Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N11: Focus Session: Aerosols, Clusters, Droplets: Physics and Chemistry of Nanoobjects I: Helium Nanodroplets I |
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Sponsoring Units: DCP Chair: Giacinto Scoles, Princeton University Room: Baltimore Convention Center 303 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N11.00001: Imaging the Photodynamics of Doped Helium Droplets Invited Speaker: During the last decade helium nanodroplets have been established as an ideal spectroscopic matrix. Helium droplets are also thought to be ideal low temperature nanoreactors because of their ability to stabilize weakly bound species. As the focus is nowadays shifting to the study of chemical reactions in liquid helium droplets, question related to the energy relaxation and solvation dynamics become more and more prominent. To address these questions experiments have been performed in with species with a well defined kinetic energy distribution have been created via the photodissociation molecules residing inside helium droplets. The velocity distributions of the photofragments that have escaped from the droplets have been determined using ion imaging techniques. The analysis of speed distributions as function of droplet size and precursor has enabled to obtain a consistent picture of the mechanisms underlying the translational motion of these non-thermal species through this quantum liquid. Additional information on the solvation dynamics could be obtained by using non-resonant ionization techniques in these experiments. More recently the translational dynamics of quasi-free electrons in helium droplets has been investigated by means of photoelectron spectroscopy. The results on these experiments indicate that the relaxation of the electrons is governed by the same mechanism responsible for the kinetic energy relaxation of non-thermal neutral molecules. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N11.00002: Photoionization and photoelectron spectroscopy of doped helium nanodroplets Daniel Neumark Photoionization and photoelectron spectra for helium nanodroplets doped with rare gas atoms and SF$_{6}$ will be reported. The experiments were conducted using tunable synchrotron radiation at the Advanced Light Source in the photon energy range of 14-26 eV. Time-of-flight mass spectra will be presented, along with photoion and photoelectron images. The results will be compared to previous electron impact ionization data. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N11.00003: Photoinitiated processes in and on liquid helium Elena Polyakova, Daniil Stolyarov, Curt Wittig Photoinitiated processes that involve molecules in He$_{n}$ droplets are examined. Excitation to states that contain Rydberg character results in repulsion between the electronically excited embedded molecules and the surrounding helium. Even after the helium has, moved further from the molecular core, the situation is unstable in the sense that the electronically excited species prefer the surface. The timescale for transport to the surface is $<$ 10 \textit{ns} duration of the laser pulse. The resulting surface-bound species can be ionized, yielding small clusters of the form He$_{m}$NO$^{+}$, where $\langle {\rm g} \quad \rangle $ is of order 10,000. The possibility of observing high Rydberg states in which the electron is outside the helium droplet will be discussed. A vastly different case of photoexcitation occurs when the excited potential is coupled to a lower one via conical intersection. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N11.00004: Wave packet propagation of alkali dimers attached to helium nanodroplets Frank Stienkemeier, Patrick Claas, Claus-Peter Schulz Real-time spectroscopy of alkali dimers attached to helium nanodroplets has been studied by femtosecond pump-probe spectroscopy. Wave packet propagation in different electronic states of Na$_2$ and K$_2$ molecules was investigated. The perturbation of the helium environment allows in particular to observe electronic ground state vibrational motion. Furthermore, for the first time wave packets in alkali dimer triplet states are observed. Finally, the slight change of the vibrational structure when desorbing from the helium droplet can be utilized to determine desorption times upon laser excitation. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N11.00005: Metal clusters in helium droplets: Fulfilling the promise Paul L. Stiles, Roger E. Miller In 2001 we demonstrated that superfluid helium droplets, coupled to high-resolution infrared spectroscopy, could be used to investigate the intermolecular interactions and structures of metal cluster-adsorbate systems. The HCN-Mg$_{n}$ (n = 1-6) clusters investigated provided several interesting surprises and taught us many valuable lessons but nevertheless remained a somewhat uninteresting system from the point of view of catalysis and reactivity. Recently, we have overcome some significant experimental challenges and are finally beginning to fulfill the promise of using superfluid helium droplet spectroscopy for the investigation of more ``chemically interesting'' systems. In this talk we will present the infrared spectra of a single HCN molecule bound to copper and silver clusters. From these spectra we were able to obtain information about the adsorbate-metal cluster interactions, as well as obtaining direct structural information through high-resolution spectra. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N11.00006: Adaptive Clustering of Adatoms Around Ionic Dopants in He Droplets: Quantum Calculations Francesco A. Gianturco The structuring and collocation of individual He atoms as quantum objects around simple atomic and molecular impurities has been the subject of a great number of studies, both experimentally and from the theoretical viewpoint [1,2] since the advent of droplets experiments, where such nanoscopic containers have been exploited to provide a sort of nanocryostat for the analysis of the dopant's spectroscopic behavior [3]. We have carried out computations of potential fields within small clusters which contain a variety of ionic dopants using post-Hartree-Fock, ab initio methods and have further endeavoured to extract from them the corresponding classical and quantum structuring of such impurities within clusters of variable size. For the latter enquiry we have employed both classical optimization methods and Quantum Diffusion Monte Carlo analysis. Results for both atomic (Li$^{+})$ and molecular (LiH$^{+}$, OH$^{+}$, OH$^{-})$ ionic dopants will be presented at the meeting. \newline \newline [1] J.P. Toennies and A.F. Vilesov, \textit{Angewandte Chemie} \textbf{43}, 2622 (2004). \newline [2] e.g. see: F. Paesani, A. Viel, F.A. Gianturco and K. Whaley, \textit{Phys. Rev. Lett.