use of a continuous high-energy electron beam meant that the data collected were averaged over periods of seconds, resulting in no information about the dynamics of the systems being studied. Nonetheless, such structures proved extremely useful in answering chemical questions related to functionality.
With the advent of ultrafast lasers came the possibility to produce pulsed short bunches of electrons, using the laser to excite electrons from a metal photocathode [2]. The same laser could be used to induce a structural change of interest, with the overall set up acting as a pump-probe experiment.
In Edinburgh we have now designed and are building an apparatus for time-resolved electron diffraction, capable of sub-picosecond resolution. Using the SIMION software package [3] much modelling has been performed to allow the electron beam profile to be optimised. These results will be discussed.
Quantum chemistry has also been used to identify suitable species for study. Typically these undergo a a large conformational change upon laser excitation, although we will also be looking at the dissociation dynamics of molecules in the gas phase. The dissociation of dimethyl disulfide, · H3CSSCH3, to give the radical species H3CS [4] is a possible initial study and will be discussed.
References
[1] H. Mark and R. Wierl, Naturwiss., 1930, 18, 205. [2] G. Sciaini and R. J. D. Miller, Rep. Prog. Phys., 2011, 74, 096101. [3] D. A. Dahl, Int. J. Mass Spectrom., 2000, 200, 3. [4] C. W. Bookwalter, D. L. Zoller, P. L. Ross and M. V. Johnston, J. Am. Soc. Mass Spectrom., 1995, 6, 872.
59 POSTER SESSION 1 P1-7
+ + Modified statistical model for the CH +H→C +H2 reaction
1 2 Tasko P. Grozdanov , Ronald McCarroll
1Institute of Physics, University of Belgrade, 11080 Belgrade, Serbia 2Laboratoire de Chimie Physique, Université Pierre et Marie Curie, 75231-Paris , France
*E-mail: [email protected] , [email protected]
Recent experiments [1] on the reaction of CH+ with H present an interesting challenge for a theoretical interpretation. In view of the existence of a deep well in the potential energy surface + (PES) of CH2 , the reaction is expected to proceed via the formation of a long-lived complex. However, the measured rate coefficient exhibits a strong decrease at low temperatures, which neither quasiclassical trajectory calculations [2,3] , nor quantum mechanical methods [3] using a full PES can explain.
It can be reasonably conjectured that the reduction of the reactivity is related to the presence of potential barriers in the near collinear configurations C-H-H and H-C-H, which hinder the formation of a complex. In this work, we investigate the possibility of an alignment of rotationally cold CH+ along the CH+-H axis using the notion of adiabatic rotation [4] or pendular [5] states. The largest alignment was found for the adiabatic rotation state labeled with (j,Ω)=(0,0) and to a somewhat lesser degree for the (1,1) and (1,0) states (Ω is the modulus of the projection of the rotational angular momentum j on to the collision axis. Thermal rate coefficients in Fig.1 are determined by standard statistical theory [6,7], using only polarization interactions in the reactant and product channels, but with a few possible energy-dependent restrictions on the reactivity of particular (j,Ω)‐states.
Fig.1. Thermal rate coefficient as a function of temperature. Experimental results: circles [1] and squares [8]. Results of standard statistical theory: full line includes all (j,Ω) states of CH+ and is in agreement with [2]. The dotted line excludes contribution of the (0,0) state assumed to be non-reactive due to alignment at collision energies Ec<2B (B is the rotation constant of CH+). The dashed line excludes contributions of (0,0) state for Ec<6B and (1,1) states for Ec<4B and the dot-dashed line excludes contributions of (0,0) state for Ec<6B and (1,1) and (1,0) states for Ec<4B .
Our calculations suggest that a very strong alignment occurs for the j=0 state and a large partial alignment of the j=1 states. This can explain very satisfactorily the main characteristics of the experimental rate coefficients. Of course a more sophisticated study is required in order to take into account of (and possibly re-examine) the detailed form of the PES close to linearity.
60 References
[1] R. Plasil, T. Mehner, P.Dohnal, T. Kotrik, J. Glosik and D. Gerlich, Ap. J. 737, 60 (2011) [2] P. Halvick, T. Stoecklin, P. Larrégaray and L. Bonnet, Phys. Chem. Chem.Phys, 9, 582 (2007) [3] R. Warmbier and R. Schneider, Phys.Chem. Chem. Phys. 13, 10285 (2011) [4] D.C. Clary, Mol. Phys. 54, 605 (1985) [5] B. Friedrich, D.P.Pullman and D.R. Herscbach, J. Phys.Chem. 95, 8118 (1991) [6] K. Park and J.C. Light, J. Chem.Phys, 127, 224101 (2007) [7] T. P. Grozdanov and R. McCarroll, J. Phys.Chem. A, 116, 4569 (2011) [8] W. Federer, H. Villinger, F. Howirka,W Lindinger, P. Tosi, D. Bassi and E. Ferguson, Phys. Rev. Lett, 52,2084 (1984)
61 POSTER SESSION 1 P1-8
Ultracold atomic hydrogen: A versatile coolant to produce ultracold molecules
Maykel L. González-Martínez1* and Jeremy M. Hutson1
1Department of Chemistry, Durham University, Durham DH1 3LE, UK
*E-mail: [email protected]
High-density samples of ultracold molecules are expected to have a profound impact in, e.g., high-precision spectroscopy, quantum information science and quantum-controlled chemistry [1]. Although proven successful, indirect methods in which molecules are created by associating ultracold atoms are fundamentally limited to molecules formed from the atomic species that can be laser-cooled [2-4]. Direct methods, although more widely applicable, are currently limited in the minimum temperatures achievable to about 10 mK [5-9]. A second-stage cooling technique bridging the gap to the micro-Kelvin regime, where most exciting applications would become possible, is therefore much needed [1]. Here, we demonstrate that sympathetic cooling [10] using ultracold spin-polarized atomic hydrogen has unique characteristics which make it likely to successfully bridge this gap. In particular, we present compelling theoretical evidence that it can be used to cool a broad range of atomic and molecular species to ultracold temperatures, using 19 2 14 3 - 16 2 F( P3/2), NH( ) and OH( 3/2) as prototypes for halogen atoms, and molecules with and ground electronic states, respectively.
References
[1] L.D. Carr, D. DeMille, R.V. Krems, and J. Ye, New J. Phys. 11, 055049 (2009). [2] J.M. Hutson and P. Soldán, Int. Rev. Phys. Chem. 25, 497 (2006). [3] T. Köhler, K. Góral, and P.S. Julienne, Rev. Mod. Phys. 78, 1311 (2006). [4] K.M. Jones, E. Tiesinga, P.D. Lett, and P.S. Julienne, Rev. Mod. Phys. 78, 483 (2006) [5] J. D. Weinstein, R. deCarvalho, T. Guillet, B. Friedrich, and J. M. Doyle, Nature 395, 148 (1998) [6] H.L. Bethlem and G. Meijer, Int. Rev. Phys. Chem. 22, 73 (2003). [7] N. Vanhaecke, U. Meier, M. Andrist, B.H. Meier, and F. Merkt, Phys. Rev. A 75, 031402(R) (2007). [8] S.D. Hogan, D. Sprecher, M. Andrist, N. Vanhaecke, and F. Merkt, Phys. Rev. A 76, 023412 (2007). [9] E. Narevicius, C.G. Parthey, A. Libson, J. Narevicius, I. Chavez, U. Even, and M.G. Raizen, New J. Phys. 9, 358 (2007). [10] P. Soldán and J.M. Hutson, Phys. Rev. Lett. 92, 163202 (2004).
62 POSTER SESSION 1 P1-9
A next-generation ultrafast detector for imaging mass spectrometry: the Pixel Imaging Mass Spectrometry (PImMS) sensor
Jason Lee1; Edward Wilman1; Jaya John John1; Andrew Clark2; Jamie Crooks2; Laura Hill1; Renato Turchetta2; Mark Brouard1; Andrei Nomerotski1; Claire Vallance1
1University of Oxford, Oxford, United Kingdom; 2STFC Rutherford Appleton Laboratory, Oxford, United Kingdom
Novel Aspect The technology of a new ultrafast multi-mass spatial imaging sensor is outlined and demonstrated with preliminary imaging mass spectrometry applications.
Introduction Imaging mass spectrometry involves combining traditional time-of-flight (TOF) mass separation with two-dimensional position sensitive detection. Such an approach can be tuned to provide information on either the spatial or velocity distribution of ions at their point of formation. An idealized instrument requires a universal ionization technique, good mass resolution, and a fast detector with good spatial resolution. Traditional imaging cameras are time-gated to image a single mass on each TOF cycle, requiring each mass to be recorded in turn. This poster outlines a new ultrafast event- triggered CMOS image sensor, which records the position and arrival time of each detected ion. The camera allows the full mass spectrum to be recorded on each TOF cycle with 25 ns time resolution.
Methods The PImMS sensor operates by timestamping detected events when signal intensity crosses a user- specified threshold. The sensor is designed using the INMAPS CMOS technology developed at the Rutherford Appleton Laboratory, and is fabricated using a 180 nm process. Each pixel contains amplification, shaping, and digitization circuitry, with four memory registers allowing multiple timestamps to be recorded within each pixel. Simulations have shown that under typical operating conditions four memory registers give a detection efficiency of greater than 99%. Data is read out from the sensor as an array of (x,y,t) data points for each detected ion, providing significant gains in efficiency over framing sensors. Post-processing allows reconstruction of all mass images and visualization of the full TOF mass spectrum.
Preliminary Data The current PImMS sensor prototype contains 72x72 70 μm diameter pixels, each with a timestamp resolution of 25 ns. The current timestamps are 12‐bit allowing ion signal acquisition for 102.4 μs each TOF cycle. A USB2 interface allows connection to any modern desktop computer and provides a maximum repetition rate of 500 Hz. Custom software and algorithms have been developed to provide control and on-the-fly processing.
The first experiment carried out to verify correct sensor operation involved imaging a 405 nm pulsed laser pattern. Laser pulses of 25 ns width at a 25 kHz frequency were passed through a diffraction grating in order to produce a two dimensional pattern, which was used to illuminate the sensor. The patterns for multiple pulses were recorded using a test timestamping resolution of 50 ns (202.8 μs total acquisition time) and summed over 3000 experimental cycles. These results clearly demonstrate acquisition of four images in the four memory registers, with the expected time resolution.
63 Following the successful ‘bench’ demonstration, the camera was installed on an imaging mass spectrometer and used to record images of photofragment velocity distributions. Dimethyl formamide (DMF) was investigated as one model compound for studying fragmentation of the peptide bond. DMF was seeded (0.3%) in 1 bar argon and photofragmented using 193 nm radiation, and the resulting fragments ionized using a single photon at 118 nm. DMF has a number of fragments each with different spatial distributions. These multiple masses were recorded in the same TOF cycle, with a significantly decreased experimental time compared to a conventional camera. The complex data set indicates multiple fragmentation pathways and formation and photolysis of small clusters.
The next generation of PImMS (PImMS2) increases pixel resolution to 324x324, while maintaining the 25 ns timestamp resolution. Sensor back-thinning will increase sensitivity by approximately five-fold.
64 POSTER SESSION 1 P1-10
Quantum dynamics studies of surface-catalysed H atom recombination
Benjamin J. Irving, Kousik Giri,† and Anthony J. H. M. Meijer‡ ∗ Dept. of Chemistry, University of Sheffield, Sheffield, S3 7HF UK
Electronic address: [email protected] † Electronic address: [email protected] ∗‡ Electronic address: [email protected]
Molecular hydrogen, H2, is one of the fundamental constituents of the universe, acting as the molecular feedstock for much of the chemistry occurring within the interstellar medium.1 Although gas phase models of the chemistry of interstellar clouds have been successful in explaining the abundances of some gas phase molecules, it has long been established that they cannot account for the large abundance of molecular hydrogen. The general consensus of the astronomical community is that carbonaceous interstellar dust grains assume a catalytic role in the formation of H2 molecules within interstellar clouds.2
First-principles calculations using the VASP (Vienna ab initio simulation package) software package have been performed in order to scrutinise the hydrogen-dust grain interaction. Furthermore, the collinearly-dominated Eley-Rideal H2 formation pathway has been studied by quantum dynamical means using the Multi Configuration Time Dependent Hartree (MCTDH) algorithm in conjunction with a novel potential energy surface. In contrast to early calculations3 our PES accounts for energy transfer from the nascent H2 bond to the dust grain surface, highlighting the importance of surface relaxation in the Eley-Rideal mechanism.4,5
References
1. D. A. Williams and T. W. Hartquist, Acc. Chem. Res., 1999, 32, 334. 2. D. Hollenbach and E. E. Salpeter, Astrophys. J., 1971, 163, 155. 3. A. J. Farebrother, A. J. H. M. Meijer, D. C. Clary, and A. J. Fisher, Chem. Phys. Lett., 2000, 319, 303. 4. M. Bonfanti, S. Casolo, G. F. Tantardini, and R. Martinazzo, Phys. Chem. Chem. Phys., 2011, 13, 16680. 5. S. Morisset, F. Aguillon, M. Sizun, and V. Sidis, J. Phys. Chem. A, 2004, 108, 8571.
65 POSTER SESSION 1 P1-11
3nj symbols and q-extensions: Basis functions in 1 and 2 discrete variables— Surprising properties and applications
R. W. Anderson University of California, Santa Cruz, California 95064, USA E-mail : [email protected]
This contribution presents recent results on 3nj Symbols on their properties, calculation, and application as discrete wave functions of one and two variables. The univariate cases are explored here as Gram polynomials, Clebsch-Gordan coefficients, 6j symbols, and their q-extensions. 9j symbols are discrete polynomials in two variables. Although 3nj symbols are well studied in quantum angular momentum theory, their application as discrete wavefunctions has only been known since the start of the Stereodynamics conferences. These applications include among others: stereo- directed bases in quantum mechanics, hyper-quantization, and smoothing and compression of univariate, bivariate, and higher dimensional data. Compression and smoothing of data is an important technique in many areas of molecular physics. Examples abound in binning of classical trajectories [1,2], smoothing and compression of experimental data, analysis of 2D and 3D images, pattern recognition, and general analysis and presentation of tabular and graphical information. There are important applications in the analysis of spin networks and quantum computing.
This report stresses the calculation of 6j and 9j symbols and their q-extensions by direct summation, recursion, and matrix diagonalization. Diagonalization in double precision arithmetic provides accurate 6j symbols for angular momenta of magnitude of several hundred, and it can easily compute either Legendre type discrete wave functions or harmonic oscillator type discrete wave functions. The 6j symbols for the former and latter cases correspond to the transformation between different coupling schemes for 3 angular momenta.
It is well known that the classical and nonclassical regions of the screen for 6j are the volume of the related tetrahedron. However for q-extensions the volume is no longer a Cayley determinant, so we have developed another method to determine the caustics. The new method agrees with the tetrahedron volume for q=1, but can also applied for q
This report will present some surprising results for the orthogonal transformations that correspond to 9j symbols. The unique structure of the transformations may have interesting applications in representing experimental data for molecular processes
References
[1] R. W. Anderson, Molec. Phys. 106, 977 (2008). [2] R. W. Anderson and V. Aquilanti, J. Chem. Phys. 124, 214104 (2006). [3] M. Ragni, A. C. P. Bitencourt, C. D. Ferreira, V. Aquilanti, R. W. Anderson, and R. G. Littlejohn, Int. J. Quantum Chem. 110, 731 (2010). [4] R. W. Anderson, V. Aquilanti, and C. da Silva Ferreira, J. Chem. Phys. 129, 161101 (2008). [5] R. W. Anderson, V. Aquilanti, and A. Marzuoli, J. Phys. Chem. A 113, 15106 (2009). [6] A. C. P. Bitencourt, A. Marzuoli, M. Ragni, R. W. Anderson, and V. Aquilanti, [B. Murgante et. al. (Eds.); ICCSA 2012, Part I, LNCS 7333, p. 723 (2012)].
66 POSTER SESSION 1 P1-12
Velocity map imaging spectrometer: An off-the-shelf system
Orla Kelly1,2*, Panos Kapetanopoulos1, Gareth Jones1, Mike NR Ashfold2, Stuart J Greaves2, Andreas M Wenge2
1Photek Ltd., 26 Castleham Road, St Leonards on Sea, East Sussex TN38 9NS, UK 2School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
*E-mail: [email protected]
Velocity Map Imaging (VMI) was first introduced in 1997 [1] and has been widely adopted in physical chemistry and laser physics experiments, where the original paper has gained more than 1000 citations in the past 15 years [2]. VMI uses optimally designed ion optics to map photoions or electrons onto a Vacuum Imaging Detector. The position of detection provides information on the initial velocity vector of the charged particle.
At present, it is not possible to buy a Velocity Map Imaging Spectrometer in the commercial market. Photek and the University of Bristol have joined forces to offer a new Velocity Map Imaging Spectrometer. Photek have extensive expertise of in-vacuum detectors, gating units and HV accessories [3], while the School of Chemistry at the University of Bristol has vast knowledge of ion optics and laser chemistry [4]. The new spectrometer offers a high spatial resolution, with the option of temporal slicing and optical zooming. This spectrometer is available as a fully characterised system, or for retro-fit into an established set up. Proof of concept experiments will be presented.
References
[1] André TJB Eppink and David H Parker, Review of Scientific Instruments, 68, 3477, (1997) [2] Google Scholar [3] www.photek.com [4] www.chm.bris.ac.uk/laser/
67 POSTER SESSION 1 P1-13
Dynamics of the reaction of muonium atoms with hydrogen molecules: Zero point energy, tunneling and vibrational adiabaticity.
