2008 APS March Meeting New Orleans, Louisiana

2008 APS March Meeting New Orleans, Louisiana http://www.aps.org/meetings/march/index.cfm i Monday, March 10, 2008 8:00AM - 11:00AM — Session A26 DCP: Focus Session: Photophysics of Cold Molecules I Morial Convention Center 218 8:00AM A26.00001 Isolation of Molecules in Helium Nanodroplets: Spectroscopy and Dynam- ics at Ultra-cold Temperatures FRANK STIENKEMEIER, University of Freiburg — The isolation of atoms, molecules, clusters or nano-sized complexes in helium nanodroplets allows detailed spectroscopic studies at temperatures in the millikelvin range. Moreover, femtosecond real-time spectroscopy has been introduced to study dynamical processes in the ultracold helium environment. On the one hand, wave packet propagation opens a window to dynamical processes, allowing even a view to superfluid properties at the nanoscale. This is exemplified at decoherence effects in the wave packet propargation of small molecules attached to the droplets. On the other hand, high-resolution mass spectra using both femtosecond photoionisation (PI) as well as electron impact ionization enable us to characterize reactive processes at temperatures in the millikelvin range. As an example, alkali cluster – water complexes are formed in helium droplets. By recording multi-photon PI spectra we can distinguish between reactive processes of the neutral clusters and ionic reactions occurring after ionisation of the alkali cluster component. These studies pave the way to time-resolved reaction dynamics at very low temperatures. 8:36AM A26.00002 Nonequilibrium magnesium complexes formed in helium nanodroplets , JOSEF TIGGESBAUMKER,¨ ANDREAS PRZYSTAWIK, SEBASTIAN GODE,¨ KARL-HEINZ MEIWES-BROER, Institute of Physics, University of Rostock, 18051 Rostock, Germany — Doping helium droplets with alkaline earth atoms is an interesting tool to investigate the interaction with the superfluid helium. Magnesium is a corner case regarding the degree of solvation in helium [1,2] which may enable the detection of quantized vortices in helium droplets. In this contribution we add another facet to the discussion. The absorption of helium droplets doped with magnesium atoms is measured with resonant two-photon ionization at different combinations of droplet size and the number of doped Mg atoms. This enables the unambiguous identification of the absorption of an isolated atom inside the droplet centered around 279 nm. When increasing the Mg content of the droplet we find evidence for the formation of metastable, weakly bound Mg complexes. After excitation of such a complex it collapses to a Mg cluster on a timescale of 20 ps. [1] J. Reho et al., J. Chem. Phys. 112, 8409 (2000) [2] Y. Ren and V.V. Kresin, Phys. Rev. A 76, 043204 (2007) 4 8:48AM A26.00003 Superfluid He density functional theory in 2-D cylindrical coordinates1 , JUSSI ELORANTA, SEAN FRENCH, Department of Chemistry and Biochemistry, California State University at Northridge, STEVEN FIEDLER, Department of Chemical Engineering, The University of Michigan — Bosonic density functional theory describing superfluid 4He is formulated in 2-D cylindrical coordinates and a numerical implementation of the model using a regular spatial grid basis is presented. The 2-D formulation has many important applications as the 1-D treatment cannot, for example, describe translational motion of atoms and molecules solvated in the liquid and the 3-D theory is usually computationally too expensive, especially when describing dynamics in bulk superfluid 4He. The theory is implemented in both real and imaginary time forms for allowing solution of both time-dependent and time-independent problems. Two test cases for the developed method are presented and the results are compared against the previously published results. Finally, the method is applied to describe solvation of single wall carbon nanotubes in superfluid 4He at 0 K and the implications of the results to dynamic liquid response are discussed. 1Support from Research Corporation is acknowledged. 9:00AM A26.00004 Photo-induced isomerization and chemical reaction dynamics in superfluid helium droplets , JEREMY MERRITT, GARY DOUBERLY, ROGER MILLER, University of North Carolina-Chapel Hill — Near threshold photo- induced isomerization and photo-induced chemical reactions have long been sough after as sensitive probes of the underlying potential energy surface. One of the most important questions asked is how the initially bright quantum state couples to the reaction coordinate, and thus relates to energy transfer in general. Helium droplets have now allowed us to stabilize entrance channel clusters behind very small reaction barriers such that vibrational excitation may result in reaction. Through two examples, namely the isomerization of the 2 binary complexes of HF-HCN {Douberly et al. PCCP 2005, 7,463}, and the induced reaction of the gallium-HCN complex {Merritt et al. JPCA 2007, DOI:10.1021/jp074981e} we will show how the branching ratios for reaction and predissociation can determined and the influence of the superfluid He solvent. 