Surface Vibrations of Large Water Clusters by He Atom Scattering

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Abstract
Weakly bonded clusters have been a focus of intense interest during the past few years. One of the major ob- jectives is to understand how the particles evolve as a function of size towards condensed phase behavior. The evolution of particle properties is influenced strongly by the large surface-to-bulk ratio, and therefore experimental tools sensitive specifically to the cluster surface properties are of interest. A unique tool of this kind has been devel- oped in our laboratory, which probes surface vibrations by inelastic energy exchange with He atoms scattered from the particles (1). In fact, we were able to demonstrate that in helium atom - argon cluster collisions surface modes were predominantly excited (2). One can distinguish be- tween single phonon and multiphonon excitation (3) us- ing the measured angular dependence, in conjunction with calculations. Here the method is applied to the investi- gation of vibrational surface properties of water particles. Water clusters are of particular interest because of the im- portance of water in terrestrial phenomena, and because of the role of icy particles in atmospheric and extraterres- trial physics and chemistry. Electron diffraction studies of sH2Odn up to sizes of several hundred molecules demon- strated a noncrystalline structure (4), while spectroscopic studies of ice nanocrystal surfaces and surface-adsorbate systems showed persistence of amorphous surface struc- ture to larger sizes, for which the particle interior is crys- talline (5). Thus, past studies indicate the properties of small ice particles that are quite distinct from those of bulk ice, and surface properties that are distinct from the interior. The present study focuses on these aspects of cluster behavior, for low energy, hydrogen bond vibration frequencies.