P. G. Jambrina1, R. Pérez de Tudela1, E. García2, V. J. Herrero3, and V. Sáez-Rábanos4, and F. J. Aoiz 1*
1Departamento de Química Física, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain. 2Departamento de Química Física, Universidad del País Vasco, Paseo de la Universidad 7, 01006 Vitoria, Spain. 3Instituto de Estructura de la Materia (CSIC), Serrano 123, 28006 Madrid (Spain). 4Departamento de Química y Bioquímica. ETS Ingenieros de Montes. Universidad Politécnica. 28040 Madrid, Spain
* E-mal: [email protected]
A fundamental issue in the field of reaction dynamics is the assessment of the importance of quantum mechanical (QM) effects such as zero point energy (ZPE) and tunneling in molecular dynamics simulations [1]. Given its low mass, reactions implying Mu (muonium) atoms appear as excellent candidates to investigate these effects [2]. To this end, we have carried out extensive calculations using accurate QM, quasiclassical trajectory (QCT) [3] and Ring Polymer Molecular Dynamics (RPMD) [4] methods for the reactions of Mu (muonium) with H2 in both v=0 and v=1 states [5]. In all the cases considered, the QM rate coefficients, k(T), are in very good agreement with the available experimental results [2,6]. In particular, the exceptionally good agreement of the RPMD calculations with the experimental and accurate QM results can be attributed to the dominant role of the ZPE in the reaction whose effect supersedes that of the possible tunneling. In contrast to the D+H2(v=0,1) and the Mu+H2(v=0) reactions, the QCT calculations for Mu+H2(v=1) predict a much smaller k(T) than that obtained with the accurate QM method. The huge vibrational enhancement in the Mu +H2 k(T) from v=0 to v=1 is indeed remarkable at low temperatures (7 orders of magnitude at 300 K) [5,6(b)]and indicates that tunneling is important.
These results can be explained by considering the vibrational adiabatic potentials along the minimum energy path. It has been found that the threshold for the reaction of Mu with H2 in both v=0 and v=1 states is the same and is given by the height of the v=0 adiabatic collinear potential (excluding the bending contribution). In contrast, the total energy threshold for the D+H2(v=0) reaction (taken as an example of a reaction with a heavier isotope) is lower than for D+H2(v=1) because the vibrational adiabaticity is preserved and the threshold for the reaction in v=1 is very close to the height of the v=1 adiabatic collinear barrier. In the Mu+H2(v=1) case, where the MuH(v=1) exit channel is closed, there has to be a breakdown of the vibrational adiabaticity. At the lowest energies the crossing from the v=1 to the v=0 adiabat takes place by tunneling through the relatively small v=1 barrier. In the classical case such tunneling cannot take place and the threshold coincides with the height of the v=1 collinear adiabat. One of the most interesting aspects of this work is that the expected tunneling enhancement of the reactivity in the reaction of Mu with hydrogen molecules occurs for H2(v=1) but not for H2(v=0).
References
[1] F. J. Aoiz, L. Bañares, and V. J. Herrero, Int. Rev. Phys. Chem., 24, 119 (2005). [2] D. G. Fleming et al., Science 331, 448 (2011).
68 [3] P. G. Jambrina, E. García, V. J. Herrero, V. Sáez-Rábanos, and F. J. Aoiz, J. Chem. Phys. 135, 034310 (2011). [4] R. Pérez de Tudela, F. J. Aoiz, Y. Suleimanov, and D. E. Manolopoulos, J. Phys. Chem. Lett. , 3, 493 (2012). [5] P. G. Jambrina, E. García, V. J. Herrero, V. Sáez-Rábanos, and F. J. Aoiz, Submitted for publication (2012). [6] (a) I. D. Reid, D. M. Garner, L. Y. Lee, M. Senba, D. J. Arsenau, and D. G. Fleming, J. Chem.Phys., 86, 5578 (1987). (b) P. Bakule, et al., Physica B, 404, 1013 (2009).
69 POSTER SESSION 1 P1-14
Laser enabled Auger decay in atoms and molecules: Probing electron correlation in inner-valence ionised states
Bridgette Cooper1 and Vitali Averbukh1
1Blackett Laboratory, Imperial College London, UK
Auger type decay processes play a fundamental role in atomic/molecular spectroscopy, surface analysis, radiation damage, etc. These transitions can be viewed as consisting of two steps. Firstly, a high energy photon produces a hole in an inner electronic shell, emitting a photoelectron with kinetic energy dependent on the incident photon. Then the hole is filled by a valence electron and a second valence electron is emitted with energy dependent on the energy levels of the singly and doubly charged ions and not the incident photon energy. Auger decay processes are considered to be an important manifestation of electron correlation as they are only possible because of electron- electron interactions.
Recently, Murnane and Kapteyn [1] have investigated Laser-Enabled Auger Decay (LEAD) in the multi- photon regime for vacancies that are not energetic enough to undergo the normal Auger decay. Here we show that if considered in the single-photon regime, the LEAD process provides a valuable insight into electron correlation in the inner valence ionised states. Firstly, we analyse the single photon LEAD for the 2s-ionised state in Ne. A detailed investigation of the mechanism of this single-photon LEAD process reveals that it is only possible because the initial 2s-1 one-hole state contains configurations of the type two holes and an electron excited to a high-energy orbital. The cross- section of the single-photon LEAD process becomes a direct measure of this configuration mixing. We use the first-principles algebraic diagrammatic construction (ADC) scheme and the Stieltjes imaging technique [2] to evaluate the single-photon LEAD cross-sections in ns-ionised states of Ne and Ar. The correlation in the inner valence ionised states of trans-1,3 Butadiene are investigated as they are an example of a molecule with strong configuration mixing resulting in the breakdown of the molecular orbital (MO) picture of ionisation [3]. We show that the breakdown of the MO picture leads to a dramatic increase of the single-photon LEAD cross-section relative to the atomic case. Finally, we propose that single photon LEAD can be a sensitive experimental probe for the attosecond hole migration triggered by the MO breakdown.
References
1. P. Ranitovic, X. M. Tong, C. W. Hogle, X. Zhou, Y. Liu, N. Toshima, M. M. Murnane, H. C. Kapteyn, Phys. Rev. Lett. 106, 053002 (2011). 2. See e.g. K. Gokhberg, V. Vysotskiy, L. S. Cederbaum, L. Storchi, F. Tarantelli, V. Averbukh, J. Chem. Phys. 130, 064104 (2009). 3. L. S. Cederbaum, W. Domcke, J. Schirmer, W. Von Niessen, Adv. Chem. Phys. 65, 115 (1986).
70 POSTER SESSION 1 P1-15
Products polarization for the H+D2(v=0, j=0) → HD(v’=4, j’)+D reaction.
J. Aldegunde1*, D. Herráez-Aguilar2, P. G. Jambrina2 and F. J. Aoiz2
1Departamento de Química-Física, Universidad de Salamanca, Salamanca (Spain) 2Departamento de Química-Física I, Universidad Complutense de Madrid (Spain)
*E-mail: [email protected]
In spite of being the most thoroughly studied chemical reaction, the H+D2 system continues rendering unexpected results that push forward our understanding of how elementary chemical reactions in gas phase take place. In particular, it has been recently found [1] that the differential cross sections for the H+D2(v=0, j=0) → HD(v’=4, j’)+D display an anomalous behavior as a function of j’; instead of becoming more sideways as the rotational excitation of the products increases, the angular distributions shift into the backward region as j’ gets larger. This has been explained in terms of a reaction barrier which, for low recoiling energies, suppress the reactivity associated with the collisions characterized by large total angular momentum and responsible for sideways scattering.
In the present work [2], the implications of this cancellation for the stereodynamics of the collisions is assessed. We concentrate on the products polarization in order to show how the values of the polarization moments that define the alignment and orientation of j’ converge to very similar values regardless of the final internal state at low enough recoiling energies, which implies that the reaction mechanism itself is exclusively determined by the shortage of radial energy available for the products.
References
[1] J. Jankunas, R. N. Zare, F. Bouakline, S. C. Althorpe, D. Herráez-Aguilar and F. J. Aoiz (work in progress) [2] D. Herráez-Aguilar, J. Aldegunde, P. G. Jambrina and F. J. Aoiz (in preparation)
71 POSTER SESSION 1 P1-16
Quantum coherent control of the lifetime of excited resonance states with laser pulses
A. García-Vela1*
1Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 123, 28006 Madrid, Spain
Quantum control of the lifetime of a system in an excited resonance state is investigated theoretically by creating coherent superpositions of overlapping resonances. The control strategy exploits the quantum interference effects that occur between the overlapping resonances [1], which can be controlled by varying the weight of the resonances in the superposition using laser pulses. Control is investigated on a realistic model of the Br2(B)-Ne predissociation decay dynamics through a three-dimensional wave packet method. Two control schemes are explored. In one of the schemes the system is excited with a single laser pulse of varying width in order to modify and control the population of the different overlapping resonances in the superposition created [2]. The second, more flexible control scheme uses a combination of two laser pulses, where the time delay and the ratio of intensities of the two pulses are the parameters modified in order to exert control [3]. Extensive control is found to be achieved on the lifetime of a single given resonance state of the superposition prepared. In particular, a strong enhancement of the lifetime by a factor of three is obtained with the two pulse scheme. Control of the resonance lifetime is possible because in the case of overlapping resonances the lifetime is no longer an intrinsic property of the resonance state, as has been recently shown [2]. An experimental realization of the control schemes is suggested.
References
[1] P.S. Christopher, M. Shapiro, and P. Brumer, J. Chem. Phys. 123, 064313 (2005). [2] A. García-Vela, J. Chem. Phys. 136, 134304 (2012). [3] A. García-Vela, J. Phys. Chem. Lett. 3, 1941 (2012).
72 POSTER SESSION 1 P1-17
Laser induced alignment of molecules in helium droplets
Dominik Pentlehner1*, Jens H. Nielsen2 and Henrik Stapelfeldt1
1 Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark 2Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark *[email protected]
Aligning molecules in the laboratory frame is desirable for the study of directionality in physical and chemical processes. Examples are the direction of transition dipole moments or the stereoselectivity of chemical reactions such as SN2 reactions. Laser induced alignment can be achieved adiabatically using long (ns) or nonadiabatically using short (fs) laser pulses with respect to the rotational period of the molecule.
So far, all studies of laser induced alignment involved isolated molecules in a cold molecular beam or in a gas cell. Here, we report the first experimental results on laser induced alignment of molecules embedded in superfluid helium droplets and thus make a step from isolated molecules in the gas phase to solvated molecules in a condensed phase.
Our studies show that adiabatic alignment of molecules in helium droplets appears similar as for the isolated molecules, whereas nonadiabatic alignment shows striking differences. (Fig.1) Nonadiabatic alignment is based on the formation of a rotational wavepacket and thus contains information about the rotational dynamics. Comparisons of the time dependence of the degree of alignment of the same molecule in a cold molecular beam and in helium droplets can thus be used to study the influence of the dissipative environment on the dynamics.
Recent data on adiabatic and nonadiabatic laser induced alignment of Iodobenzene (C6H5I), Methyliodide (CH3I) and Carbondisulfide (CS2) in the gas phase and inside helium droplets will be presented and discussed.
Fig.1. Time dependent alignment of CH3I in the gas phase (black) and in helium droplets (red) following a 0.5 ps alignment pulse (green).
73 POSTER SESSION 1 P1-18
3 The dynamics of the O( P) + CH4 → OH + CH3 reaction is similar to that of a triatomic reaction
Rodrigo Martínez,1 Pedro A. Enríquez,1 María P. Puyuelo,1 Miguel González2,*
1Depto. de Química, Univ. de La Rioja, C/ Madre de Dios, 51, 26006 Logroño (Spain) 2Dept. de Química Física i IQTC, Univ. de Barcelona, C/ Martí i Franquès, 1, 08028 Barcelona (Spain)
*E-mail: [email protected]
3 The O( P) + CH4 reaction has been investigated [1] using the quasi-classical trajectory (QCT) method and an ab initio pseudo-triatomic potential energy surface (PES) [2]. This has been mainly motivated by very recent experiments [3] which support the reliability of the triatomic modeling even at high -1 collision energy (
This work was supported by the Spanish Ministry of Science and Innovation (projects CTQ2008- 06805-C02-01 and CTQ2011-27857-C02-01). Thanks are also given to the “Generalitat de Catalunya” (Autonomous Government of Catalonia, ref. 2009SGR 17) for some support.
References
[1] Martínez, R.; Enríquez, P. A.; Puyuelo, M. P.; González, M. J. Phys. Chem. A, 2012, 116, 5026, and refs. therein. [2] González, M.; Hernando, J.; Millán, J.; Sayós, R. J. Chem. Phys. 1999, 110, 7326. [3] Zhang, J; Lahankar, S. A.; Garton, D. J.; Minton, T. K.; Zhang, W.; Yang, X. J. Phys. Chem. A 2011, 115, 10894. [4] Zhang, B.; Liu, K. J. Phys. Chem. A 2005, 109, 6791. [5] Troya, D.; Millán, J.; Baños, I.; González, M. J. Chem. Phys. 2002, 117, 5730, and refs. therein. [6] Troya, D.; Millán, J.; Baños, I.; González, M. J. Chem. Phys. 2004, 120, 5181.
74 POSTER SESSION 1 P1-19
High resolution imaging of ion-molecule reactions
M.Stei, E. Carrascosa, J. v. Vangerow, R. Otto a, A. Kelkar, S. Trippel b, J. Cox c , F. Hochheimer, J. Wildauer, T. Best and Roland Wester
Institut f. Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstrasse 25/3, A- 6020 Innsbruck, Austria
Over the past decade velocity map imaging (VMI) [1] has emerged as a well established and widely used technique for studying molecular reaction dynamics. With an improved crossed beam 3D-VMI setup we have measured vibrational state-to-state differential cross-sections of the ion- molecule + + reaction Ar + N2 → Ar + N2 with significantly increased resolution. Scattering into v’=1 has been found to be dominant only in forward direction. Higher vibrational excitation up to v’=6 has been detected for larger scattering angles. All kinematically allowed quantum states are populated. Scattering into higher scattering angles is increased for decreasing collision energy. We find qualitative agreement with the theoretical predictions of Candori et al. [2], thus approaching a consistent description of this long-puzzling reaction [3].
The electrostatic lens arrangement of a VMI spectrometer also provides the flexibility to operate in spatial map imaging (SMI) mode. We will present simulations on the influence of different parameters on the imaging resolution and a parametric characterization by a Taylor expansion. Experimental measurements using multi-photon ionisation of toluene in a focused laser beam validate the simulations. A spatial resolution of less than 5 μm in a volume of 4 mm diameter could be demonstrated. Using a simultaneous time-of-flight measurement 3D-SMI detection was achieved. Different ionization and fragmentation products could be resolved with a mass resolution better than m/Δm = 800. Possible applications will be discussed.
We will give an outlook on our plans to further increase the resolution using rotational state control with a Stark deflector [4] and preparation of selected vibrational states with a narrowband infrared laser.
References
[1] A. T. Eppink, D. H. Parker, Rev. Sci. Instrum. 1997, 68 [2] R. Candori, S. Cavalli, F. Pirani, A. Volpi, D. Cappelletti, P. Tosi, and D. Bassi , J. Chem. Phys. 2001, 115 [3] J. H. Futrell, Adv. Chem. Phys. 1992, 82 [4 ] F. Filsinger, J.Küpper, G. Meijer, L. Holmegaard, J. H. Nielsen, I. Nevo, J. L. Hansen, and H. Stapelfeld, J. Chem. Phys 2009, 131 anow at: Dept. of Chem. and Biochem., University of California, San Diego, USA bCenter for Free Electron Lasers, DESY 22706 Hamburg, Germany c Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany
75 POSTER SESSION 1 P1-20
Towards cold chemistry with magnetically decelerated hydrogen atoms
Katrin Dulitz1*, Michael Motsch2, Hansjürg Schmutz2 and Timothy P. Softley1
1Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, United Kingdom, 2Laboratorium für Physikalische Chemie, ETH Zürich, CH-8093, Switzerland
*E-mail: [email protected]
Zeeman deceleration is an experimental technique which makes it possible to manipulate the velocity of open-shell atoms and molecules in a supersonic beam [1, 2]. The method is based on the Zeeman interaction between paramagnetic particles and time-dependent, inhomogeneous magnetic fields generated by pulsing high currents through an array of solenoid coils.
Here, we present progress on the deceleration of hydrogen atoms in a 12-stage Zeeman decelerator that is being set up in Oxford. Results from previous Zeeman deceleration experiments [3] strongly suggest that nonadiabatic transitions, induced by a rapid reversal of the magnetic field direction, can lead to a significant loss of decelerated particles. Experiments to further quantify these effects are currently underway.
In the future, the Zeeman decelerator will be combined with other sources of cold ions and molecules [4] to study chemical collisions at low temperatures. The work will contribute towards the understanding of chemical reactivity in the low-temperature regime and it will provide fundamental tests for chemical reaction theories.
References
[1] N. Vanhaecke et al., Phys. Rev. A 75, 031402 (2007). [2] E. Narevicius et al., Phys. Rev. A 77, 051401 (2008). [3] S.D. Hogan et al., Phys. Rev. A 76, 023412 (2007). [4] M.T. Bell et al., Mol. Phys. 107, 99-132 (2009).
76 POSTER SESSION 1 P1-21
Imaging mass spectrometry using electron impact ionization: Efficiencies and dynamics
James N Bull, Jason W L Lee, Claire Vallance
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, United Kingdom
Collision of free electrons with molecular targets leading to ionization represents one of the most fundamental processes in particle physics. This process constitutes the basis of oldest and most common universal fragmentation mechanism utilized in small molecule mass spectrometry, having been first suggested by J. J. Thomson nearly 100 years ago. Despite wide application in traditional mass spectrometry, the detailed dynamics of EI ionization of molecular systems containing more than a few atoms remains a largely empirical and unstudied field due to the complexity of the many-body processes involved. Our group is performing a series of experimental and theoretical studies concerned with the three general properties of molecular EI ionization; total ion production efficiency, fragment ion production efficiency, and kinetic and internal energy disposal in nascent ions. The overall thrust is to obtain more comprehensive molecular fingerprints and molecular dynamics than available from traditional mass spectrometry. The primary species of interest are those having atmospheric or biological importance, such as halocarbons and prototypes for the peptide bond or nucleotide bases.
This poster outlines partial EI ionization cross-sections for six perfluorocarbons (PFCs), CF4,C2F6,C3F8, C3F6, CF2CFCFCF2 and CF3CCCF3, measured from near threshold to around 210 eV using an open- architecture ion source coupled with a Vacuum Generators SXP-100 quadrupole mass spectrometer. Ion extraction potentials were tuned to extract all ions based on reproducing total (gross) ionization efficiency curves previously measured using an instrument with essentially unit detection efficiency, and supported by SIMION simulations. Normalized mass spectra at 70 eV are in very close agreement with those from the NIST mass spectrum database, and varying agreement with other experimental cross-section determinations. It is argued that these measurements may represent the most precise to date. Theoretical modelling of the total electron impact ionization cross-section using the binary- encounter Bethe (BEB) model provides further evidence to the previous contention that the model overestimates by assuming that all energy in excess of ionization threshold is channelled into ionization. A substantial fraction of the overestimation appears to arise from neutral dissociation accompanying ionization, which is apparently especially pronounced for PFCs. Analysis of the mass spectrum fragmentation patterns combined with ab initio calculations supports that dissociative ionization is the dominant process for the first three saturated PFCs, while the latter three unsaturated PFCs exhibit ionization channels involving energetically-favourable intramolecular specific rearrangements through, apparently, F- migrations.
