Structural Dynamics and Energy Flow in Rydberg-Excited Clusters of N,N

THE JOURNAL OF CHEMICAL PHYSICS 135, 044319 (2011) Structural dynamics and energy flow in Rydberg-excited clusters of N,N-dimethylisopropylamine Sanghamitra Deb, Michael P. Minitti,a) and Peter M. Weberb) Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA (Received 14 April 2011; accepted 13 June 2011; published online 29 July 2011) In molecular beams, the tertiary amine N,N-dimethylisopropyl amine can form molecular clusters that are evident in photoelectron and mass spectra obtained upon resonant multiphoton ionization via the 3p and 3s Rydberg states. By delaying the ionization pulse from the excitation pulse we follow, in time, the ultrafast energy relaxation dynamics of the 3p to 3s internal conversion and the ensuing cluster evaporation, proton transfer, and structural dynamics. While evaporation of the cluster occurs in the 3s Rydberg state, proton transfer dominates on the ion surface. The mass-spectrum shows protonated species that arise from a proton transfer from the alpha-carbon of the neutral parent molecule to the N-atom of its ionized partner in the dimer. DFT calculations support the proton transfer mechanism between tightly bonded cluster components. The photoelectron spectrum shows broad peaks, ascribed to molecular clusters, which have an instantaneous shift of about 0.5 eV toward lower binding energies. That shift is attributed to the charge redistribution associated with the induced dipoles in surrounding cluster molecules. A time-dependent shift that decreases the Rydberg electron binding energy by a further 0.4 eV arises from the structural reorganization of the cluster solvent molecules as they react to the sudden creation of a charge. © 2011 American Institute of Physics. [doi:10.1063/1.3609110] I. INTRODUCTION the cluster dynamics of species excited to molecular valence states. Molecular clusters, loosely bound aggregates of In our recent work we have found that Rydberg levels molecules that are bound by van der Waals or electric are intriguingly capable reporters of the structure of isolated multipole interactions, are intriguing intermediates between molecules.18, 19 The Rydberg electron, orbiting a positively the isolated molecules of the gas phase and the densely charged ion core at a large distance, probes the molecular packed systems of condensed phases.1–4 Frequently observed structure because part of its wave function penetrates the ion in the cold environments of molecular beams,5, 6 the study core. This leads to structure-specific phase shifts in the wave of clusters sheds light on the molecular basis of essential function that are reflected in the Rydberg electron’s binding chemical phenomena such as solvation and intermolecular energy. The binding energy can be conveniently measured us- interactions that precede chemical reactions. ing photoelectron spectroscopy. Implemented with ultrashort While many clusters have been studied using a vari- laser pulses, the ionization from Rydberg states is a very ca- ety of spectroscopic techniques,2, 7–11 the dynamical, struc- pable tool for studying the structural dynamics of molecular tural motions of clusters and their molecular components systems. have remained difficult to unravel. Multiphoton ionization Because the potentials governing the vibrational motions coupled with mass spectrometry, which can be implemented of the ion core barely change when the Rydberg electron with ultrashort laser pulses to allow the observation of time- is ejected, the resulting spectra are free of vibrational pro- dependent phenomena, can be difficult to interpret because gression but show the structure-specific ionization transitions the clusters observed at the detector may not necessarily from the Rydberg states. For rigid molecules, the resulting be the ones of which the dynamics is followed during ion- spectra feature lines with spectral widths that are determined ization. In comparison, time-resolved photoelectron spec- only by the bandwidth of the ionizing pulsed laser or the pho- troscopy (TRPES) has the advantage that the electrons are toelectron spectrometer. For flexible molecules we have found ejected at the time of ionization, yielding a very direct signa- that different conformeric forms give rise to separate spectral ture of the time-dependent dynamical processes. During the peaks, and that the width of the lines reflect the structural dis- last decade, TRPES has also been applied to isolated clusters persion of the Rydberg excited molecules.20 This has enabled in gas phase.11–15 TRPES is also useful because it directly in- us to observe the structural dynamics of conformational trans- terrogates the evolving electronic structure along an excited- formations in molecular systems with internal rotations about state reaction coordinate.16, 17 Most of those studies explored single bonds.21, 22 Our goal in the present work is to extend the explo- a) Present address: LCLS Laser Department, SLAC National Accelerator ration of structural dynamics in floppy systems to molecular Laboratory, Menlo Park, California 