} \textbf{90}, 073401 (2003). \newline [3] J.P. Toennies and A.F. Vilesov, {\{}$\backslash $it Ann. Rev. Phys. Chem.{\}} {\{}$\backslash $bf 49{\}}, 1 (1998). [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N11.00007: Electron-impact ionization mass-spectrometry of molecules and clusters in a pulsed helium droplet source Shengfu Yang, Scott Brereton, Andrew M. Ellis A pulsed helium droplet source has been developed and characterized. The nozzle geometry was found to be critical in allowing controlled tuning of helium nanodroplet size by variation of the stagnation pressure and temperature. The average droplet size scales according to a simple {\{}$p$,$T${\}} scaling law, placing pulsed helium nanodroplet sources on a par with cw sources for the first time. Using this pulsed source, the ability of helium nanodroplets to impede ion fragmentation in electron impact mass spectrometry has been explored. A number of haloalkanes and C$_{1}$--C$_{6}$ alcohols were selected as the target species. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others. Parent ion intensities are also enhanced by the helium for alcohols, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. The results obtained are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:24AM |
N11.00008: Electron and Ion Emission from Clusters exposed to Strong Laser Fields Invited Speaker: When clusters interact with intense optical laser pulses energetic and highly charged atomic fragment ions e.g. are generated$^1$. In contrast to atoms the efficiency of the process could be enhanced by choosing a pair of optical delayed pulses instead of a single but more intense femtosecond pulse$^2$. In metals the stronger charging of the clusters can qualitatively be explained by a plasmon enhanced ionization process. We extended our studies and have made a compared analysis of the emission of highly charged ions and energetic electrons the interaction dynamics of intense femtosecond laser fields with nanometer-sized silver clusters. Using a pair of laser pulses with variable optical delay the time-dependent cluster response is resolved. A dramatic increase both in the atomic charge state of the ions and the maximum electron kinetic energy is observed for a certain delay of the pulses. Corresponding Vlasov calculations on a metal cluster model system indicate that enhanced cluster ionization as well as the generation of fast electrons coincide with resonant plasmon excitation.$^3$ \begin{enumerate} \item L.~K{\"o}ller, M.~Schumacher, J.~K{\"o}hn, S.~Teuber, J.~Tiggesb{\"a}umker, and K.-H. Meiwes-Broer, Phys. Rev. Lett. {\bf 82}, 3783 (1999). \item T.~D{\"o}ppner, Th. Fennel, Th. Diederich, J.~Tiggesb {\"a}umker, and K.-H. Meiwes-Broer, Phys. Rev. Lett. {\bf 94}, 013401 (2005). \item Th.~Fennel, G.F. Bertsch, and K.-H. Meiwes-Broer, Eur. Phys. J. D {\bf 29}, 367 (2004). \end{enumerate} [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N11.00009: Energy and angular momentum densities of states of ripplons on the surfaces of helium nanodroplets and bubbles Vitaly Kresin, Michael Johnson, Klavs Hansen We present an analytical evaluation of the statistical densities of states of surface excitations (“ripplons”) of (1) isolated liquid-drop helium nanoclusters and (2) large multielectron bubbles in bulk liquid helium [1]. For the former case, the calculation of the energy density of states, $\rho (E)$, can be accurately performed in a microcanonical ensemble formalism [2] and yields an expression which is extremely close both to the exact numerical calculation and to its fitted form [3]. For case (2) the canonical ensemble formulation is appropriate. For both systems, the calculation is then extended to yield the energy- and angular-momentum- resolved density of states $\rho (E,L)$ (c.f. [3]); in other words, the ripplon moment of inertia is described. \newline \newline [1] J.Tempere, I.F.Silvera, J.T.Devreese, Phys.Rev.Lett. {\bf 87}, 275301 (2001). \newline [2] J.U.Andersen, E.Bonderup, K.Hansen, J.Chem.Phys. {\bf 114}, 6518 (2001). \newline [3] K.K.Lehmann, J.Chem.Phys. {\bf 119}, 3336 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N11.00010: Excited states of He$_{N}$H$^{- }$Clusters Paulo Acioli, Frederico V. Prudente We use correlation function quantum Monte Carlo (CFQMC) method to compute the excited states of the weakly bonded Helium clusters with the H$^{-}$ impurity (He$_{N}$ H$^{-}$, N=1,{\ldots},5). The methodology was tested through comparison with previously published results for the ground state of the system with N=1-11. Our test basis set consists of a standard pair-product ground state multiplied by a polynomial. Our tests for HeH$^{-}$ and He$_{2}$H$^{-}$ demonstrated very good agreement with previously published discrete variable representation (DVR) results. We believe the lowest excited states of the larger clusters to be of similar quality and they can reveal important properties of these weakly bound systems, mainly on the effect of the impurity on the cluster and vice versa. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N11.00011: Microwave Spectroscopy in Helium Nanodroplets. Rudolf Lehnig, Wolfgang Jaeger We have implemented a microwave resonator, i.e. a Fabry-Perot cavity, into a helium nanodroplet instrument. The cavity consists of two spherical aluminum mirrors with radius of curvature of 13 cm and diameter of 10 cm. The cavity is mounted in a coaxial fashion into the instrument to maximize the interaction length between radiation and doped helium droplets. The helium droplet beam enters and exits the cavity through a hole in each of the mirrors. One of the mirrors can be adjusted to tune the cavity into resonance. The output of a cw microwave synthesizer can be amplified by a traveling wave tube amplifier to powers of about 25 Watt and is coupled into the cavity through a simple wire hook antenna. Detection is accomplished using the depletion technique. We have measured the spectrum of the J=2-1 transition of carbonylsulfide demonstrating the sensitivity of this method. Power saturation was observed and will be analyzed as will be the observed line width of the transition. [Preview Abstract] |
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