This poster also outlines the first preliminary measurements recorded using a new electron impact ionization velocity map imaging (VMI) mass spectrometer, recently constructed in our laboratory. Although VMI detection has, in some respects, revolutionized molecular dynamics studies of laser- induced ionization and dissociation of small molecules, adoption to other fields has been slow. The spectrometer utilizes a LaB6 single-crystal electron gun capable of 250 meV resolution (FWHM) in the kinetic energy range of 5 – 100 eV. Electron beam modulation is achieved by pulsing of a normally- earthed focusing lens near the end of the electron gun optics with MOSFET switches capable of 80% rise and fall times of about 20 ns concomitant with typical beam currents. Pulsed ion extraction and
77 velocity mapping is achieved with a standard three-electrode lens assembly and two Behlke model HTS-81 fast high voltage transistor switches. Unlike laser experiments that have repetition rates limited by laser operation at typically 10 Hz, the electron gun and pulsed ion extraction combined with a suitable piezoelectric molecular beam source allows, in principle, the instrument to operate at kilohertz frequencies. Moreover, inverting the polarity of the VMI optics potentially allows direct measurement (imaging) of both (e, 2e) distributions accompanying ionization and the study of negative ion formation resulting from electron capture. PFC species represent an ideal molecular targets for imaging experiments due to the high degree of internal excitation following ionization, large fragment ion kinetic energy recoils (10+ eV with 70 eV ionizing electrons), and large vertical and adiabatic electron affinities.
78 POSTER SESSION 1 P1-22
Time-domain ultrafast vibrational spectroscopy of chemical reaction dynamics
Matz Liebel1 and Philipp Kukura 1*
1 Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ
*E-mail: [email protected]
The advancement in ultrafast laser technology has made it possible to record vibrational spectra in the time domain over the full vibrational manifold. A long standing goal is to use time-domain based Raman spectroscopy to record vibrational spectra of excited electronic states. One of the major challenges is the decomposition of the recorded spectra into ground and excited state contributions. This has led to some controversy if it is at all possible to excite high frequency vibrational coherences on excited states. Isolating the pure excited state coherence would enable one to follow chemical reaction dynamics with the ultimate temporal and spatial resolution.
Here we describe a new approach towards ISRS (Impulsive Stimulated Raman Spectroscopy) that allows one to obtain excited state-only vibrational spectra. We employ a three pulse scheme (ps- pump, fs -pump, probe) to measure the vibrational spectrum of beta-carotene in it’s first excited electronic state with high spectral resolution (<10 cm-1). We also discuss preliminary results on an extension of this technique that allowed us to ultimately proof that it is possible to directly excite and detect high frequency (1800 cm-1) coherences on excited states in a standard two-pulse transient absorption experiment. By employing ISRS we are able to show that the strongly forbidden and previously unreported S0-S1 transition in beta-carotene does take place, albeit at a probability too small to be detected by electronic spectroscopy. Our results demonstrate the feasibility of performing time-domain vibrational spectroscopy with high-sensitivity purely based on electronic resonances decoupling the observation of vibrational structural dynamics from the necessity of large infrared or Raman scattering cross sections.
79 POSTER SESSION 1 P1-23
Ultrafast intersystem crossing in the DNA nucleobase cytosine
Martin Richter*1, Philipp Marquetand2, Jesús González-Vázquez3, Ignacio Sola3, and Leticia González2
1Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena 2Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna 3Departamento de Química Física I, Universidad Complutense, 28040 Madrid, Spain
*E-mail: [email protected]
Singlet-triplet transitions in organic molecules are commonly believed not to compete with femtosecond processes such as internal conversion [1,2]. A recently developed mixed quantum- classical dynamics method called SHARC [3], that allows treating non-adiabatic couplings and spin- orbit couplings on the same footing in combination with ab initio calculations on the CASSCF level of theory was used to unravel the deactivation of the DNA nucleobase cytosine after absorption of UV light. The results reveal the presence of an unprecedented ultrafast intersystem crossing that directly competes with internal conversion on the femtosecond time scale. The most important singlet-triplet population transfer is found between states showing weak spin-orbit coupling but being very close in energy. This deactivation mechanism is different from that previously proposed using quantum chemical calculations alone [4]. The ultrafast nature of this process opens a new view on the importance of intersystem crossing in general.
References
[1] E. Nir et. al Chem. Phys. Lett. 2002, 355, 59-64 [2] K. Kosma et. al J. Am. Chem. Soc. 2009, 131, 16939-16943 [3] M. Richter et. al J. Chem. Theory Comput. 2011, 7(5), 1253-1258 [4] M. Merchán et. al J. Am. Chem. Soc. 2005, 127, 1820-1825
80 POSTER SESSION 1 P1-24
Infrared driven cluster surface reactions
Alexander C. Hermes,a Suzanne M. Hamilton,a Graham Cooper,a Stuart R. Mackenziea & Dan J Harding,b Christian Kerpal,b Gerard Meijerb and André Fielickeb
a Department of Chemistry, PTCL, University of Oxford, UK b Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
[email protected] [email protected]
In recent years infrared multiple photon dissociation spectroscopy (IR-MPD) has been employed to great effect to study the vibrational spectroscopy of small (<20 atoms) transition metal clusters in the gas phase. Used in conjunction with the inert messenger technique, this action spectroscopy takes advantage of the sensitivity of mass spectrometric detection to compensate for the low number densities of clusters generated. Comparison of IR-MPD spectra with vibrational spectra simulated by density functional theory has permitted the assignment of low lying structural isomers in a number of metal cluster systems.1
Recently, we have employed a variant of this technique to study infrared driven chemical reactions on the surface of size- and structure-selected metal clusters. In these experiments, IR radiation is absorbed by a chromophore on the cluster surface raising the internal energy of the cluster until reactive processes occur.2,3 Our initial experiments have focused on the decomposition of + molecularly adsorbed N2O on rhodium clusters, Rhn (n=4-8) where we showed that the ratio of reaction (i.e., N2O decomposition) versus N2O desorption was heavily dependent on both cluster size and oxygen coverage.
Here, we summarise some of these results, and present the first bimolecular reactions to be studied + in this manner – infrared driven CO oxidation on PtnOm clusters. IR-MPD spectra provide conclusive evidence of the precursor cluster structures present in our molecular beam, including evidence for meta-stable peroxo-type structures. The data presented provide convincing evidence of efficient bimolecular reactivity across the range of cluster sizes studied (n=3-7) which is supported by DFT calculations of the reaction pathway.
Infrared-driven CO oxidation on platinum cluster oxides
References
1 see, for example, Harding et al., J. Chem. Phys., 132, 011101 (2010); ibid. 133, 214304 (2010) 2 Hamilton, et al., J. Am. Chem. Soc., 132, 1448 (2010) 3 Hamilton, et al., J. Phys. Chem. A, 115, 2489 (2011)
81 POSTER SESSION 1 P1-25
Rotational angular momentum polarisation effects in the inelastic scattering of fully quantum state selected NO(X) with the rare gases.
M. Brouard1*, H. Chadwick1, C. J. Eyles1, B. Hornung1, B. Nichols1, F. J. Aoiz2, S. Stolte3
1Department of Chemistry, University of Oxford, United Kingdom 2Departmento de Química Física, Universidad Complutense, Spain 3Atomic and Molecular Physics Institute, Jilin University, China
* Email: [email protected]
Inelastic scattering measurements have been made for the NO(X)-rare gas systems using a crossed molecular beam apparatus coupled with velocity mapped ion imaging, for a selection of spin-orbit conserving and spin-orbit changing transitions. The initial state of the NO(X) was selected using a hexapole field and the final state by (1+1') REMPI, allowing full Λ‐doublet resolution of both the initial and final quantum state.
The state to state differential cross-sections for scattering with xenon have been measured, building on previous work on NO(X)-Ar1-3 and NO(X)-He4. The same parity dependent effects were observed with xenon as have been seen previously for NO(X)-Ar. Complementary quantum mechanical calculations have also been performed and the agreement between the experimental and theoretical differential cross-sections is reasonable.
Measurements of the product rotational angular momentum alignment following collisions of NO(X) with argon and krypton have also been performed experimentally. The results obtained can be explained classically by considering a simple kinematic apse5 model, in which the projection of j onto the apse is conserved throughout the collision. Again, the polarisation parameters obtained from the experimental data are in good agreement with those obtained theoretically.
References
[1] C. J. Eyles, M. Brouard, C.‐H. Yang, J. Kłos, F. J. Aoiz, A. Gijsbertsen, A. E. Wiskerke and S. Stolte, Nature Chemistry, 3, 597, (2011) [2] C. J. Eyles, M. Brouard, H. Chadwick, B. Hornung, B. Nichols, C.‐H. Yang, J. Kłos, F. J. Aoiz, A. Gijsbertsen, A. E. Wiskerke and S. Stolte, Phys. Chem. Chem. Phys., 14, 5403, (2012) [3] C. J. Eyles, M. Brouard, H. Chadwick, F. J. Aoiz, J. Kłos, A. Gijsbertsen, X. Zhang and S. Stolte, Phys. Chem. Chem. Phys., 14, 5420, (2012) [4] A. Gijsbertsen, H. Linnartz, G. Rus, A. E. Wiskerke, S. Stolte, D. W. Chandler and J. Kłos, J. Chem. Phys., 123, 224305, (2005) [5] A. J. McCaffery, M. J. Proctor and B. J. Whitaker, Annu. Rev. Phys. Chem., 37, 223, (1986)
82 POSTER SESSION 1 P1-26
Charge transfer dynamics of Rydberg hydrogen atoms at surfaces
M Dethlefsen, J Gibbard, E So, S Ganashalingam, X Li, T P Softley
Department of Chemistry, University of Oxford, United Kingdom
In a Rydberg states a valence electron of an atom or molecule is excited to a high principle quantum number. These states exhibit interesting properties like weak binding energy, high polarisability and are very susceptibility to external electric and magnetic fields. An interesting field of study is the charge transfer of the Rydberg electron into the conduction band of surfaces. In this poster results of the interaction of hydrogen Rydberg states with gold metal surfaces are presented. Differences to previous experiments using non hydrogen Rydberg states (e.g., H2) are illustrated and the possibility to control the ionisation behaviour with spectroscopic methodes is shown. Furthermore, results of charge transfer experiments at semiconductor and nanoparticle surfaces are shown, demonstrating the possible application of Rydberg states as surface probes.
83 POSTER SESSION 1 P1-27
Two-colour resonant four-wave mixing: A perspective tool to study molecular collisions and reactions
P. Radi1, P. Maksyutenko1, D. Kozlov2, A. Kouzov3*
1Paul Scherrer Insitute, 5232 Villigen, Switzerland 2A.M. Prokhorov General Physics Institute, 119991 Moscow, Russia 3Saint-Petersburg State University, 198504 Saint-Petersburg, Russia
Coherent responses produced by resonant four-wave mixing in a weakly absorbing medium carry valuable information on the intrinsic properties and dynamics of the quantum states involved. Potentialities of the generalized (two-colour) RFWM set-ups to study dynamical state characteristics were revealed by pioneering experimental [1] and theoretical [2] investigations in the Sandia Laboratories. Detection of weaker satellite resonances in the continuous TC-RFWM spectra [3] which are entirely induced by the rotational collisional transfer marked a further progress. Subsequent studies proved the unique TC-RFWM ability to produce the state-to-state resolved picture of the rotational relaxation, both in the frequency [4] and time [5] domains.
Vaccaro and co-workers applied the degenerate (DG) RFWM technique to molecules with angular momentum anisotropy and derived expressions for the TC-RFWM amplitudes for a particular (SEP) three-level excitation scheme [6]. Here we present the TC-RFWM theory which is extended to an arbitrary excitation scheme of the rotationally anisotropic states and includes DG-RFWM as a limiting case. New experimental DG-RFWM patterns from the OH radicals produced by laser photolysis of H2O2 are reported, too. Their polarization dependence and Doppler line structure show evidence on the pronounced anisotropy of angular momentum (J) and velocity (v) distributions as well as on the J-v correlation.
The work was supported by the Swiss Federal Office of Energy, the Swiss National Science Foundation (200020_124542/1), and by the Russian Foundation for Basic Research, grants № 11‐02‐ 01296 and 11-03-00448.
References
[1] S. Williams, J.D. Tobiason, J.R. Dunlop, and E.A. Rohlfing, J.Chem. Phys. 102, 8342 (1995). [2] S. Williams, E. A. Rohlfing, L. A. Rahn, and R.N. Zare, J. Chem. Phys. 106, 3090 (1997). [3] P. P. Radi, H.-M. Frey, B. Mischler, A. P. Tzannis, P. Beaud, and T. Gerber, Chem. Phys. Lett. 265, 271 (1997). [4] A.P. Kouzov and P. P. Radi, Phys. Rev. A 63, 010701 (2000). [5] X. Chen, T.B. Settersten, and A.P. Kouzov, J. Raman Spectrosc. 40, 847 (2009). [6] D. Murdock, L.A. Burns, and P.H. Vaccaro, J. Phys. Chem. A 113, 13184 (2009).
84 POSTER SESSION 1 P1-28
REMPI spectroscopy of NO-alkane complexes
Joe Harris
School of Chemistry, University of Nottingham, UK
Spectra of NO—methane, NO—ethane, NO—propane, and NO—n-butane have been recorded using
Resonance Enhanced Multi-Photon Ionisation (REMPI) spectroscopy via the 3s (Ã) state. D0 values have been obtained for the excited state complexes, and thermodynamic cycles employed to appraise D0 for the ground state complexes. Geometry optimisations were also performed using MP2 theory.
85 POSTER SESSION 1 P1-29
Femtosecond time-resolved Photodissociation Dynamics of ClN3
David Staedter1, Nicolas Thire12, Petros Samartzis3 – Valerie Blanchet1
1Addresses1Universite de Toulouse, UPS, 118 route de Narbonne, F-31062 Toulouse, France 2Institut National de la Recherche Scientifique, Université du Québec, 1650 Blvd Lionel-Boulet, Varennes, J3X1S2, Québec, Canada 3The Institute of Electronic Structure and Laser, Foundation of Research and Technology Hellas, Iraklion 71110, Greece
*E-mail: [email protected]
Azide (X-N3) UV photochemistry (where X can stand for H, I, Cl, Br, F) proceeds through two pathways producing NX + N2 and X + N3 where N3 can appear not only in a linear geometry but also as cyclic-N3, a unique all-nitrogen ring [1]. Although N3 is expected to be produced in a cyclic geometry, spectroscopic studies reveal mostly its linear geometry [2]. Previous work in the nanosecond regime determined the dissociation limit D0 of ClN3 to Cl radical and N3(lin) to 1.86 ± 0.5 eV [3] and found evidence for cyclic-N3 production below 250 nm (4.96 eV), with its yield increasing as wavelength is decreased. The dissociation limit for N3 + Cl where N3 is produced in a cyclic configuration lies typically 1.3 eV higher in energy compared to the dissociation limit for N3 in a linear configuration [4]. 1 + 2 Cyclic N3 is stabilized by a spin-orbit forbidden dissociation onto N2(X Σ g )+N( D), an endoenergetic 1 + 4 barrier to N2(X Σ g )+N( S) of 1.42 eV. The relaxation onto its linear form is characterized by a frustrating barrier of 1.37 eV [5].
Here we report the first time-resolved study of photochemistry of ClN3 by femtosecond velocity map imaging (fs-VMI). Goal of the experiment is to elucidate the ultrafast dynamics that lead to a cyclic-N3 production. The photodissociation of ClN3 is studied at two different energies, namely around 4.6 eV (268 nm) where only linear N3 is produced and around 6.2 eV (200 nm) where mainly cyclic-N3 is produced. In order to detect this elusive N3 cyclic radical, we utilize the small difference in ionization potential (~0.47 eV) between the cyclic and linear geometry. Besides dissociation, time-resolved photofragment images also provide information on ClN3 dissociative ionization dynamics.
References
[1] P. C. Samartzis and A. M. Wodtke, Phys. Chem. Chem. Phy, 9, 3054-66,(2007). [2] A. E. Douglas and W. J. Jones, Can. J. Phys, 43, 2216, (1965). [3] A. Quinto-Hernandez et al., Int. J. Mass spectrom., 265, 261-266, (2007). [4] N. Hansen and A. M. Wodtke, J. Phys. Chem. A, 107, 10608-10614, (2003). ϱD ͘ŝƩĞƌĞƌŽǀĄ͕, ͘P ƐƚŵĂƌŬ, and T. Brinck, J. Phys. Chem. A, 116, 9740,(2002).
86 POSTER SESSION 1 P1-30
3 Crossed molecular beam study of O( P) + H3CCCH reaction
F. Leonori1, N. Balucani1, V. Nevrly2, S. Falcinelli3, D. Stranges4, P. Casavecchia1*
1ŝƉĂƌƟŵĞŶƚŽĚŝŚŝŵŝĐĂ͕hŶŝǀĞƌƐŝƚăĚŝWĞƌƵŐŝĂ, Via Elce di Sotto 8, 06123 Perugia, Italy 2Faculty of Safety Engineering, VSB-Technical University of Ostrava, Lumirova 13, Ostrava-Vyskovice, 70030, Czech Republic 3Dipartimento di Ingegneria Civile e Ambientale, Universita` di Perugia, Perugia, Italy 4ŝƉĂƌƟŵĞŶƚŽĚŝŚŝŵŝĐĂ͕hŶŝǀĞƌƐŝƚă͞>Ă^ĂƉŝĞŶnjĂ͟, P.le A. Moro 5, Rome I-00185, Italy
*E-mail: [email protected]
Reactions of unsaturated hydrocarbons with oxygen atoms are important elementary steps of detailed combustion mechanisms. The reaction of propyne (methylacetylene) with O(3P) was investigated in crossed molecular beam (CMB) experiments and compared with previous results [1] on the reaction with the allene isomer in order to explore the effect of molecular structure on chemical reactivity. Mass-spectroscopic detection of reaction products with time-of-the-flight (TOF) analysis was utilized in order to characterize the open reaction channels, their relative yields (branching ratio) and reaction mechanisms. The unique assignment of individual features in TOF spectra was enabled based on the combination of tuneable soft-electron impact ionization, mass- specific selection of reaction products as well as scattering distributions.