94025, USA. b)Author to whom correspondence should be addressed. Electronic mail: clusters. Because the Rydberg electron wavefunction is large [email protected]. Fax: +1-401-863-2594. compared to typical molecular dimensions, even in clusters 0021-9606/2011/135(4)/044319/10/$30.00135, 044319-1 © 2011 American Institute of Physics Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp 044319-2 Deb, Minitti, and Weber J. Chem. Phys. 135, 044319 (2011) of modest size, the Rydberg electron binding energy is sensi- tive to the global molecular structure. We therefore expect the cluster formation to have a profound impact on the Rydberg states. The structural sensitivity also offers the opportunity to observe, in real time, how the global structure of the cluster system evolves. For our studies we choose N,N-dimethylisopropylamine (DMIPA), a tertiary amine in which the nitrogen atom acts as an ionization chromophore and the aliphatic substituents provide for interesting structural and chemical dynamics. DMIPA is an attractive model system because the structural and electronic relaxation dynamics of the monomer are well understood.23 Excitation in the UV, at 209 nm, leads to excitation of the 3p Rydberg states, which rapidly decays to 3s. Because the ground state potential features a double well that is well known from ammonia,24 while in the Rydberg state the molecule is close to planar, excitation in the Franck-Condon region inserts a large amount of energy into vibrational co- ordinates. This energy, plus the energy that is converted in the electronic relaxation from 3p to 3s, is available to induce FIG. 1. The two-color ionization scheme to probe the dynamics of DMIPA fragmentation of the α carbon-carbon bond. However, studies in the Rydberg states. The molecules are excited to 3p by absorption of a have shown that, in the parent molecule, the lifetime of 3s is photon at 209 nm, and ionized by a time-delayed 418 nm photon. The kinetic energy spectrum of the ejected electrons is recorded as a function of time. As so short that any such fragmentation occurs in the ion state revealed by those spectra, the parent ion P+ generated by ionization out of 3p 23 25 instead. In the related work on ammonia clusters and on has insufficient energy to fragment, while the ion generated by ionization out mixed clusters with ammonia26 evidence for proton/hydrogen of 3s can do so. transfer has been observed. Being amongst the most impor- tant and ubiquitous phenomena, proton/hydrogen transfer reactions have been widely studied in clusters.27–32 We have laser pulse overlap was determined by monitoring the cross- found that DMIPA easily forms molecular clusters when ex- correlation time between the fourth harmonic excitation and panded in argon as a carrier gas, permitting us to study their second harmonic ionization pulses using the molecular re- dynamics from a structural point of view. sponse function of 1,4-dimethylpiperazine, which has a very In our experiments, we excite the molecules and the clus- fast response time. ters from the ground state to the 3p Rydberg state using a The molecular beam is generated by expanding DMIPA single photon at 209 nm (Fig. 1) and ionize it using a time- vapor through a nozzle orifice and a skimmer with diameters delayed probe photon at 418 nm. The kinetic energy of the of 100 μm and 150 μm, respectively. To vary the admixture ejected photoelectron yields the electronic state at the time of of DMIPA in the carrier gas, we bubble the rare gas at 1.15 bar ionization, as well as the structure-specific electron binding through the reservoir of liquid sample at various temperatures. energy of the Rydberg orbital. The mass spectrum reveals the At reservoir temperatures of −30 ◦C and 0 ◦C, the vapor pres- cluster composition, although the flight time to the spectrom- sure of DMIPA is 5 Torr and 35 Torr, respectively.35 At either eter is so large that fragmentation during the flight can mask temperature, helium expansions result in beams consisting al- the true cluster composition at the time of ionization. most exclusively of parent molecules. For argon as a carrier gas we observe the formation of molecular clusters, with a II. EXPERIMENT mass distribution that depends on the reservoir temperature. The mass resolution of our instrument was m/m = 300. The photoelectron-mass spectroscopy apparatus and the Anhydrous DMIPA (99.8%) was purchased from Sigma- ultrafast laser system used to generate femtosecond laser Aldrich and used without further purification. Our mass spec- pulses have been described previously.20, 33, 34 For the present tra show no signs of impurities. time-resolved experiments the regenerative amplifier, operat- ing at 5 kHz repetition rate, produces pulses with ∼70 fs du- ration at 836 nm. The output of the amplifier is upconverted III. RESULTS AND DISCUSSION to the second and fourth harmonics using β-barium borate crystals, providing pulses at 418 and 209 nm that are used The ejection of a photoelectron is fast compared to the as the probe and pump pulses, respectively.

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