Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session H1: Spectroscopy of Molecular Complexes, Clusters, and Aggregates |
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Chair: Y. Xu, University of Alberta Room: TELUS Convention Centre Macleod BC |
Thursday, June 7, 2007 10:30AM - 11:06AM |
H1.00001: Spectroscopy of molecular clusters in He droplets Invited Speaker: Molecular spectroscopic experiments in He droplets will be reviewed. Results of the recent infrared laser study of ammonia and water clusters in He droplets will be presented. Hydrogen bonding in the studied clusters causes an enhancement of the intensity of the hydrogen stretching bands. Two types of the clusters show qualitatively different size dependence of the infrared intensity per hydrogen bond. In ammonia dimers and trimers it is close to the crystal value. In water clusters it increases monotonically with cluster size being in tetramers a factor of two smaller than in the ice. Thus larger water clusters are required in order to obtain the electronic distribution in clusters, which is characteristic for the bulk ice. Study of heterogeneous clusters such as containing water and HCl molecules will also be introduced. In the second part of the talk the utility of the helium droplet technique for growth and study of large atomic and molecular clusters of up to about 10$^{4}$ particles is discussed. Size dependence of the spectra, allows studying the evolution of the system from single molecules to molecular solid and provides information on the structure of the large clusters formed in He droplets. [Preview Abstract] |
Thursday, June 7, 2007 11:06AM - 11:42AM |
H1.00002: Interplay between rotational dynamics, quantum solvation and superfluid response in doped helium clusters Invited Speaker: We show how state-of-the-art Quantum Monte Carlo methods assist in revealing the interplay between rotational dynamics, quantum solvation and superfluid response in doped helium clusters. While strong correlations exist between the size variation of the rotational constant and changes in structure, exchange effects are crucial to explain a non-monotonic evolution of the rotational constant of the complexes. We demonstrate that exchanges facilitate the decoupling between a dopant molecule and helium for cluster sizes where the effective anisotropy of the helium-dopant interaction is reduced due to structural changes. In addition, for the class of molecular dopants with a T-shaped He-molecule dimer configuration, the variations in the rotational constant can be unambiguously related to the non-trivial evolution of the helium superfluid response. This allows one to use a molecular dopant as an experimental probe of superfluidity at the microscopic level. We show that experimental superfluid response builds up in stages correlated with the filling of the solvation layers around the dopant molecule. [Preview Abstract] |
Thursday, June 7, 2007 11:42AM - 12:18PM |
H1.00003: Spectroscopy of Hydrogen clusters : Non-rigidity of large parahydrogen clusters at low temperatures Invited Speaker: Interest in quantum clusters has increased over the last decade. Especially, clusters of molecular hydrogen have been attracted attention because of the possible superfluid phase of hydrogen clusters. Here, we have studied hydrogen clusters (N=1 - 1000) embedded in superfluid He nano-droplets at 0.4 K. Laser induced fluorescence of Mg-phtalocyanine simultaneously doped in droplets showed clear evidence of non-rigidity of parahydrogen clusters at 0.4 K. We will discuss the difference of para and ortho hydrogen clusters as well as Ar clusters in He nano-droplets. Spectra in hydrogen droplets (N=$10^5$, T=4K) will also be discussed. [Preview Abstract] |
Thursday, June 7, 2007 12:18PM - 12:54PM |
H1.00004: Vibrational evidence for chiral recognition phenomena \textit{in vacuo} Invited Speaker: Molecules and molecular conformations which cannot be superimposed on their mirror image are chiral. Chirality or handedness plays an important role throughout the life sciences. When two molecules interact, they can sense their relative handedness, giving rise to spectroscopic signatures of \textit{chiral recognition}. This is often mediated by hydrogen bonds, most versatile and directional intermolecular interactions. If the distinction between the homo- and heteroconfigurational pairs of molecules is large enough, \textit{chiral discrimination}, i.e. differences in abundance may occur. The contact between two flexible, transiently chiral molecules may induce a matching of their handedness, i.e. \textit{chirality synchronization}. Such phenomena are best studied at low temperatures in vacuum isolation, without perturbing interactions [1]. Structural information on the isolated molecular complexes can be obtained by rotational spectroscopy [2], if there is a sufficient dipole moment. Vibrational spectroscopy [3] provides a more universal, but also more coarse-grained access to these phenomena. Our group has reported the first spectroscopic example of chiral recognition between constitutionally identical molecules in the gas phase [4]. We have found a case of chiral discrimination in tetrameric aggregates of methyl lactate, where the relative configuration controls the hydrogen bond topology [5]. In the case of alcohols, we have observed different degrees of chirality synchronization up to a quantitative chirality matching in dimers of trifluoroethanol [6]. These discoveries became possible through the use of a powerful combination of FTIR spectroscopy and high-throughput, pulsed supersonic nozzle expansions into large vacuum chambers [7]. The isolated and elementary character of the investigated molecular assemblies is favourable for quantum chemical treatments [8]. Valuable benchmarks for the modeling of more complex chiral recognition phenomena are thus established. \newline [1] A. Al-Rabaa, E. Br\'{e}h\'{e}ret, F. Lahmani, A. Zehnacker, Chem. Phys. Lett. 1995, 237, 480 \newline [2] J.P.I. Hearn, R.V. Cobley, B.J. Howard, J. Chem. Phys., 2005, 123, 134324; Z. Su, N. Borho, Y. Xu, J. Am. Chem. Soc. 2006, 128, 17126 \newline [3] K. Le Barbu, F. Lahmani, A. Zehnacker, J. Phys. Chem. A, 2002, 106, 6271 \newline [4] N. Borho and M. A. Suhm, Phys. Chem. Chem. Phys., 2002, 4, 2721 \newline [5] N. Borho and M. A. Suhm, Org. Biomol. Chem., 2003, 1, 4351 \newline [6] T. Scharge, T. H\"{a}ber, M. A. Suhm, Phys. Chem. Chem. Phys., 2006, 8, 4664 \newline [7] N. Borho, M. A. Suhm, K. Le Barbu-Debus, A. Zehnacker, Phys. Chem. Chem. Phys., 2006, 8, 4449 \newline [8] T. B. Adler, N. Borho, M. Reiher, M. A. Suhm, Angew. Chem. Int. Ed., 2006, 45, 3440 [Preview Abstract] |
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