3 Previous study of O( P)+H2CCCH2 reaction [1] revealed the central importance of inter-system crossing (ISC). Chain-terminating character of that reaction is relevant to major yields of stable products, which are formed following the transition from the initial triplet potential energy surfaces 3 (PES) to the underlying singlet PES. On the other hand, the results on O( P)+HCCCH3 show rather chain-branching effect due to formation of radical species (such as H+H3CCCO and CH3+HCCO), as primary products. The overall ratio of products from the singlet PES to those from the triplet PES becomes smaller moving from allene to propyne, Qualitative analysis of singlet and triplet PESs provided for the given systems [2, 3] rationalizes such behaviour. It can be concluded that the 3 reactions of O( P) with the two C3H4 isomers allene and propyne are obviously different, in spite of the apparent similarities. Comparisons with the results of dynamical QCT calculations on full dimensional triplet and singlet coupled PESs (with inclusion of ISC effects), as recently reported for 3 3 the related O( P)+C2H4 reaction [4], are desirable also for the O( P)+C3H4 isomeric reactions in order to quantitatively rationalize the variation of the branching ratios and of the extent of ISC with molecular complexity and structure.
NV acknowledges the COST Action CM0901—Detailed Chemical Models for Cleaner Combustion, for support of a one-month short-term mission to Perugia.
References
[1] F. Leonori, A. Occhiogrosso, N. Balucani, A. Bucci, R. Petrucci, P. Casavecchia, J. P. C. Lett. 3, 75 (2012). [2] T.L. Nguyen, J. Peeters, L. Vereecken, J. Phys. Chem. A 110, 12166 (2006). [3] S. Zhao,W. Wu, H. Zhao, H. Wang, C. Yang, K. Liu, H. Su, J. Phys. Chem. A 113, 23 (2009). [4] B. Fu, Y-C. Han, J.M. Bowman, L. Angelucci, N. Balucani, F. Leonori, P. Casavecchia, PNAS 109 (25) 9733-9738 (2012).
87 POSTER SESSION 1 P1-31
2 Dynamics study of the Cl( P) + NH3 reaction with QCT calculations: Role of vibrational and translational energy
J. Espinosa-Garcia,1 C. Rangel, J.C. Corchado, M. Monge-Palacios
Departamento de Química Física, Universidad de Extremadura 06071 Badajoz (Spain)
1e-mail: [email protected]
The dynamics connected with vibrationally excited molecules represents a very interesting field of research, which is related to issues such as bond and mode selectivity or the problem of which motion (vibration or translation) is more effective in driving the reaction, Polanyi’s rules.
The hydrogen abstraction reaction from ammonia by a chlorine atom presents a very complicate potential energy surface (PES) with several maxima and minima, which may have significant influence on the dynamics. These systems present a severe challenge to developing PESs, and carrying out dynamics calculations.
Based on an analytical PES recently developed in our group, using exclusively high-level ab initio 2 calculations at the CCSD(T)=FULL/aug-cc-pVTZ level, for the Cl( P) + NH3(v) HCl + NH2 gas-phase reaction, quasi-classical trajectory (QCT) calculations were performed on the NH3(v) vibrational ground-state and on the independent one-quantum vibrational excitation of all NH3(v) modes. The reaction cross sections, the mode selectivity, and the effects of equivalent amounts of energy as vibration or translation in overcoming the reaction barrier were analysed. Unfortunately, there are neither experimental nor theoretical values for comparison, but the influence of the reactant channel well in the QCT results was analysed based on a modified surface (PES-mod), also developed in our group, in which the reactant channel well has been removed.
This work was partially supported by the Gobierno de Extremadura, Spain, and Fondo Social Europeo, Proj. nº. IB10001.
88 POSTER SESSION 1 P1-32
Parity-dependent oscillations in the transfer of orientation in inelastic collisions of CN(A2Π) with Ar
Stephen J. McGurk, Kenneth G. McKendrick and Matthew L. Costen1*
1Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS
*E-mail: [email protected]
We present measurements of the propensity to conserve orientation in inelastic collisions of CN(A2Π, v = 4) with Ar. CN(X2Σ+) was prepared by photolysis of ICN, and allowed to translationally and rotationally relax in a bath of Ar. Oriented samples of CN(A2Π, v = 4) were then prepared in rotational and parity-resolved levels (specifically j = 6.5 F1e, and j = 10.5 F2f) by circularly-polarized excitation on specific R or P branch transitions of the A-X(4,0) band. The orientation of the prepared level, and that of product rotational levels populated by collisions with the Ar bath gas, was measured using Frequency Modulated Stimulated Emission Spectroscopy on the A-X(4,2) band.[1] The degree of orientation transferred in the collision may be quantified by the multipole transfer efficiency, ( )( ), which ranges from +1 (total conservation) to -1 (complete change of sign of the orientation).ଵ ᇱ ܧ ݆ǡ݆
We see striking oscillations in ( )( , ) as a function of the final rotational level,ଵ spin-orbitᇱ and lambda-doublet state. Collisionsܧ ݆݆ that conserve total rotational parity preferentially conserve orientation, whilst parity-changing collisions are found to result in significant loss, or even reversal in the sign, of the orientation. As far as we are aware, these are the first ever experimental observations of such parity- dependent oscillations in a transferred polarization, which are also predicted by quantum scattering calculations, but await more qualitative explanation. [2]
References
[1] S. J. McGurk, K.G. McKendrick, M. L. Costen, D. I. G. Bennett, J. Kłos, M. H. Alexander and P. J. Dagdigian, J. Chem. Phys. 136 164306 (2012) [2] S. J. McGurk, K.G. McKendrick, M. L. Costen, M. H. Alexander and P. J. Dagdigian, In preparation
89 POSTER SESSION 1 P1-33
Conformer-specific reactions with Coulomb-crystallised ions
D. Rösch 1*, S. Willitsch1, Y.-P. Chang2*, J. Küpper2,3
1Department Chemie, Universität Basel, 4056 Basel, Switzerland, 2Center for Free-Electron Laser Science, DESY, 22607 Hamburg, Germany, 3Department of Physics, University of Hamburg, 22761 Hamburg, Germany
*E-mail: [email protected] and [email protected]
Many molecules have multiple conformations (rotational isomers) which can exhibit different reactivities, opening up perspectives to manipulate the outcome of chemical reactions by selecting specific molecular conformations [1]. However, a detailed understanding of the role of conformations in gas-phase chemical reactions still has to be established. We present a study of reactive collisions between conformer-selected neutral molecules [2] and Coulomb crystals of laser-cooled Ca+ ions [3] to explore the conformation dependence of bimolecular reactions.
In the present study, the reaction of 3-aminophenol with Coulomb-crystallised Ca+ ions was chosen as a model system. 3-Aminophenol exhibits two different conformations (cis and trans) with different permanent electric dipole moments. Their interaction with external inhomogeneous electric fields enables the two conformers to be spatially separated in a molecular beam passing through an electrostatic deflector [2]. Coulomb- crystals of spatially localised Ca+ ions stored in an ion trap [3] provide a suitable stationary target for the conformer-selected molecular beams enabling the study of conformer-specific reactive collisions with extremely high sensitivities down to the level of single reaction events.
Electronic structure calculations using DFT methods indicate that both conformers react through the same transition state and show similar dynamics at short range. However, preliminary results from adiabatic-capture theory calculations predict that the reaction rate for the cis-conformer should be larger than for the trans-conformer, due to the differences in the long-range ion-molecule interaction potentials of the two conformers [4].
The experimental realization was developed in two stages. First, a molecular-beam machine with an electrostatic deflector was built [2], and the spatial separation of cis- and trans-3-aminophenol seeded in He or Ne was characterized. Subsequently, the molecular-beam apparatus was combined with an ion trap for the laser cooling of Ca+ ions [3]. By tilting the molecular-beam assembly mechanically, conformationally pure components of the molecular beam can be overlapped with the spatially localised ions in the collision region. The progress of the reaction is monitored by imaging the laser- induced fluorescence of unreacted ions. On the poster, we will introduce our experimental method and present first results.
90 References
[1] F. Filsinger, U. Erlekam, G. von Helden, J. Küpper, and G. Meijer, Phys. Rev. Lett. 100, 133003 (2008) [2] F. Filsinger, J. Küpper, G. Meijer, J. L. Hansen, J. Maurer, J. H. Nielsen, L. Holmegaard, and H. Stapelfeldt, Angew. Chem. Int. Ed. 48, 6900 (2009). [3] S. Willitsch, Int. Rev. Phys. Chem. (2012), DOI :10.1080/0144235X.2012.667221 [4] D.C. Clary, J. Chem. Soc. Faraday Trans. 88, 901 (1992).
91 POSTER SESSION 1 P1-34
Hot molecules in helium nanodroplets: a new route to optical spectra
Cheng Feng, Benjamin Shepperson, Adrian Boatwright, Daniel Spence, Shengfu Yang and Andrew Ellis
Department of Chemistry University of Leicester
The S1-S0 electronic excitation of toluene in helium nanodroplets is found to alter the branching ratio + + for production of the C7H7 and C5H5 fragment ions when the droplets are subjected to subsequent electron impact. This is attributed to the intersystem crossing from the S1 state into a long-lived triplet state T1, which then delivers metastable toluene molecules into the ion source of the mass + spectrometer. The abundance of C7H7 ions decreases by this optical excitation process whereas the + abundance of C5H5 ions increases. Compared with the commonly used process for recording optical spectra of molecules in helium nanodroplets, i.e. to the measurement of the reduction in ion signal produced by the contraction in size of the helium droplet induced by light absorption and subsequent dissipation of the added energy in the form of heat, the mechanism in operation in the current study is different and has potential for recording spectra of molecules whenever optical excitation delivers a relatively long-lived (ms or longer) excited state.
92 POSTER SESSION 1 P1-35
Cold charge-transfer reactions with a Stark decelerated molecular beam
Lee Harper, Nabanita Deb, Brianna Heazlewood, James Oldham, Chris Rennick, Martin Bell and Tim Softley
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, United Kingdom
Reactive collisions between xenon ions and ND3 molecules are investigated under cold conditions. An ion trap is loaded with calcium ions, which are subsequently Doppler cooled to adopt a “Coulomb crystal” phase. Xenon molecules leaked into the reaction chamber are then ionised, enabling them to be sympathetically cooled into the crystal through their Coulomb interaction with the laser-cooled calcium ions. The incorporation of Xe+ ions into the crystal is monitored through spatial changes in the fluorescence emitted by the trapped Ca+ ions, indicated by a flattening of the observed crystal shape. To investigate the charge-exchange reaction between trapped cold xenon ions and cold neutral molecules, a Stark decelerated molecular beam of ND3 molecules is introduced to the + + reaction chamber. The resulting ND3 ions, formed following charge-exchange with Xe , can be sympathetically cooled into the centre of the Coulomb crystal. Through monitoring and modelling the changes in the spatial location of the fluorescing Ca+ ions within the crystal, the charge exchange reaction can be examined and characterised.
93 POSTER SESSION 2 P2-1
Anatomy of the OH + D2 → HOD(v1’,v2’,v3’) + D benchmark reaction from molecular beams and QCT-GB calculations
J. D. Sierra,1 D. R. Albert,2 H. F. Davis,2 L. Bonnet,3 M. González4,*
1Depto. de Química, Univ. de La Rioja (Spain) 2Dept. of Chemistry and Chemical Biology, Cornell Univ. (USA) 3Institut des Sciences Moléculaires, Univ. de Bordeaux I (France) 4Dept. Química Física i IQTC, Univ. de Barcelona (Spain)
*E-mail: [email protected]
For the OH + D2 benchmark reaction [1] we extended a recent quasiclassical trajectory-gaussian binning (QCT-GB) study performed on it at Ecol=0.28 eV (employing the WSLFH potential energy surface) [2] by calculating 102 million trajectories (i.e., one order of magnitude more trajectories than in Ref. [2]). There it was shown that the strong differences observed between QCT and experimental results mainly come from an inadequate quantization of HOD, and it was obtained one of the best agreements between theory and experiment reported so far in polyatomic reaction dynamics [2]. Even though the center of mass angle-velocity distribution and HOD vibrational populations determined here are essentially coincident with those of Ref. [2], now we compared theory with experiment in an even deeper way than before. Thus, the T.O.F. and total angular distributions in the laboratory (D atom) were simulated using the QCT-GB center of mass differential cross section and translational energy distribution of products. There is a good agreement between these experimental data [3,4] and the simulations. Moreover, other relevant properties not reported in Ref. [2] were also investigated here, including, e.g., vibrational state-specific cross sections and rotational energy distributions. Finally, the large improvement of the statistics allowed us to analyze the microscopic mechanism (direct reaction of rebound type) to try to understand why is so difficult to generate HOD in bending excited states.
This work was supported by the Spanish Ministry of Science and Innovation (projects CTQ2008- 06805-C02-01 and CTQ2011-27857-C02-01) and by the Office of Science, U.S. Department of Energy, under Grant DE-FG02-00ER15095. Thanks are also given to the Autonomous Government of Catalonia (ref. 2009SGR 17) for some support.
References
[1] Sierra, J. D.; Martínez, R.; Hernando, J.; González, M. Phys. Chem. Chem. Phys. 2009, 11, 11520. [2] Sierra, J. D.; Bonnet, L.; González, M. J. Phys. Chem. A 2011, 115, 7413. [3] Strazisar, B. R.; Lin, C.; Davis, H. F. Science 2000, 290, 958. [4] Strazisar, B. R. Ph. D. Thesis, Cornell University, 2000.
94 POSTER SESSION 2 P2-2
‐ ‐ Photodetachment of cold OH and H3O2
Alexander von Zastrow*, D. Hauser, Rico Otto, Katharina Geistlinger, Eric Endres, Thorsten Best and Roland Wester
Institute for Ion Physics and Applied Physics, University of Innsbruck, Technikerstraße 25/3, 6020 Innsbruck, Austria
E‐mail: [email protected]
In order to gain fundamental understanding of processes such as ion‐molecule reaction dynamics, laser‐induced reactions, internal quantum state population and thermalisation processes, and astrochemistry of ions, multipole radiofrequency ion traps have become an essential tool in the last decade [1]. Ions are efficiently trapped in a 22‐ pole ion trap and cooled by buffer gas collisions to a controllable temperature.
‐ ‐ Here results on the near‐threshold photodetachment spectroscopy of OH and H3O2 are presented. Using ion density tomography, the absolute photodetachment cross sections have been obtained as a function of frequency above the detachment threshold and for different buffer gas temperatures. For OH‐, several steps in the cross section are observed, which are determined by the population of the different rotational states in the anion. This has allowed us to determine the rotational temperature of the trapped ‐ anions. For H3O2 we found a linear dependence of the cross section, which indicates a strong influence of the vibrational Franck‐Condon factor on the cross section. This is supported by the observed large change of the absolute cross section with trap temperature, which also shows the change of the vibrational temperature of the H3O2 [2].
In the future we plan to use the internal‐state dependence of the photodetachment cross section to study resonant two‐photon photodetachment. As an example, in OH‐ a terahertz photon can drive the transition from the rotational ground state to the first excited state, which is subsequent detached by a second photon. This will provide a new way to perform terahertz spectroscopy of ions and will also allow studies on rovibrationally inelastic collisions of cold trapped ions.
* now at: Department of Molecular and Laser Physics, Radboud University Nijmegen, 6500 GL Nijmegen, the Netherlands
References
[1] R. Wester, J. Phys. B 42, 154001 (2009) [2] R. Otto etal., in preparation [3] J. Mikosch etal., Phys. Rev. A 78, 023402 (2008)
95 POSTER SESSION 2 P2-3
New theoretical method for calculating the radiative association cross section - of a triatomic molecule: Application to N2-H
T. Stoecklin1, F. Lique2 and M. Hochlaf3
1 Institut des Sciences Moléculaire, UMR5255-CNRS, Université de Bordeaux, 351 cours de la Libération, 33405 Talence Cedex, France 2 LOMC - UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 540, 76058, Le Havre, France 3 Université Paris-Est, Laboratoire Modélisation et Simulation Multi Echelle, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallée, France.
Radiative association in ion-molecule collisions is considered to be an important step of the synthesis of polyatomic species in interstellar clouds [1]. The cosmic rays ionise the atoms and the molecules which can then recombine by radiative association to produce new molecules. About 14 positive ions have been detected in the Interstellar Medium as well as several carbon chain anions. However, whereas the radiative association of diatomic molecules is the object of many theoretical [2] studies the radiative association of a triatomic molecule has up to very recently not received much interest compared to photodissociation. The main reason of this lack of studies is the experimental difficulty to measure these cross sections. The first and only theoretical study of radiative association of a - triatomic molecule was done in 2011 by Ayouz et al [3] for the H3 anion. We present here a new method to treat the atom diatom radiative association within a time independent approach. This method is an adaptation of the driven equations method developed for photodissociation. In the - second part of this paper, this approach is applied to the radiative association of the N2H anion. The bound states energies and wave functions of this anion which we calculated exactly in a recent study [4] are used to propagate the overlap with the initial scattering wave function. The main features of the radiative association cross sections are analysed and the magnitude of the calculated rate - coefficient at 10 k is used to discuss the existence of the N2H in the interstaller medium which could - be used as a tracer of both N2 and H .
References
1 D. R. Bates, E. Herbst, in ‘Rate coefficients in Astrochemistry’,T. J. Millar, D. A. Williams editors, Kluwer Academic, Dordrecht p 17 (1998) 2 O. J. Bennett, A. S. Dickinson, T. Leininger and F. X. Gadea, Mon. Not. R. Astron. Soc. 341, 361 (2003) 3 M. Ayouz, R. Lopes, M. Raoult, O. Dulieu, and V. Kokoouline, Phys. Rev. A. 83, 052712 (2011) 4 F. Lique, P. Halvick, T. Stoecklin and M. Hochlaf, J. Chem. Phys. 138, 244302 (2012)
96 POSTER SESSION 2 P2-4
Describing temporary anion states in diazines
Zdeňek Mašín and Jimena D. Gorfinkiel*
Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
*E-mail: [email protected]
The damaging effect of low energy electrons on DNA strands was established over a decade ago [1]. The basics of the mechanism are also well known: the electron attaches itself to the DNA molecule forming a temporary negative ion (TNI) that decays (at least partially) via the molecule breaking-up while the electron remains attached to one of the fragments. The detection and characterization of these TNIs has therefore become of significant interest: experimental work on low energy electron scattering from DNA and its constituents has shed light on the contribution that low energy electrons make to radiation-induced DNA damage. Theoretical work has been scarcer due to the computational resources that scattering calculations, even for isolated DNA constituents (for example, DNA bases) require.
