Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A26: Focus Session: Photophysics of Cold Molecules I |
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Sponsoring Units: DCP Chair: Andrey Vilesov, University of Southern California Room: Morial Convention Center 218 |
Monday, March 10, 2008 8:00AM - 8:36AM |
A26.00001: Isolation of Molecules in Helium Nanodroplets: Spectroscopy and Dynamics at Ultra-cold Temperatures Invited Speaker: 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. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A26.00002: Nonequilibrium magnesium complexes formed in helium nanodroplets Josef Tiggesb\"aumker, Andreas Przystawik, Sebastian G\"ode, Karl-Heinz Meiwes-Broer 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. \newline [1] J. Reho \emph{et al}., J. Chem. Phys. {\bf 112}, 8409 (2000) \newline [2] Y. Ren and V.V. Kresin, Phys. Rev. A {\bf 76}, 043204 (2007) [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A26.00003: Superfluid $^4$He density functional theory in 2-D cylindrical coordinates Jussi Eloranta, Sean French, Steven Fiedler Bosonic density functional theory describing superfluid $^4$He 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 $^4$He. 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 $^4$He at 0 K and the implications of the results to dynamic liquid response are discussed. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A26.00004: Photo-induced isomerization and chemical reaction dynamics in superfluid helium droplets Jeremy Merritt, Gary Douberly, Roger Miller 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. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
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*He$_{N}$ ($N$ = 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*He$_{N}$ 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. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A26.00006: Photoelectron imaging of doped helium nanodroplets Daniel Neumark 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. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A26.00007: High Resolution Fluorescence Excitation and Dispersed Emission Spectra of Organic Molecules in Superfluid Helium Nanodroplets Alkwin Slenczka, Ricarda Riechers, Dominik Pentlehner, Alexander Vdovin 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. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A26.00008: Coherent boson dynamics in strongly localized potentials - helium excitations at planar aromatic molecules and trapped cold atoms Invited Speaker: 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. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A26.00009: Fragmentation dynamics inside helium nanodroplets: new theoretical results Nadine Halberstadt, David Bonhommeau, Marius Lewerenz 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 that the dopant can be ejected from the helium droplet. These results will be presented and discussed. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A26.00010: Suppressing the fragmentation of fragile molecules in helium nanodroplets by co-embedding with water: Possible role of the electric dipole moment Yanfei Ren, Vitaly Kresin When fragile molecules are embedded in liquid helium nanodroplets, electron-impact ionization usually leads to fragmentation which is as extensive as for isolated gas-phase molecules. This occurs because of the energy release accompanying charge transfer from the impurity to the He$^{+}$ hole created by electron bombardment. However, in experiments with glycine, polyglycine peptide chains, and alkane hydrocarbon chains we found that if a few molecules of water are co-embedded with these molecules, the fragmentation of the latter is drastically reduced or completely eliminated. On the other hand, the fragmentation of alkanethiols remains unaffected. On the basis of these observations, it is proposed that the fragmentation ``buffering'' effect may correlate with the magnitude of the impurity's electric dipole moment, which steers the migration of the ionizing He$^{+ }$hole through the droplet. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A26.00011: Path integral investigation of the electronic spectra of He-tetracene clusters Heather D. Whitley, K. Birgitta Whaley Planar aromatic molecules (PAMs) are nanoscale precursors to bulk graphite. Their electronic spectra have been extensively studied in $^{4}$He nanodroplets and show a number of unusual spectroscopic features. We have conducted many-body quantum simulations of tetracene in He nanodroplets to probe the 1.1 cm$^{-1}$ spectral splitting of the electronic origin seen for this PAM. We calculate spectral shifts and He density profiles via path integral quantum Monte Carlo simulations. The spectral splitting is examined using a path integral correlation function approach to determine the lowest-lying vibrational excitation frequencies for small He$_{N}$-tetracene clusters. Simulations in the S$_{1}$ state of tetracene utilize a semi-empirical perturbative interaction potential for a He atom with a PAM. Results for the splitting of the electronic origin and the spectral shifts are in good agreement with experiment. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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