9:12AM A26.00005 Rydberg States of Na-doped Helium Nanodroplets , MARCEL DRABBELS — The dynamics of excited states of Na atoms deposited on the surface of helium nanodroplets has been investigated with velocity map ion imaging, photoelectron spectroscopy and time-of-flight mass-spectroscopy. For the first time, the excitation spectra of Na-doped helium nanodroplets corresponding to Rydberg states of Na atoms have been measured from the lowest excited 3p state up to the ionization threshold. All lines in the excitation spectra are shifted and broadened with respect to the corresponding atomic lines. In addition to bare Na* atoms also Na*HeN (N = 1-6) exciplexes are detected upon excitation. Photoelectron spectroscopy reveals the desorption of Na* not only in the initially excited states but also in lower lying states, indicating that relaxation plays an important role. The recorded velocity distributions show interesting characteristics: for the lowest states the mean kinetic energy of Na* increases linearly with excitation energy. The velocity distributions of Na*HeN exciplexes do not manifest such remarkable properties. The observations can be largely explained by assuming that the interaction of Na* with the helium nanodroplet can be described by the sum of Na*-He pair potentials. 9:24AM A26.00006 Photoelectron imaging of doped helium nanodroplets , DANIEL NEUMARK, University of California, Berkeley — Photoelectron images of helium nanodroplets doped with Kr and Ne atoms are reported. The images and resulting photoelectron spectra were obtained using tunable synchrotron radiation to ionize the droplets. Droplets were excited at 21.6 eV, corresponding to a strong droplet electronic excitation. The rare gas dopant is then ionized via a Penning excitation transfer process. The electron kinetic energy distributions reflect complex ionization and electron escape dynamics. 9:36AM A26.00007 High Resolution Fluorescence Excitation and Dispersed Emission Spectra of Organic Molecules in Superfluid Helium Nanodroplets1 , ALKWIN SLENCZKA, RICARDA RIECHERS, DOMINIK PENTLEHNER, ALEXANDER VDOVIN, Institue for Physical and Theoretical Chemistry, University of Regensburg — Superfluid helium droplets serve as a very gentle cryogenic matrix for molecular spectroscopy. The low temperature and high thermal conductivity of superfluid helium droplets are of great advantage for the investigation of dispersed emission spectra of molecules. As a complement to the fluorescence excitation spectrum the emission spectra provide important details on dynamic processes of intramolecular as well as intermolecular nature. This will be demonstrated for various examples such as intramolecular proton tunnelling, isomeric van der Waals complexes, tautomerization and microsolvation. 1A Humboldt fellowship for A.V. is gratefully acknowledged 9:48AM A26.00008 Coherent boson dynamics in strongly localized potentials - helium excita- tions at planar aromatic molecules and trapped cold atoms , K. BIRGITTA WHALEY, University of California, Berkeley — Planar aromatic molecules provide strongly localizing potentials for helium that considerably modify the local superfluid properties of a solvating helium environment. I shall describe some of the effects of these interactions on the solvation structure and spectroscopy of tetracene and phthalocyanine in helium droplets, comparing results of zero and finite temperature quantum Monte Carlo simulations with experimental data. The helium atoms closest to the molecule are seen to show similarities to trapped cold atoms in multi-well potentials. Studies of cold bosons with attractive and repulsive interactions in double well potentials will also be presented, showing formation of squeezed and quantum superposition states of cold atoms. 10:24AM A26.00009 Fragmentation dynamics inside helium nanodroplets: new theoretical results , NADINE HALBERSTADT1, LCAR-IRSAMC, CNRS and Universite Toulouse, DAVID BONHOMMEAU2, University of Minnesota, MARIUS LEWERENZ3, Universite Paris Est, Laboratoire de Chimie Theorique — We present a theoretical study on the effect of a helium nanodroplet environment on the fragmentation dynamics of embedded rare gas cluster ions. The helium atoms are treated explicitly, with zero-point effects taken into account through an effective helium-helium interaction potential. All the nonadiabatic effects between electronic states of the ionized rare gas cluster are taken into account. Our results reveal new mechanisms for the cooling by helium, and show

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