Three DNA/RNA bases are pyrimidine derivatives. Pyrimidine can therefore be used as a model molecule in both theoretical and experimental studies of electron scattering from DNA bases: its higher symmetry makes the calculations more manageable and allows for a more detailed study of the collision process. We have performed a high-level, comprehensive theoretical, ab-initio study of electron collisions with pyrimidine and its isomers pyrazine and pyridazine [2-4]. We have focused on the identification and characterization of the electronic resonances in these systems and paid special attention to developing an understanding of the quantum chemical models (basis set, configuration interaction model, etc.) that best describe the TNIs in these molecules. Using the well- established R-matrix method [5] we have determined that a large number of resonances is present in the diazines. We will discuss all the collisional data obtained and link our findings to those from DNA base studies.
References
[1] Boudaiffa B, Cloutier P, Hunting D, Huels M A and Sanche L, Science 287, 5458 (2000) [2] Z. Mašín and J. D. Gorfinkiel, J. Chem. Phys. 135, 144308 (2011). [3] Z. Mašín, J. D. Gorfinkiel, D. B. Jones, S. M. Bellm, and M. J. Brunger, J. Chem. Phys. 136, 144310 (2012) [4] Z. Mašín and J. D. Gorfinkiel, submitted to J. Chem. Phys. [5] P. Burke, R-matrix theory of Atomic Collisions (Springer Series on Atomic, Optical, and Plasma Physics), 2011.
97 POSTER SESSION 2 P2-5
Classical and semi-classical approaches to radiative association
Magnus Gustafsson , Sergey V. Antipov, and Gunnar Nyman ⋆ Department of Chemistry and Molecular Biology, University of Gothenburg, SE-412 96 Gothenburg, Sweden
E-mail: [email protected] ⋆ Radiative association is a mechanism which contributes to the production of molecules in interstellar clouds. It is especially important in low-density and dust-poor regions where reactions due to ternary collisions are rare [1]. The rate coefficients for radiative association are typically orders of magnitude lower than those of other chemical processes, such as exchange reactions, and direct laboratory measurements have so far only been possible to carry out for certain ionic systems [2]. Thus astronomers who wish to model the chemical evolution in interstellar space often have to rely on theoretical calculations of radiative association rates.
Semiclassical calculations of radiative association from an upper to a lower lying electronic state [3] have proven to give good estimates of rate constants. Furthermore, they provide a cross section baseline, to which a quantum mechanical resonance contribution can be added by means of Breit– Wigner theory [4] for better accuracy. It turns out, however, that the conventional semiclassical theory fails in predicting the baseline of the cross section at energies above the Franck–Condon limit, which is given by the maximum of the difference between the upper and the lower potential curves for a given geometry. We present a modified semiclassical expression to resolve this failure, and thereby also improve predictions of the high temperature rate constant [5].
For radiative association with no change in the electronic state a classical theory has so far been missing. The advantages with such a theory would be the same as for the case described above, i.e. reasonable estimates of rate constants to which resonance contributions can be added according to Breit–Wigner theory. A recent study [6] indicates that it is indeed possible to obtain quite good results for this class of processes using calculations based on classical mechanics and classical electromagnetism.
We have computed radiative association cross sections and rate constants for a few relevant diatomic systems (LiH, HD, CO, CN, and SiN) to study how well the methods work.
Acknowledgments: This work was supported by the Swedish Research Council and the ASTRONET CATS collaboration.
References
[1] J. F. Babb and K. P. Kirby, in The Molecular Astrophysics of Stars and Galaxies, edited by T. W. Hartquist and D. A. Williams (Clarendon Press, Oxford, 1998), pp. 11 – 34. [2] D. Gerlich and S. Horning, Chem. Rev. 92, 1509 (1992). [3] D. R. Bates, Mon. Not. R. Astron. Soc. 111, 303 (1951). [4] R. A. Bain and J. N. Bardsley, J. Phys. B 5, 277 (1972). [5] M. Gustafsson, S. V. Antipov, J. Franz, and G. Nyman, submitted to J. Chem. Phys. (2012). [6] M. Gustafsson, (2012), manuscript.
98 POSTER SESSION 2 P2-6
Hydrogen diffusion on interstellar ices 1* Bet hmini Senevirathne, Stefan Andersson and Gunnar Nyman
1 Department of Chemistry and Molecular Biology, University of Gothenburg, 41296 Gothenburg, Sweden
*E-mail: [email protected]
H2 is the most abundant molecule in interstellar space. Its formation in gas phase is too slow to explain its abundance. It is believed that formation on grain surfaces is the dominant route. Such surfaces are often covered with a mantle of water ice. If hydrogen atom diffusion is fast on such surfaces this could lead to adsorbed hydrogen atoms moving around to find each other to form hydrogen molecules, which may then desorb from the surface. Experiments on how fast hydrogen atom diffusion is on amorphous solid water gave apparently conflicting results [1-4]. Recent work by N. Watanabe et al [5] provides an explanation that hinges on the binding energies of the adsorption sites. Here we investigate theoretically the hydrogen atoms diffusion. We are able to verify the interpretation of Watanabe. We further calculate barrier heights and rate constants for hopping between local minima on the surface of both crystalline and amorphous ice.
Fig 1. Rate constants for hydrogen atom hopping on amorphous ice obtained with classical transition state theory (qq‐‐‐HTST) and a quantum version that accounts for tunneling (HQTST). Tc is the cross over temperature below which tunneling dominates.
References
[1] G. Manico, G. Ragunı, V. Pirronello, J. E. Roser and G. Vidali; 2001, ApJ, 548, L253. [ 2 ] H. B. Perets, O. Biham, G Manico, V. Pirronello, J. Roser, S Swords and G. Vidali; 2005, ApJ, 627, 850. [ 3 ] L. Hornekær, A. Baurichter, V. V. Petrunin, D. Field and A. C. Luntz; 2003, Science, 302, 1943. [4] E. Matar, E. Congiu, F. Dulieu, A. Momeni, and J. L. Lemaire; 2008, A&A, 492, L17. [ 5 ] N. Watanabe, Y. Kimura, A. Kouchi, T. Chigai, T. Hama and V. Pirronello; 2010, ApJ, 714, L233.
99 POSTER SESSION 2 P2-7
Theoretical study of the dynamics and kinetics of the O + CS → CO + S chemical laser reaction, where CO shows a very high vibrational excitation
Pablo Gamallo,1 Rafael Francia,2 Rodrigo Martínez,2 Ramón Sayós,1 Miguel González1,*
1Dept. de Química Física i IQTC, Univ. de Barcelona, C/ Martí i Franquès, 1, 08028 Barcelona (Spain) 2Depto. de Química, Univ. de La Rioja, C/ Madre de Dios, 51, 26006 Logroño (Spain)
*E-mail: [email protected]
The dynamics and kinetics of O(3P)+CS(X1) was studied theoretically in detail for the first time, as a function of Ecol (0.0388-2.0 eV) [1]. This was made using the QCT method and employing the best ab initio analytical ground PES (13A') available [2]. Scalar and vector properties were determined, including also the reaction mode. The behaviours observed and the formation of OCS collision complexes were interpreted from some characteristics of the PES [early barrier, shallow minimum in the exit valley, and high exoergicity (channelled into CO vibration; up to ≈80% of the available energy)] and the HHH kinematics. The QCT vibrational and rotational CO populations and kk’, kj’, and k’j’ angular correlations show a quite good agreement with experiments [3-5], but the QCT rate constant disagrees with the measured one [6]. To better account for the kinetics, we performed ab initio CASPT2 calculations on the minimum energy path of the ground and first excited (13A'') PESs. Based on these results, the transition state theory, which can be satisfactorily applied here (recrossing and tunnelling play a small role), leads to rate constant values [1] that are quite close to the measured ones [6]. We expect that these results will encourage further theoretical and experimental developments.
This work was supported by the Spanish Ministry of Science and Innovation (projects CTQ2008- 06805-C02-01, CTQ2011-27857-C02-01, and CTQ2009-07647). Thanks are also given to the “Generalitat de Catalunya” (Autonomous Government of Catalonia, ref. 2009SGR 17) for some support.
References
[1] Gamallo, P.; Francia, R.; Martínez, R.; Sayós, R.; González, M. J. Phys. Chem. A (submitted). [2] González, M.; Hijazo, J.; Novoa, J. J.; Sayós, R. J. Chem. Phys. 1996, 105, 10999. [3] Hancock, G.; Ridley, B. A.; Smith, I. W. M. J. Chem. Soc. Farad. II 1972, 68, 2117. [4] Summerfield, D.; Costen, M. L.; Ritchie, G. A. D.; Hancock, G.; Hancock, T. W. R.; Orr-Ewing, A. J. J. Chem. Phys. 1997, 106, 1391. [5] Costen, M. L.; Hancock, G.; Orr-Ewing, A. J.; Summerfield, D. J. Chem. Phys. 1994, 100, 2754. [6] Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson Jr, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe J. Atmos. Chem. Phys. 2004, 4, 1461.
100 POSTER SESSION 2 P2-8
Formation of highly excited atoms in the fragmentation of strongly driven molecules
Agapi Emmanouilidou
1Department of Physics and Astronomy, University College London, Gower Street, London WC1E-6BT, United Kingdom 2Chemistry Department, University of Massachusetts at Amherst, Amherst, Massachusetts 01003, USA
*E-mail: [email protected]
We present a theoretical quasiclassical study of the formation, during Coulomb explosion, of highly excited neutral H atoms for strongly driven H2 molecules. This process' where after the laser field is turned off an electron occupies a Rydberg state of a hydrogen atom, was reported 1n a recent experimental study [1]. We find that two-electron effects are important in order to correctly account for the formation of neutral atoms [2]. We show that the formation of a single excited neutral atom H* is possible via two pathways (A and B), where either the first or the second ionization step is "frustrated". These two single H* formation pathways have distinct traces in the probability distribution of the escaping electron momentum components. We further show that the formation of two highly excited atoms is also possible [3], via a pathway where both ionization steps are "frustrated". This latter double H* formation pathway is similar with one of the two single H* formation pathways. Moreover, we compute the screened nuclear charge that drives the Coulomb explosion of the nuclei during the double H* formation.
Figure 1 The 2D electron momentum distribution of the escaping electron for the single H* formation: for pathway A (a), and for pathway B (b). The mean total kinetic energy of the nuclei,
References
[1] B. Manschwetus et al., Phys. Rev. Lett. 102, 113002 (2009) [ 2] A. E m m a n ou ilid ou et a l., Phys. Rev. A !"! #$$%#&'() (2012) [3] A. Emmanouilidou et al., Submitted in New J. Phys. (2012), arxiv: 1206.6746
101 POSTER SESSION 2 P2-9
UV photodissociation of N,N-dimethylformamide studied by velocity-map imaging
Sara H. Gardiner, Laura Lipciuc and Claire Vallance
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford, OX1 3TA, United Kingdom
Fragmentation studies on large molecules have traditionally employed collisionally-induced dissociation (CID) or infra-red multiphoton dissociation (IRMPD), but currently there is considerable interest in ultra-violet (UV) photodissociation. This is often a very rapid process in peptides [1], occurring on a much faster timescale than intramolecular vibrational redistribution and leading to ‘non-statistical’ fragmentation patterns. For example, stronger bonds may be cleaved, while weaker ones remain intact, and the fragments are produced with non-statistical internal energy distribution. N,N-dimethylformamide (DMF), with its carbonyl-nitrogen bond linkage, is a model for the peptide bond. This study is the first in a series which aims to investigate the UV photodissociation of the peptide bond; future studies will explore the dissociation of di- and tri-peptides.
Forde et al. have previously undertaken an investigation into the 193 nm photodissociation of neutral DMF [2], determining recoil translation energy distributions of the nascent photofragments from time-of-flight spectra. Following an initial excitation to the second excited singlet state [3], three competing primary dissociation pathways were identified, namely, HCONCH3 + CH3, and two channels 2 2 2 2 in which the ‘peptide’ bond is cleaved, HCO (X A') + N(CH3)2 (X B1), and HCO (X A') + N(CH3)2 (A A1).
We report here our results from the photodissociation of neutral DMF by means of the velocity-map imaging technique [4]. The dissociation was carried out at a number of different wavelengths, including; 193 nm, 225 nm, 230 nm and 258 nm. 118 nm light was used to ionize all resulting photofragments produced on dissociation of the neutral molecule and was also used to produce the DMF cation.
References
[1] R. Antoine, M. Broyer, J. Chamot-Rooke, C. Dedonder, C. Desfrancois, P. Dugourd, G. Grégoire, C. Jouvet, D. Onidas, P. Poulain, T. Tabarin, and G. van der Rest, Rapid Commun. Mass Spectrom. 2006, 20, 1648. [2] N. R. Forde, L. J. Butler, S. A. Abrash, J. Chem. Phys. 1999, 110, 8954-8968. [3] M. Eckert-Maksic, I. Antol, J. Chem. Phys. A. 2009, 113, 12582-12590 [4] A. T. J. Eppink, D. H. Parker, Rev. Sci. Instrum. 1997, 68, 3477-3484
102 POSTER SESSION 2 P2-10
Excitation dependence of the ultrafast THz photoconductivity decay of bulk in heterojunction materials
Carlito S. Ponseca Jr.1, Arkady Yartsev1, Ergang Wang2, Mats R. Andersson2, Dimali Vithanage1, and Villy Sundström1*
1Division of Chemical Physics, Lund University, Box 124, 221 00 Lund, Sweden 2Department of Chemical and Biological Engineering/Polymer Technology, Chalmers University of Technology, 412 96 Göteborg, Sweden
*E-mail: [email protected]
The ultrafast THz photoconductivity dynamics of polymer:PCBM blends has been extensively studied in past several years and was assigned to either exciton-exciton annihilation, interfacial charge transfer and/or competition between them depending on the concentration of PCBM1 (other groups reported that changing the PCBM concentration should not affect the ultrafast decay2,3), cooling of highly mobile charges4, and lifetime of the coupled polaron pair5. In Figure 1, the ultrafast THz photconductivity decay of the TQ1:PCBM blend is shown to have a strong fluence dependence with fluency and at the lowest excitation density it is almost entirely eliminated. Similar dependence of the ultrafast decay with pump intensity was observed in APFO3-PCBM (not shown here). At high excitation conditions (~1014- 1015 ph/cm2 per pulse), the distance between photogenerated charge pairs is so small (~4 nm) that very little charge diffusion is required for recombination, i.e. non-equilibrated charge pairs recombine or “annihilate” immediately after their formation.6 On the other hand, at low enough excitation density (~1012-1013 ph/cm2 per pulse), i.e. fewer photoexcited carriers, annihilation is almost eliminated. Our transient absorption measurements revealed that charge concentration at this intensity and time scale is constant leading us to conclude that very high carrier mobility prevails for long time, of importance for free charge formation in organic solar cell.7
References
[1] X. Ai, et. al., J. Phys. Chem. B, 110, 25462, 2006. [2] O. Esenturk, et. al., J. Appl. Phys., 103, 023102, 2008. [3] P. D. Cunningham, et. al., J. Phys. Chem. C, 112, 7928, 2008. [4] H. Němec, et. al., J. Phys. Chem C, 112, 6558, 2008. [5] H. Němec, et. al., Phys. Rev. B, 79, 245326, 2009. [6] S. Pal, et al., J. Am. Chem. Soc., 132, 2010. [7] C. Jr. S. Ponseca, et al., J. Am. Chem. Soc. (Comm.), in press, 2012.
103 POSTER SESSION 2 P2-11
Hyperspherical harmonics representation of binary interactions H2O–H2,N2, and O2. A minimal model for the H2O – rare-gas-atom systems.
Patricia R. P. Barreto1*, Alessandra F. Albernaz2, Federico Palazzetti3, Andrea Lombardi3, Gaia Grossi3, Vincenzo Aquilanti3
1Instituto Nacional de Pesquisas Espaciais (INPE/MCT), Laboratório Associado de Plasma (LAP), São José dos Campos, SP, CEP 12247-970, CP515, Brazil. 2Instituto de Fìsica, Universidade de Brasìlia, CP04455, Brasília, DF, CEP 70919-970, Brazil. 3Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
*E-mail: [email protected]
In this work we report the explicit representation of the potential energy surfaces based on orthogonal vectors of binary systems involving the rigid molecules H2O and H2,N2, and O2. [1,2] The interaction potential is given as an expansion in hyperspherical harmonics and the functional form depends on a radial coordinate R between centers-of-mass of the two molecules and on four angles: the polar angles , 1, 2, and on the dihedral angle . A minimal model has also been illustrated to represent the intermolecular potentials for H2O – rare-gas-atom systems, the reduced dimensionality make the functional form depending on only three coordinates: the distance, R, between the center- of-mass of the molecule and the atom, and the polar angles and . The molecular geometries have been optimized at the CCSD(T)/aug-cc-pVXZ level of calculation, with X = D, T, Q, and 5. The potential energy surfaces were calculated by ab initio calculations at CCSD(T)/aug-cc-pVTZ level, for about one hundred points of the surface for each of the fifteen leading configurations of the H2O–X2 system and the four leading configurations of the H2O–rare-gas atom systems. The potential energy surfaces are in format to be employed in molecular dynamics simulations [3] and in the analysis of experimental results [4]. The hyperspherical harmonics expansion allows compactness, full account of symmetries, and to obtain realistic results with a low computational cost. [5]
References
[1] P. R. P. Barreto, A. F. Albernaz, A. Capobianco, F. Palazzetti,. A. Lombardi, G. Grossi, V. Aquilanti Comp. Theor. Chem. 990 (2012) 53. [2] P. R. P. Barreto, V. W. Ribas, F. Palazzetti J. Phys. Chem. A 113 (2009) 15047. [3] A. Lombardi, F. Palazzetti, G. S. Maciel, V. Aquilanti, M. B. Sevryuk Int. J. Quant. Chem. 111 (2011) 1651. [4] A. Bartocci, B. Brunetti, P. Candori, S. Falcinelli, F. Palazzetti, F. Pirani, F. Vecchiocattivi Chem. Phys. Lett. (submitted). [5] F. Palazzetti, M. Elango, A. Lombardi, G. Grossi, V. Aquilanti Int. J. Quant. Chem. 111 (2011) 318.
