Surface Vibrations of Large Water Clusters by He Atom Scattering

Abstract

The low energy intermolecular vibrational modes of water clusters for the average size n 110 have been measured by high resolution inelastic helium atom scattering. The water clusters are generated in adiabatic expansions through conical nozzles. By accompanying quantum and classical calculations the excited mode at 5.0 meV has been identified as O? ?O? ?O bending motion between adjacent hydrogen bonds, involving 3-coordinated water molecules on the amorphous cluster surface. Experiments for different sizes show that the frequency and thus the force constant of this mode increase for larger cluster sizes. [S0031-9007 (98)05739-1] PACS numbers: 36.40.Sx, 34.50.Dy, 68.35.JaWeakly bonded clusters have been a focus of intense interest during the past few years. One of the major objectives 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 developed 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 between single phonon and multiphonon excitation [3] using the measured angular dependence, in conjunction with calculations. Here the method is applied to the investigation of vibrational surface properties of water particles. Water clusters are of particular interest because of the importance of water in terrestrial phenomena, and because of the role of icy particles in atmospheric and extraterrestrial physics and chemistry. Electron diffraction studies of ͑H 2 O͒ n up to sizes of several hundred molecules demonstrated a noncrystalline structure [4], while spectroscopic studies of ice nanocrystal surfaces and surface-adsorbate systems showed persistence of amorphous surface structure to larger sizes, for which the particle interior is crystalline [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.For bulk hexagonal ice these modes have been measured by neutron scattering. In the range from 2 to 40 meV the peaks are attributed to the hindered intermolecular motion and interpreted as acoustical (7.1 meV) and optical (28.4 and 37.9 meV) modes [6], the splitting between the latter ones still being a subject of controversy [6,7]. The modes near 7.1 meV have also been interpreted as O? ?O? ?O bending between adjacent hydrogen bonds [8], while the higher frequency modes have been assigned to hydrogen bond stretching [6,8]. For clusters, extensive calculations are available for some sele...

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