104 POSTER SESSION 2 P2-12
Laser-induced non-adiabatic processes in molecular systems
G. J. Halász1, N. Moiseyev2, L. S. Cederbaum3 and A. Vibók4
1Department of Information Technology, University of Debrecen, H-4010 Debrecen, PO Box 12, Hungary 2Schulich Faculty of Chemistry and Minerva Center of Nonlinear Physics in Complex Systems, Technion - Israel Institute of Technology, Haifa 32000, Israel 3Theoretische Chemie, Physikalish-Chemisches Institut,Universität Heidelberg, H-69120, Germany 4Department of Theoretical Physics, University of Debrecen, H-40410 Debrecen, PO Box 5, Hungary
e-mail: [email protected]
A few years ago we have started a systematic study of the nonadiabatic effect induced by laser waves in diatomic molecules. It has been shown that light-induced conical intersections (LICIs) can be formed in a molecular system either by standing or by running laser waves [1,2]. The energetic and internuclear positions of these CIs are dependent on the laser frequencies while the strength of their non-adiabatic couplings can be modified by the field intensities. The impact of these LICIs on different physical properties of the diatomics has been discussed in several papers [1-8]. Among others it was shown that the appearance of the LICIs can significantly reduce the magnitude of the trapping effect of cold diatomic molecules in the lowest electronic state. For the case of running laser waves, it was demonstrated that LICIs have a very strong impact on the molecular spectrum and spectral properties. The topological or Berry phase has also been calculated for LICI situations. The results obtained clearly show that the phase behaves the same in diatomics with a LICI as it does for “atural” CIs, i.e. for CIs of polyatomic molecules in free space.
Wave packet calculations have been performed which demonstrate that LICIs exerts strong effects on the quantum dynamics even for weak laser fields [5,7]. The impact of LICIs on another process of interest has also been studied, namely on the spatial alignment of diatomics in fields of moderate intensity (I ≈ 108 to 1010 W/cm2) [6]. An important message of that work [6] is that LICIs can substantially influence molecular alignment. Calculating, for example, the population on the first excited electronic state of aligned molecules, there is a large difference between the results obtained in the presence of the LICI and those obtained by employing the “standard rigid rotor” one dimensional model. Since the electronic population is a relevant measurable quantity, we hope that our work concerning the molecular alignment [6] will stimulate experimental investigations in the near future.
References
[1] N. Moiseyev, M. Sindelka, and L. S. Cederbaum, J. Phys. B: At. Mol. Opt. Phys. 41, 221001 (2008). [2] N. Sindelka, N. Moiseyev, and L. S. Cederbaum, J. Phys. B: At. Mol. Opt. Phys. 44, 045603 (2011). [3] N. Moiseyev, and M. Sindelka, J. Phys B: At. Mol. Opt. Phys. 44, 111002 (2011). [4] L. S. Cederbaum, Y. C. Chiang, P. V. Demekhin, and N. Moiseyev, Phys. Rev. Lett. 106, 123001 (2011). [5] G. J. Halász, A. Vibók, M. Sindelka, N. Moiseyev, and L. S. Cederbaum, J. Phys B: At. Mol. Opt. Phys. 44, 175102 (2011). [6] G. J. Halász, A. Vibók, M. Sindelka, N. Moiseyev, and L. S. Cederbaum, Chem. Phys. doi:10.1016/j.chemphys.2011.06.038 (2012). [7] G. J. Halász, M. Sindelka, N. Moiseyev, L. S. Cederbaum and A. Vibók, J. Phys. Chem. A. 116, 2636 (2012). [8] G. J. Halász, A. Vibók, N. Moiseyev and L. S. Cederbaum: Light-induced conical intersections for short and long laser pulses: Floquet and rotating wave approximations versus numerical exact results. in press J. Phys. B.
105 POSTER SESSION 2 P2-13
Accelerated DVR methods for use in chemical physics
Anthony J. H. M. Meijer* and Rebecca Hylton
Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, United Kingdom
* Electronic address: [email protected]
A time-dependent quantum dynamics program spends most of the computational time on the following operation to propagate the wave packet in time (assuming orthogonal coordinates):
( ) matrix,൱Ȳǡ and V the potential energy ܄ ܂Ȳ kineticൌ ൭ energy ܄the total܂where H is the Hamiltonian matrix,۶Ȳ ൌ T matrix. Because there are no cross-terms in the kinetic energy operator we can write T as a sum over kinetic energy matrices for each of the degrees-of-freedom separately, .
To evaluate the fundamental operation, one often uses a suitable܂ discrete variable repre- sentation of the kinetic energy operator. In this basis, the second derivative of the wave function (x ) at point is: ܂ Ψ
୧ (Ψ ݔ + , (1 ே ୀଵ where Δj are the DVR weights,ȲԢԢሺݔ ሻ whichൌ Ȳሺݔ take)Δ the∑ usualሺȲ formsሺݔ for,݆݄e.g.ሻ,Ȳ aሺ sinc-DVRݔ െ ݆݄1ሻሻorȟ݆ a Gauss-Legendre DVR.2 N is the size of the basis and h is the grid spacing.
In order to be able to tackle larger systems than currently possible, we have been investigating ways to turn a full N×N sinc-DVR matrix, which is used in Eq. (1) in case of scattering or vibrational coordinates, into a banded matrix with a band width 2n+1 << N. Within the confines of a DVR-based method most promising in this regard are methods, which effectively restrict the calculation to a subset of the full momentum space. A number of these methods have been reported on in the literature.3–6
The main drawback of these methods is the need to determine beforehand how much of the kinetic energy matrix can be removed, while still keeping the total error under control. This contribution will focus on a recent extensive study of two of these accelerated DVR methods, which has enabled us to resolve this drawback and increase the applicability of these methods for scattering calculations in particular.
References
1 D. T. Colbert and W. H. Miller, J. Chem. Phys., 1992, 96, 1982. 2 S. E. Choi and J. C. Light, J. Chem. Phys., 1989, 90, 2593. 3 J. P. Boyd, Comput. Methods. Appl. Mech. Eng., 1994, 103, 1. 4 D. A. Maziotti, J. Chem. Phys., 2002, 117, 2455. 5 S. K. Gray and E. M. Goldfield, J. Chem. Phys., 2001, 115, 8331. 6 D. Morgan, A. J. H. M. Meijer, and R. J. Doyle, J. Chem. Phys., 2009, 130, 084114.
106 POSTER SESSION 2 P2-14
Potential energy surface for H2 HX system, with X=H, F, Cl, Br
Patrícia RP Barreto1*, Alessandra F Albernaz2, Amedeo Capobianco3
1Instituto Nacional de Pesquisas Espaciais (INPE/MCT), Laboratório Associado de Plasma (LAP), São José dos Campos, SP, CEP 12247-970, CP515, Brazil. 2Instituto de Fìsica, Universidade de Brasìlia, CP04455, Brasília, DF, CEP 70919-970, Brazil, 3Dipartimento di Chimica e Biologia. Università di Salerno, Via Ponte don Melillo, I-84084 Fisciano, Italy
*E-mail: [email protected]
In this work we report the explicit representation of the potential energy surfaces for the H2 HX systems, with X = H, F, Cl or Br, via a harmonic expansion functional form for rigid diatom-diatom interactions [1,2]. The framework of the supermolecular approach was used, as well as the counterpoise-corrected interaction energies at CCSD(T)/aug-cc-pVQZ levels. The energies were calculated in nine leading configurations according to the orientation of the dimers, depending on a radial coordinate R between centers-of-mass of the two molecules, and the polar and diedral angles
(1,2,). The analytical form of the potential energy surfaces, for each of the leading configurations, is constructed by fitting the energies to a fifth degree generalized Rydberg function. Comparing the fitted energies with the CCSD(T)/aug-cc-pVQZ one for the H2 HX systems, with X=H, F, Cl or Br we -1 found a maximum rms error of 21.1 cm for Lb configuration of H2 HBr and a minimum rms error -1 of 1.13 cm for H configuration of H2 H2. This study predicts the T-shaped structures to be the most stable configurations for the above dimers, but with different orientation. For H2 HF and -1 H2 HBr it is the Ta configuration (/2,0,0), with energy of 361.3 and 170.1 cm , respectively, while -1 the Tc configuration (0,/2,0) is the most stable for the H2 HCl, with energy of 202.4 cm . For the -1 H2 H2 the T-shape has a minimum with energy of 33.3 cm . For all dimer the Lb configuration is the less stable (0,,0), with has a repulsive characteristic. The isotropic term of the potential are 20.3, -1 36.7, 53.1 and 74.7 cm for H2 HX systems, with X=H, F, Cl or Br, respectively.
The authors are grateful to the Fundação de Amparo de Pesquisa do Estado de São Paulo (FAPESP) for the support.
References
1. V. Aquilanti, D. Ascenzi, M. Bartolomei, D. Cappelletti, S. Cavalli, M. C. V\`itores, and F. Pirani, J. Am. Chem. Soc., 121, 10794, (1999). 2. V. Aquilanti, M. Bartolomei, D. Cappelletti, E. Carmona-Novillo and F. Pirani, Phys. Chem. Chem. Phys., 3, 3891 (2001)
107 POSTER SESSION 2 P2-15
Detailed resonance analysis in the + + Ne + H2 (v0=2,j0=1) NeH (v'=0,j') + H proton transfer reaction. A challenge for experiment
Pablo Gamallo,1 Fermín Huarte-Larrañaga,1 Miguel González1,*
1Dept. de Química Física i IQTC, Univ. de Barcelona, C/ Martí i Franquès, 1, 08028 Barcelona (Spain)
*E-mail: [email protected]
The dynamics of the title proton transfer reaction was studied in previous works in a wide collision energy (Ecol) interval by means of several methodologies, quantum and classical, and using different potential energy surfaces (see, e.g., Refs. [1,2]). In all previous quantum studies a notorious resonance pattern characterized the system’s reactivity, even at the total cross section level. The case of the v0=2 reactant vibrational state was highlighted as particularly noteworthy. Thus, in a recent work [2], using the ab initio based analytical PES of Ref. [3], we obtained total cross sections that reproduced the available experimental data [4,5]. In the present exact (coupled-channel) quantum dynamics contribution, we complemented our recent exact time-dependent real wave-packet (TD) dynamic study [2] with extensive time-independent (TI) exact calculations, employing the ABC code [6] and using a very dense grid in the low collision energy range (Ecol < 0.3 eV). The data coming out from the TI calculations were employed to produce initial state specific total cross sections that were validated with the TD ones. Next, the corresponding TI state-to-state total cross sections were + generated and the resonance peaks appearing in the H2 (v0=2, j0=1) reactivity were assigned to specific reactive transitions. Finally, we looked for the signature of these resonances in the TI opacity function. The results of the present investigation will probably encourage the experimentalists for further work on this interesting system.
This work was supported by the Spanish Ministry of Science and Innovation (projects CTQ2008- 06805-C02-01, CTQ2011-27857-C02-01 and CTQ2009-12215). Thanks are also given to the “Generalitat de Catalunya” (ref. 2009SGR 17) for some support.
References
[1] Mayneris, J.; Sierra, J. D.; González, M. J. Chem. Phys. 2008, 128, 194307 and refs. therein. [2] Gamallo, P.; Defazio, P.; González, M. J. Phys. Chem. A 2011, 115, 11525. [3] Lv, S.-J.; Zhang, P.-Y.; Han, K. L.; He, G.-Z. J. Chem. Phys. 2010, 132, 014303. [4] Zhang, T.; Qian, X.-M.; Tang, X. N.; Ng, C. Y.; Chiu, Y.; Levandier, D. J.; Miller, J. S.; Dressler, R. A. J. Chem. Phys. 2003, 119, 10175. [5] Herman, Z.; Koyano, I. J. Chem. Soc., Faraday Trans. 2 1987, 83, 127. [6] Skouteris, D.; Castillo, J.F.; Manolopoulos, D.E., Comp. Phys. Comm. 2000, 133, 128.
108 POSTER SESSION 2 P2-16
Collisional dynamics of OH(A) + Kr and OH(A) + H2: theory and experiment
M. Brouard1*, H. Chadwick1, J. Lawlor1, T. Perkins1, S. A. Seamons1, P. Stevenson1, F. J. Aoiz2, J. F. Castillo2, D. Herraez2, J. Kłos3
1Department of Chemistry, University of Oxford, The Physical and Theoretical Chemistry Laboratory, South Parks Rd, Oxford, OX1 3QZ, UK 2Departamento de Química Física, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain 3Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021, USA
2 + The collisional dynamics of the OH(A ) + H2 and OH(A)+Kr systems have been investigated experimentally and through the use of quasi-classical trajectory (QCT) theory. Both of these systems display significant non-adiabatic effects, and a focus of this work is the effect of electronic quenching on the excited state dynamics. OH(A) + H2 in particular has been widely studied and is a ‘benchmark’ four-atom system in the literature, while OH(A) + Kr is more theoretically tractable, builds on earlier OH(A)-rare gas studies in our groups, and is the subject of current investigations in several laboratories.
Cross-sections for electronic quenching, rotational energy transfer and collisional depolarisation (elastic and inelastic in N) have been measured using Zeeman quantum beat spectroscopy. The measurement of vector correlations – in this case, the j-j’ correlation – gives another level of insight into the collision process over and above scalar energy transfer rates, and is a stringent test of theory.
QCT theory has been performed on the excited state PESs of Bowman et al. (H2) and Kłos et al. (Kr), and has been found to agree well with experiment in the range of N where quenching is negligible. However, where quenching is significant, experiment and theory display some disagreement.
It is suggested that the reason for this is that the signature of non-adiabatic effects is visible in the results presented here, and, for the OH(A) + Kr system, surface-hopping QCT calculations are performed on the new coupled PESs of Kłos et al. in order to investigate this further. The inclusion of non-adiabatic effects improves the agreement between experiment and theory, and accounts more fully for the trends observed in the data.
109 POSTER SESSION 2 P2-17
Spatial and velocity map imaging with a new surface imaging instrument
Cécilia Cauchy 1*, Gabrielle de Wit, Jason Lee 1, Scott Hopkins 2, and Claire Vallance1
1Chemisry Research Laboratory, University of Oxford 2University of Waterloo, Ontario, Canada
*E-mail: [email protected]
The first data from a new surface imaging instrument is presented. The instrument consist of a position-sensitive time-of-flight detector (TOF) detector1, which records the x,y and t (time) coordinates of incident ions, generated from desorption/ionization off a surface. Using modified Wiley-McLaren ion optics, in which the sample plate also functions as the repeller electrode, Spatial Imaging or Velocity Map Imaging can be performed. Furthermore, swapping between imaging modes is readily achieved by changing the voltage ratio on the repeller and extractor lenses. In this way, the instrument performs as a TOF mass spectrometer, with correlated spatial, (or velocity) information.
Results from the preliminary design2 and calibration of an optimized three-electrode system for Spatial Imaging will be presented. Several sample deposition techniques have also been implemented, including vacuum sublimation3, high resolution printing4, electrospray5, and the use of an electroformed nickel mesh. The advantages and disadvantages of each technique will be discussed and compared.
Initial Velocity Map Imaging experiments have also been undertaken, with a view to investigating the MALDI ablation processes, as well as studying surface dynamics in biological and chemical systems. In contrast to gas phase studies, surface imaging allows examination of dynamic processes in an environment closely resembling the natural state in which the chemistry occurs.
References
[1] M. Brouard, A.J. Johnsen, A. Nomerotski, C.S. Slater, C. Vallance, and W.H. Yuen, Journal of Instrumentation 6, C01044 (2011). [2] SIMION http://simion.com/ [3] J.A. Hankin, R.M. Barkley and R.C. Murphy, J. Am. Soc. Mass Spectrom., 18, 1646–1652. (2007) [4] D.L. Baluya, T.J. Garrett, and R.A. Yost, Anal. Chem. 79, 6862–6867 (2007). [5] Built in house by the Brouard Group and the Physical & Theoretical Chemistry workshops at the University of Oxford.
110 POSTER SESSION 2 P2-18
Prospects for producing ultracold methyl halide molecules CH3X (X = F, Cl, Br,I) via sympathetic cooling with alkali metals A (A = Li, Na, K, Rb): A survey of interaction potentials and reaction pathways using Ab Initio methods.
Jesse J. Lutz' and Jeremy Hutson', *
1Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom
The possibility of producing ultracold methyl halide molecules CH3X (X = F, Cl, Br, I) by sympathetic cooling in a bath of ultracold alkali metal atoms A (A = Li, Na, K, Rb) is investigated by performing ab initio computations to predict which, if any, of these atom-molecule combinations has a barrier to reaction. To begin, single-reference coupled-cluster methods are employed to obtain non-reactive intermolecular PESs, facilitating the identification and characterization of the key stationary points of the approach. The mechanism and energetics of the only exothermic reactive channel available, CH3X + A CH3 + AX, is explored by constructing reactive PESs as a function of the C-X stretch and the intermolecular separation using the complete-active-space self-consistent field method. We demonstrate that this class of dissociative charge transfer reactions must be initiated by a pre- stretching of the CH3X molecule and that the amount of stretching required is dependent on the intermolecular distance. Coupled-cluster and multi-reference perturbation theory calculations are then performed to obtain highly-accurate minimum- minimum-energy reaction profiles connecting the entrance and exit valleys. The resulting activation energies are further refined by extrapolation to the complete basis set limit. For every combination of methyl halide and alkali metal atom positive activation energies are predicted, indicating that reactive events at extremely low temperatures are unlikely and motivating future non-reactive quantum scattering computations on these systems. Finally, factors influencing the frequency of unfavorable inelastic sympathetic cooling collisions are discussed and used to propose a few particularly promising sympathetic cooling partners to focus on in future work.
* Corresponding author; e-mail: [email protected]
111 POSTER SESSION 2 P2-19
Ready - a reactive dynamic simulation for hydrogen combustion
César Mogo and João Brandão
CIQA, Dept. Química e Farmácia FCT Universidade do Algarve 8005-139 Faro, Portugal
In this work we present the program READY, aiming to reproduce the mechanism of hydrogen combustion using accurate potential energy surfaces.
Starting from a bulk mixture of H2 and O2 molecules, the program integrates the equations of motion in a potential build from the several reactive potential energy surfaces such as H4[1], H3O[2], HO3[3] and H2O[4,5] among others that should play a role in the combustion process. Non reactive collisions are treated using repulsive potentials in order to account for the energy transfer between colliding partners.
Using different initial conditions, composition, temperature and volume we can study the time evolution of the reactive mixture, i.e. the rate of intermediate formation and consumption as well as product formation.
Acknowledgements: This work was supported by the FCT under the PTDC/QUI-QUI/100089/2008 research project, co-financed by the Euro-pean community fund, FEDER.
References
[1] A. I. Boothroyd, P. G. Martin, W. J. Keogh, and M. J. Peterson; J. Chem. Phys. 116, 666 (2002) [2] Wu, G.-S., Schatz, G. C., Lendvay, G., Fang, D.-C. & Harding, L. B.; J. Chem. Phys. 113, 3150–3161 (2000) [3] A. J. C. VARANDAS & H. G. YU; Molecular Physics: An International Journal at the Interface Between Chemistry and Physics, 91:2, 301-318 (1997) [4] João Brandão, César Mogo, and Bruno C. Silva; J. Chem. Phys. 121, 8861 (2004) [5]João Brandão and Carolina M. A. Rio; J. Chem. Phys. 119, 3148 (2003)
112 POSTER SESSION 2 P2-20
Coherent spectroscopy with quantum cascade lasers
James Kirkbride1, Sarah E. Causier1, Damien Weidmann1,2 & Grant A.D Ritchie*1
1Department of Chemistry, Physical & Theoretical Chemistry Laboratory, The University of Oxford, South Parks Road, Oxford, UK 2 Space Science & Technology Department, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0QX, UK
*E-mail: [email protected]
High power single mode quantum cascade lasers (QCLs) open up the possibility for rovibrational state preparation of gas phase molecules in low frequency vibrational modes [1]. For example, recently we have used two cw QCLs operating at 5 µm to perform a counter-propagating pump and probe experiment on a low pressure sample of nitric oxide [2] in which the strong pump field excites a fundamental rovibrational transition and the weaker probe field is tuned to an appropriate rotationally resolved hot band transition. When both light fields are in resonance, rapid passage is observed in the hot band absorption lineshape arising from a minimally damped and velocity- selected sample of molecules in the v = 1 state.
In these first experiments a commercial external cavity QCL (140 mW power, linewidth = 2 MHz Daylight Solutions) was used as the pump laser while the probe laser was a distributed feedback QCL (7 mW, linewidth = 16 MHz, Alpes Lasers). Most recently, the weak probe has been replaced with a higher powered (up to 190 mW) narrow bandwidth (800 kHz) device and new examples of pump and probe data with these cw QCLs will be presented. In particular the temporal decay of the rapid passage signals, which is a function of the total sample pressure, is found to be subtly different for each doublet of the pumped rotational state indicating that the underlying hyperfine structure plays a role in determining the dephasing of the signals.
References
[1] E.A. McCormack, H. Lowth, M.T. Bell, D. Weidmann, G.A.D. Ritchie – J Chem. Phys. 137 034306 DOI: 10.1063/1.4734020 (2012). [2] R.J. Walker, J.H. van Helden, J. Kirkbride, E.A. McCormack, M.T. Bell, D. Weidmann, G.A.D. Ritchie – Optics Letters 36 4725 (2011).
113 POSTER SESSION 2 P2-21
Structured pseudopotential correction to the jellium model for fullerenes
A.V. Verkhovtsev1,2, R.G. Polozkov1, V.K. Ivanov1*, A.V. Korol2, A.V. Solov’yov2
1St.Petersburg State Polytechnic University, Politekhnicheskaya ul. 29, 195251 St.Petersburg, Russia 2Frankfurt Institute for Advanced Studies, Ruth-Moufang-Str. 1, 60438 Frankfurt am Main, Germany
*E-mail: [email protected]
A new type of a correction for a more accurate description of fullerenes within the spherical jellium model is considered. The correction represents a pseudopotential which originates from the comparison between an accurate ab initio and the jellium model calculations.
Contemporary software for the quantum-chemical calculations (e.g., [1]) provides an accurate quantitative description of the ground state of many-particle systems and allows one to obtain information on geometrical and chemical properties of the system. Meanwhile, the description of dynamic properties, which play an important role in the process of photoionization, by means of such programs faces significant difficulties. Thus, collective electron excitations in fullerenes, which lie in the continuous spectrum, have not been described so far by means of quantum-chemical programs. However, this can be achieved within simplified model approximations, e.g. the jellium model. The ground state properties of fullerenes cannot be described properly by the standard jellium model which produces, in particular, unreliable values for the total energy [2].
In this contribution we introduce a structured pseudopotential [3] which originates from the comparison of an accurate ab initio calculation with the jellium-based one. It is shown that such correction to the standard jellium model allows one to account, at least partly, for the sp2- hybridization of carbon atomic orbitals. Therefore, it may be considered as a more physically meaningful correction as compared to the structureless square well pseudopotential which has been widely used earlier (see, e.g., [3]).
The results of calculations of the ground state of the C60 and C20 fullerenes within this “modified” jellium model, and the Local density and Hartree-Fock approximations are discussed.
References
[1] Frisch M J et al 2009 Gaussian 09 Revision A.1, Gaussian Inc. Wallingford CT [2] Yannouleas C and Landman U 1994 Chem. Phys. Lett. 217 175 [3] Verkhovtsev A V, Polozkov R G, Ivanov V K, Korol A V and Solov’yov A V, submitted to J. Phys. B: At. Mol. Opt. Phys (2012) (see also arXiv:1206.5105v1 [physics.atm-clus])
114 POSTER SESSION 2 P2-22
Collisional activation of small size-selected transition metal clusters
Imogen Parry,a Aras Kartouzian,a Suzanne M. Hamilton,a Stuart R. Mackenziea and Martin K. Beyerb and O. Petru Balajb
a Department of Chemistry, PTCL, University of Oxford, UK bDepartment of Chemistry, Christian Albrechts University, Kiel, Germany
[email protected] [email protected]
We report the results of experiments performed using ion cyclotron resonance mass spectrometry + to study collisional activation of small size-selected RhnN2O clusters.
+ We have recently shown that infrared excitation of RhnN2O clusters via the N2O chromophore is effective in inducing decomposition of the nitrous oxide adsorbate.1,2 in order to test the thermal nature of this process we have performed a series of collisional experiments designed to study the same chemistry. Specifically, we have used Ar- collision induced dissociation (CID) and CO chemisorption to raise the internal energy of the parent complex to initiate surface processes. In each case the progress of reaction is followed mass-spectrometrically and the kinetics fit to plausible reaction mechanisms. These experiments show qualitative agreement with the infrared activated processes but also highlight significant differences.
In addition, we have observed evidence of blackbody infrared dissociation (BIRD) in this system following storage under UHV conditions for extended periods of time.
References
1 Hamilton, et al., J. Am. Chem. Soc., 132, 1448 (2010) 2 Hermes et al., J. Phys. Chem. Lett., 2, 3053 (2011)
115 POSTER SESSION 2 P2-23
Multi-mass microscope mode imaging mass spectrometry: an application of the Pixel Imaging Mass Spectrometry (PImMS) sensor
Mark Brouard1*, Edward Halford1, Craig Slater1, Alexandra Lauer1, Benjamin Winter1, WeiHao Yuen1, Jaya John John2, Andrew Clark3, Jamie Crooks3, Renato Turchetta3, Iain Sedgwick3, Andrei Nomerotski2, Jason Lee1
1The Department of Chemistry, University of Oxford, The Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, UK 2The Department of Physics, University of Oxford, UK 3The Rutherford Appleton Laboratory, STFC, UK
*Email: [email protected]
Spatial imaging mass spectrometry is a powerful technique that correlates the chemical information of a sample to its spatial coordinate [1]. It can be performed in either microprobe or microscope mode [2]. In microprobe mode, only a small portion of the sample is irradiated with each laser pulse, and the time-of-flight (TOF) mass spectrum is obtained for that area; it is necessary to raster over the entire sample surface to build a complete image. In microscope mode, usually only one ion mass to charge ratio is imaged at once, so a series of measurements is required to obtain full mass information over the whole sample.
The novel Pixel Imaging Mass Spectrometry (PImMS) sensor overcomes these shortcomings by recording the complete mass spectrum over the entire sample in each TOF cycle [3]. This technology offers much higher throughput for samples in many areas including biological tissue analysis [4], and the analysis of micro-arrays or lab-on-chip experiments.
Data has been taken using a velocity-map ion imaging apparatus modified to achieve microscope mode spatial imaging of a large sample area (approximately 5x5 mm2) [5]. A number of dyes in different spatial configuration, as well as some preliminary biological samples, have been imaged using both a conventional CCD camera and PImMS in order to determine the specifications of the spectrometer and the applicability of the new sensor.
References
[1] R.M. Caprioli, T.B. Farmer, and J. Gile, Anal. Chem 69, 4751 (1997). [2] S.L. Luxembourg, T.H. Mize, L.A. McDonnell, and R.M.A. Heeren, Anal. Chem. 76, 5339 (2004). [3] A. Nomerotski, M. Brouard, E. Campbell, A. Clark, J. Crooks, J. Fopma, J.J. John, A.J. Johnsen, C. Slater, R. Turchetta, C. Vallance, E. Wilman, and W.H. Yuen, Journal of Instrumentation 5, C07007 (2010). [4] K. Chughtai and R.M.A. Heeren, Chem. Rev. (Washington, DC, U.S.) 110, 3237 (2010). [5] M. Brouard, A.J. Johnsen, A. Nomerotski, C.S. Slater, C. Vallance, and W.H. Yuen, Journal of Instrumentation 6, C01044 (2011).
116 POSTER SESSION 2 P2-24
Electrostatic extraction of buffer gas cooled beams for studying ion-molecule chemistry at low temperatures
Kathryn Twyman, Laura Pollum, Brianna Haezlewood, Martin Bell, Tim Softley
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, United Kingdom
A source of cold and slow molecular beams has been constructed and characterized in order to study cold ion-molecule reactions. The apparatus combines buffer gas cooling and a bent quadrupole velocity selector to cool both the translational and rotational energy distributions of the molecules.
The velocity distribution of the ND3 and CH3F beams have been measured and indicate a translational temperature of approximately 9 K under optimized conditions. REMPI characterization of the ND3 beam shows that only a few low lying rotational states are populated. A linear Paul ion trap has been designed and constructed for use with the cold molecule source to study cold chemical reactions.
117 POSTER SESSION 1 P2-25
The role of orbiting resonances in the vibrational relaxation of I2(B,v'=21) by collisions with He at very low energies: A theoretical and experimental study
A. García-Vela1*, I. Cabanillas-Vidosa2, J.C. Ferrero2, and G.A. Pino2
1Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 123, 28006 Madrid, Spain 2Centro Láser de Ciencias Moleculares, INFIQC, Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Pabellón Argentina, Ciudad Universitaria, 5000 Córdoba, Argentina
The low-energy collisions of I2(B,v'=21) with He involving collision-induced vibrational relaxation of I2 are investigated both experimentally and by means of wave packet simulations [1]. The theoretical cross sections exhibit a structure of peaks originated by orbiting resonances of the I2(B,v'=21)-He van der Waals complex formed in the I2 + He collisions. Such a structure has similar characteristics as the structure of peaks found in the experimental cross sections [2]. In fact, four out of the five peaks of the measured cross sections appear at positions nearly coincident with those of four of the peaks found in the theoretical cross sections. Thus this result confirms the experimental finding [2,3] that enhancement of I2 vibrational relaxation is caused by population of I2(B,v'=21)-He orbiting resonances populated upon the low-energy collisions. The possibility of using this mechanism in the vibrational cooling of diatomic molecules is discussed.
References
[1] A. García-Vela, I. Cabanillas-Vidosa, J.C. Ferrero, and G.A. Pino, Phys. Chem. Chem. Phys. 14, 5570 (2012). [2] I. Cabanillas-Vidosa, G.A. Pino, C.A. Rinaldi, and J.C. Ferrero, Chem. Phys. Lett. 429, 27 (2006). [3] I. Cabanillas-Vidosa, C.A. Rinaldi, G.A. Pino, and J.C. Ferrero, J. Chem. Phys. 129, 144303 (2008).
118 POSTER SESSION 2 P2-26
The electric deflector – a simple state selector for complex molecular systems
Y.-P. Chang1*, S. Trippel1, T. Mullins1, S. Stern1, L. Holmegaard1, K. Długołęcki1, J. Küpper1,2
1Center for Free-Electron Laser Science, DESY, 22607 Hamburg, Germany, 2Department of Physics, University of Hamburg, 22761 Hamburg, Germany
*E-mail: [email protected]
Size, isomer and state selection of neutral molecules by means of electric deflection is an important, recently rejuvenated topic in molecular beam studies. The force exerted on neutral polar molecules by an inhomogeneous electric field can be exploited to spatially separate quantum states and isomers due to their distinct effective dipole moments [1-3]. This technique has been used to create extremely polar samples for alignment and orientation control [2,4], as well as the separation of structural isomers of large molecules [3] for a forthcoming dynamical application [5]. In this contribution, we extend the state-of-the-art experiment to the size selection of more complex systems, and indole(H2O)n clusters with different sizes and structures are used as prototypes.
The electric deflection of complex and non-rigid systems, such as indole(H2O)n clusters, has been regarded as difficult due to chaotic rotational motions in the electric field, enhanced by the asymmetry and rotational-vibration coupling of the system [6]. In particular, torsional motions for an internal group, such as the water moiety in indole(H2O)1, can create more complexities. This internal rotation can couple to the overall rotation of the system, and also create tunneling splittings of energy levels. These effects, which may perturb rotations and projections of dipole moments in the field, cause potential difficulties of deflecting “floppy” molecules.
Here we present our experimental results of separating a single species from a modestly cold (~5 K) molecular beam of water, indole, indole(H2O)1, indole(H2O)n (n≥2) and similar species. Specifically, we have created a pure sample of the indole-water 1:1 complex with a spatial separation (~1 mm) from other species, in spite of potential difficulties mentioned above. The obtained state selection is analyzed using trajectory calculations as well as measured rotational envelopes of electronic transitions. These purified and state-selected indole(H2O)1 samples will facilitate further applications, such as the control of the molecular axes [2,3] and the study of the half-collision dynamics in the molecular frame.
References
[1] F. Filsinger, U. Erlekam, G. von Helden, J. Küpper, and G. Meijer, Phys. Rev. Lett. 100, 133003 (2008); F. Filsinger, J. Küpper, G. Meijer, J. L. Hansen, J. Maurer, J. H. Nielsen, L. Holmegaard, and H. Stapelfeldt, Angew. Chem. Int. Ed. 48, 6900 (2009). [2] Holmegaard, PRL 2009; F. Filsinger, J. Küpper, G. Meijer, L. Holmegaard, J. H. Nielsen, I. Nevo, J. L. Hansen, and H. Stapelfeldt, J. Chem. Phys. 131, 064309 (2009). [3] J. Küpper, F. Filsinger and G. Meijer, Faraday Discuss. 142, 155 (2009). [4] I. Nevo, L. Holmegaard, J. H. Nielsen, J. L. Hansen, H. Stapelfeldt, F. Filsinger, G. Meijer, and J. Küpper, Phys. Chem. Chem. Phys. 11, 9912 (2009). [5] See another poster – “Conformer-specific reactions with Coulomb-crystallised ions” by D. Rösch et al. [6] R. Antoine, M. Abd El Rahim, M. Broyer, D. Rayane, and Ph. Dugourd, J. Phys. Chem. A 110, 10006 (2005) and references within.
119 POSTER SESSION 2 P2-27
Quantum electrostatic analysis of dielectric materials
Gerardo Raggi
Supervisors: Professor A.J. Stace and Dr. E. Bichoutskaia
There are many instances in which the polarization of particles explains why coalesce one another even with the same type of charge. We developed a model in which we can see the polarization of a particle due the presence of an external charge, first using the Legendre polynomial expansion and we compare the results with quantum DFT calculations. The results were we expect according to the model and we conclude the population analysis we used was the appropriate.
120 POSTER SESSION 2 P2-28
3 Experimental and theoretical study of O( P) + C2H4 multichannel reaction dynamics
F. Leonori1, L. Angelucci1, N. Balucani1, A. Occhiogrosso1, R. Petrucci1, P. Casavecchia1*, B. Fu,2 Y.-C. Han,2 J. M. Bowman2
1 Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy 2 Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, USA
*E-mail: [email protected]
Small unsaturated hydrocarbons (C2H2,C2H4,C3H4) are crucial intermediates in hydrocarbon combustion. Their dominant consumption pathways are their reactions with O(3P). In combustion modelling, among the most important information needed for each of these elementary polyatomic multichannel reactions, are (a) the overall rate constant, (b) the identity of the primary products and (c) the branching ratios (BRs) possibly as a function of collision energy Ec (temperature). While kinetics experiments are able to satisfy point (a), point (b) and (c) still represent a challenge for kinetics, although considerable progress has recently been made. The method most suitable to tackle this challenge is the “crossed molecular beams” (CMB) scattering technique with “universal” mass spectrometric detection and time-of-flight analysis, based on “soft” ionization by tunable low energy electrons [1,2] or VUV synchrotron radiation [3].
3 Here we report on a combined experimental and theoretical study of the reaction O( P)+C2H4 as a function of Ec. Experimentally, we have investigated the reaction at a new Ec (8.4 kcal/mol), characterized the product angular and translational energy distributions in the center-of-mass system, and derived BRs for the five competing channels leading to H+CH2CHO, H+CH3CO, H2+CH2CO, CH3+HCO, and CH2+HCHO [4]. The derived BRs are analyzed together with those obtained previously at higher Ec [2] and from kinetics studies at room temperature (Ec 0.9 kcal/mol) [5]. The combined kinetics and dynamics results allow us to examine the BRs and the extent of ISC in a wide range of Ec (temperature), from about 1 kcal/mol (300 K) up to about 13 kcal/mol (4300 K). Theoretically, full dimensional potential energy surfaces (PESs) for the triplet and singlet states as well as spin-orbit couplings between them have been calculated and half a million of surface hopping quasiclassical trajectories have been run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross sections at the different Ecs [4,6]. Good agreement is observed between experiment and theory.
References
[1] P. Casavecchia, F. Leonori, N. Balucani, R. Petrucci, G. Capozza, E. Segoloni, PCCP 11, 46-65 (2009). [2] P. Casavecchia et al., J.Phys Chem. A 109, 3527 (2005). G. Capozza et al., J.Chem.Phys. 120, 4557 (2004). [3] S.-H. Lee, W.-K. Chen, W.-J. Huang, J. Chem. Phys. 130, 054301 (2009). [4] B. Fu, Y.-C.Han, J.M. Bowman, L.Angelucci, N.Balucani, F.Leonori, P.Casavecchia, PNAS 109, 9733 (2012). [5] T. L. Nguyen, et al., J. Phys. Chem. A, 109, 7489 (2005); A. Miyoshi et al., PCCP 11, 7318 (2009). [6] B. Fu, Y.-C. Han, J. M. Bowman, F. Leonori, L. Angelucci, N. Balucani, A. Occhiogrosso, R. Petrucci, P. Casavecchia, J. Chem. Phys., submitted.
121 POSTER SESSION 2 P2-29
A classical versus quantum mechanics study of the OH + CO H + CO2 (J=0) reaction
E. Garcia1*, F.J. Aoiz2, A. Laganà3
1Dpto. Quimica Fisica. Universidad del Pais Vasco (UPV/EHU), Vitoria, Spain 2Dpto. Quimica Fisica. Universidad Complutense, Madrid, Spain 3Dipartimento di Chimica, Università di Perugia, Perugia, Italy
*E-mail: [email protected]
The comparison of calculated and experimental data of the kinetics and dynamics of a chemical reaction is the usual way to validate its potential energy surface (PES) and rationalize its dynamical behavior. The title reaction is an ideal benchmark for this purpose because, due to its importance in modeling combustion and atmospheric chemistry, a great deal of kinetics and dynamical results are available. In spite of the theoretical endeavor, the existing calculations are not capable of reproducing all the information derived from experiments [1]. Kinetics and dynamics calculations were carried out using mainly quasiclassical trajectory (QCT) calculations. Recently, however J=0 full- dimensional state-to-state time-dependent wave packet QM calculations were performed [2]. In particular, besides the evaluation of the reaction probabilities as a function of the collision energy for the ground ro-vibrational and the first excited vibrational states of both reactants, these calculations provided the average ro-vibrational energy of CO2 as a function of the collision energy, plus the separated product vibrational (PVD) and rotational (PRD) distributions at some collision energies. In this work, QCT calculations have been performed for the OH+CO2 reaction at zero total angular momentum and collision energy ranging up to 0.4 eV in order to asses to what extent QCT results can be taken as a proper means for comparison between theory and experiment. The resulting quasiclassical total reaction probability as well as the PRDs and PVDs have been analyzed and compared with the outcomes of a pseudo-quantization treatment of the product vibrational energy [3]. Classical trajectories reproduce fairly well all the quantum features but the oscillatory patterns.
References
[1] Laganà, A, Garcia E, Paladini A, Casavecchia P, Balucani N (2012) Faraday Discuss (in press) [2] Liu S, Zhang DH (2011) J Chem Phys 135:234307; Liu S, Xu X, Zhang DH (2012) Theor Chem Acc 131:1068; Wang C, Liu S, Zhang DH (2012) Chem Phys Lett 537:16 [3] Garcia E, Corchado J, Espinosa-Garcia J (2012) Comput Theor Chem 990:47
122 POSTER SESSION 2 P2-30
Infrared emission following the quenching of electronically excited NO and OH
Julian Few, Gus Hancock, Sarah Gowrie, Maximiliano Burgos-Paci
Oxford University, Department of Chemistry, Physical and Theoretical Chemistry Laboratory
Time resolved FTIR emission has been used to determine the fates of electronically excited NO and OH in their A2Σ states in collisions with a variety of quenching molecules. Quenching of A2Σ v=0 by 2 CO2 yields emission from NO(X Π, v=1‐20), but the dominant quenching process involves the formation of NO with no vibrational excitation. Intense emission from highly vibrationally excited CO2 is seen with a distribution which is hotter than statistical. Reaction is also seen to produce ground vibrational state CO, and a minor channel forming NO2 is also observed. This behaviour contrasts with that for quenching by N2O, where the dominant process is cleavage of the N2-O bond, with very little energy transfer to form vibrationally excited N2O. With O2 as the quencher, the dominant channel is vibrational energy transfer, with only a minor route leading to reaction to form O atoms. The behaviour of these and other quenchers will be discussed for both the v=0 and v=1 states of NO A2Σ, together with preliminary data on the behaviour of the same levels in the A2Σ state of the OH radical, where an unexpectedly intense emission is seen on the fundamental (1,0) band within the A2Σ state following irradiation of A2Σ (v=1)
123 POSTER SESSION 2 P2-31
QM/MM trajectory surface hopping approach to photochemical dynamics of indolylmaleimide
T. Murakami1*, A. Kondorskiy2, T. Ishida3, S. Nanbu1
1Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-Cho, Chiyoda-ku, Tokyo 102-8554, Japan. 2P. N. Lebedev Physical Institute, Leninsky pr., 53, Moskow 119991, Moscow Institute of Physics and Technology, Institutsky per., 9, Dolgoprudny, Moscow Region 141700, Russia. 3Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-nishihirakicho, Kyoto 606- 8103, Japan
*E-mail: [email protected]
Recently, on-the-fly nonadiabatic ab initio molecular dynamics simulations were carried out for three anionic species of indolylmaleimides (3-(1H-3-indolyl)-2,5-dihydro-1H-2,5-pyrroledione, IM) to clarify the mechanisms of photochemical reactions.[1,2] The results were obtained for (i) a monovalent anion with a deprotonated indole NH group (IM-″), (ii) a monovalent anion with a deprotonated maleimide NH group (IM-″) and (iii) a divalent anion with doubly deprotonated indole and the maleimide NH groups (IM2-). Quantum chemical calculations are treated at the three state averaged complete-active space self-consistent field level for 6 electrons in 5 orbitals with the cc- pVDZ basis set (CAS (6, 5) SCF/cc-pVDZ). Molecular dynamics simulations are performed with electronically nonadiabatic transitions included using the Zhu–Nakamura version of the trajectory surface hopping (ZN-TSH) method. It was found that the nonadiabatic transitions occur accompanied by the stretching and shrinking motions of the CN bond at indole moiety in the case of IM-″and the CN bond at maleimide part in IM2- rather than the twisting motion of the dihedral angle. We also found that the ultrafast S2 -S1 nonadiabatic transitions occur through the conical intersection (CoIn) - 2- right after photoexcitation to S2 in IM ″ and IM . Furthermore, the S1 -S0 nonadiabatic transitions are found to take place in IM-″. It was concluded that IM2- would mainly contribute to the - photoemission, because the S1 ← S0 and S2 ← S0 transitions of IM ″ were dipole‐forbidden 2- transitions and, moreover, IM was found to be the only species to stay in the S1 state without non- radiative decay. Additionally, we will discuss design and evaluation of theoretical absorption spectrum.
References
[1] M. Nakazono, A. Jinguji, S. Nanbu, R. Kuwano, Z, Zheng, K. Saita, Y. Oshikawa, Y. Mikuni, T. Murakami, Y. Zhao, S. Sasaki and K. Zaitsu, Phys. Chem. Chem. Phys., 2010, 12, 9783. [2] T. Murakami, M. Nakazono, A. Kondorskiy, T. Ishida, S. Nanbu, Phys. Chem. Chem. Phys., in press.
124 POSTER SESSION 2 P2-32
Photostop: Magnetic trapping of cold SH radicals produced by photodissociation
L. Deng,1 A. M. Rowland,1 M. L. Lambert,1 D. Carty,1,2 and E. Wrede1*
1JQC Durham-Newcastle, Department of Chemistry, Durham University, U.K. 2JQC Durham-Newcastle, Department of Physics, Durham University, U.K.
*E-mail: [email protected]
We have recently demonstrated that photofragments can be produced at rest in the laboratory frame after the photodissociation of a precursor molecule—a technique we call photostop. If the recoil speed of the photodissociation event is matched to the speed of the precursor molecules in a molecular beam those photofragments that recoil opposite to the precursor velocity will be created at zero velocity. Molecular and atomic fragments (NO from NO2 dissociation and Br from Br2 dissociation) have been successfully stopped and their free evaporation from the probe volume monitored via resonance‐enhanced multi‐photon ionization for up to 10 and 100 μs after the dissociation, respectively [1,2].
At this conference we will present our latest data of cold SH (SD) radicals produced with the photostop technique and hope to show preliminary results of trapped SH molecules inside a static magnetic trap monitored via laser-induced fluorescence. We will compare our experimental data to simulations of the photodissociation and evaporation processes in free space and within the trap and discuss the prospect of the accumulation of molecules created by subsequent laser pulses.
References
[1] A. Trottier, E. Wrede and D. Carty, Mol. Phys. 109, 725 (2011). [2] W. G. Doherty, M. T. Bell, T. P. Softley, A. M. Rowland, E. Wrede and D. Carty, PCCP 13, 8441 (2011).
125 POSTER SESSION 2 P2-33
Towards merged-beam experiments of neutral particles
Benjamin Bertsche, Andreas Osterwalder*
EPFL, Station 6, CH-1015 Lausanne, Switzerland
Collision experiments with fast beams can be performed at low collision energies if the beams move parallel in the same direction. Compared with crossed beam setups this leads to enhanced interaction volumes which make the detection of smaller cross sections possible.
In order to completely overlap the two beams and to assure their parallel movement, a high level of control over the motion of the particles is mandatory. So far, this could only done when at least one of the beams consists of charged particles. Progress in using the Stark and Zeeman effect to manipulate neutral particles have now reached a level where merged-beam experiments for neutral- neutral collisions become possible.
We are presently constructing a setup where two guides, one electric and one magnetic, for fast particles from supersonic expansions are merged which will enable the study of a very wide range of collision systems between polar and paramagnetic particles in a range of relative velocities between zero and several 100 m/s.
This planned setup will be presented as well as our approaches to extract the maximum information from such an experiment.
126 POSTER SESSION 2 P2-34
Hyperspherical harmonics representation of binary interactions H2O–H2,N2, and O2. A minimal model for the H2O – rare-gas-atom systems.
Patricia R. P. Barreto1*, Alessandra F. Albernaz2, Federico Palazzetti3, Andrea Lombardi3, Gaia Grossi3, Vincenzo Aquilanti3
1Instituto Nacional de Pesquisas Espaciais (INPE/MCT), Laboratório Associado de Plasma (LAP), São José dos Campos, SP, CEP 12247-970, CP515, Brazil. 2Instituto de Fìsica, Universidade de Brasìlia, CP04455, Brasília, DF, CEP 70919-970, Brazil. 3Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy
*E-mail: [email protected]
In this work we report the explicit representation of the potential energy surfaces based on orthogonal vectors of binary systems involving the rigid molecules H2O and H2,N2, and O2. [1,2] The interaction potential is given as an expansion in hyperspherical harmonics and the functional form depends on a radial coordinate R between centers-of-mass of the two molecules and on four angles: the polar angles , 1, 2, and on the dihedral angle . A minimal model has also been illustrated to represent the intermolecular potentials for H2O – rare-gas-atom systems, the reduced dimensionality make the functional form depending on only three coordinates: the distance, R, between the center-of-mass of the molecule and the atom, and the polar angles and . The molecular geometries have been optimized at the CCSD(T)/aug-cc-pVXZ level of calculation, with X = D, T, Q, and 5. The potential energy surfaces were calculated by ab initio calculations at CCSD(T)/aug-cc-pVTZ level, for about one hundred points of the surface for each of the fifteen leading configurations of the H2O–X2 system and the four leading configurations of the H2O–rare-gas atom systems. The potential energy surfaces are in format to be employed in molecular dynamics simulations [3] and in the analysis of experimental results [4]. The hyperspherical harmonics expansion allows compactness, full account of symmetries, and to obtain realistic results with a low computational cost. [5]
References
[1] P. R. P. Barreto, A. F. Albernaz, A. Capobianco, F. Palazzetti,. A. Lombardi, G. Grossi, V. Aquilanti Comp. Theor. Chem. 990 (2012) 53. [2] P. R. P. Barreto, V. W. Ribas, F. Palazzetti J. Phys. Chem. A 113 (2009) 15047. [3] A. Lombardi, F. Palazzetti, G. S. Maciel, V. Aquilanti, M. B. Sevryuk Int. J. Quant. Chem. 111 (2011) 1651. [4] A. Bartocci, B. Brunetti, P. Candori, S. Falcinelli, F. Palazzetti, F. Pirani, F. Vecchiocattivi Chem. Phys. Lett. (submitted). [5] F. Palazzetti, M. Elango, A. Lombardi, G. Grossi, V. Aquilanti Int. J. Quant. Chem. 111 (2011) 318.
127 POSTER SESSION 2 P2-35
Potential energy surface for H2 HX system, with X=H, F, Cl, Br
Patrícia RP Barreto1*, Alessandra F Albernaz2, Amedeo Capobianco3
1Instituto Nacional de Pesquisas Espaciais (INPE/MCT), Laboratório Associado de Plasma (LAP), São José dos Campos, SP, CEP 12247-970, CP515, Brazil. 2Instituto de Fìsica, Universidade de Brasìlia, CP04455, Brasília, DF, CEP 70919-970, Brazil, 3Dipartimento di Chimica e Biologia. Università di Salerno, Via Ponte don Melillo, I-84084 Fisciano, Italy
*E-mail: [email protected]
In this work we report the explicit representation of the potential energy surfaces for the H2 HX systems, with X = H, F, Cl or Br, via a harmonic expansion functional form for rigid diatom-diatom interactions [1,2]. The framework of the supermolecular approach was used, as well as the counterpoise-corrected interaction energies at CCSD(T)/aug-cc-pVQZ levels. The energies were calculated in nine leading configurations according to the orientation of the dimers, depending on a radial coordinate R between centers-of-mass of the two molecules, and the polar and diedral angles
(1,2,). The analytical form of the potential energy surfaces, for each of the leading configurations, is constructed by fitting the energies to a fifth degree generalized Rydberg function. Comparing the fitted energies with the CCSD(T)/aug-cc-pVQZ one for the H2 HX systems, with X=H, F, Cl or Br we -1 found a maximum rms error of 21.1 cm for Lb configuration of H2 HBr and a minimum rms error -1 of 1.13 cm for H configuration of H2 H2. This study predicts the T-shaped structures to be the most stable configurations for the above dimers, but with different orientation. For H2 HF and -1 H2 HBr it is the Ta configuration (/2,0,0), with energy of 361.3 and 170.1 cm , respectively, while -1 the Tc configuration (0,/2,0) is the most stable for the H2 HCl, with energy of 202.4 cm . For the -1 H2 H2 the T-shape has a minimum with energy of 33.3 cm . For all dimer the Lb configuration is the less stable (0,,0), with has a repulsive characteristic. The isotropic term of the potential are -1 20.3, 36.7, 53.1 and 74.7 cm for H2 HX systems, with X=H, F, Cl or Br, respectively.
The authors are grateful to the Fundação de Amparo de Pesquisa do Estado de São Paulo (FAPESP) for the support.
References
1. V. Aquilanti, D. Ascenzi, M. Bartolomei, D. Cappelletti, S. Cavalli, M. C. V\`itores, and F. Pirani, J. Am. Chem. Soc., 121, 10794, (1999). 2. V. Aquilanti, M. Bartolomei, D. Cappelletti, E. Carmona-Novillo and F. Pirani, Phys. Chem. Chem. Phys., 3, 3891 (2001)
128 POSTER SESSION 2 P2-36
H + n-C4H10 → H2 + 1-C4H9/2-C4H9 reactions: Testing a new potential energy surface construction method
Xiao Shan*, David C. Clary
Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK
*E-mail: [email protected]
An efficient method to construct two dimensional potential energy surfaces (PESs) for chemical reactions was recently suggested by von Hoesten et al. [1] In the same paper, it was successfully applied to the quantum scattering calculations for various H-abstraction reactions from C1-C3 alkane molecules. In this poster, we demonstrate an alternative PES construction method, which utilizes the minimum energy path and one-dimensional Morse potential function. We then test this method against the H-abstraction reaction from primary and secondary position of n-C4H10 by H-atom. The geometry optimizations and frequency calculations are done at MP2/cc-pVTZ level of theory, while the energies are calculated with CCSD(T) method with the same basis set. We compare in the poster our reduced dimensionality quantum scattering results of rate constants for the two reaction channels with classical transition state theory results as well as previous experimental data. In addition, we show in the branching ratio analysis that at low temperature the overall reaction is dominated by the abstraction of secondary H in n-C4H10, while the reaction rates for the two channels become more comparable at higher temperature.
References
[1] H. F. von Horsten, S. T. Banks, and D. C. Clary, J. Chem. Phys., 2011, 135, 094311
129 POSTER SESSION 2 P2-37
Cold, magnetically-trapped bromine atoms
Jessica Lam, Will Doherty, Chris Rennick and Tim Softley
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, United Kingdom
Photodissociating molecular bromine near threshold produces a pair of bromine atoms that recoil along the polarisation axis of the laser. At an appropriate wavelength, the velocity vector of one of the atoms can be aligned to exactly oppose the molecular beam velocity; this atom will then be stopped in the lab frame. The stopped atoms are probed by delayed multiphoton ionisation.
We form a 0.2 Tesla-deep magnetic trap from two opposing neodymium iron boride bar magnets, and align its potential minimum to overlap the photodissociation volume. Sufficiently slow ground state bromine atoms in the low field--seeking Zeeman substate are trapped in the potential minimum; the maximum trappable velocity is 7 m/s, corresponding to an equivalent temperature of 235 mK. We have trapped atoms for periods extending to 99 ms, and have modelled the loss mechanisms accounting for ballistic expansion of fast atoms, collisions with residual gas pressure in the vacuum chamber, and Majorana transitions as the atoms move through the magnetic field minimum. Our measured trap loss rate shows that the primary mechanism over our experimental time scales is kinetic energy transfer via elastic collisions with the residual pressure of argon in the vacuum system.
130 131 We wish to thank the following for their contributions towards the success of this conference:
Photonic Solutions: Supplier of opto-electronics to The journal Physical Chemistry Chemical Physics the photonics market of the Royal Society of Chemistry.
Photonis: Industry | Science | Medical Laser 2000: Advanced Solutions for Photonics
Nature Chemistry: Nature Publishing Group Oerlikon: Oerlikon Leybold Vacuum UK Ltd
Taylor & Francis: Publishing group Allectra Ltd
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