The vibrational OH stretch spectra have been measured for the size-selected pure water clusters (H2O)7. In contrast to (H2O)n, n=8–10 clusters, which exhibit three distinct bands corresponding to three distinct types of OH bonds, the heptamer spectrum displays seven bands spanning the range from 2935 to 3720 cm−1. Calculations suggest that the spectra originate from two isomers, derived from the S4 octamer cube by removal of either one double donor or one double acceptor water molecule.

The vibrational OH-stretch spectra of size-selected pure water (H 2 O) 9 clusters, measured at two different temperatures, have been used to identify isomeric transitions. The broad, featureless high-temperature, liquidlike spectrum resembles that of water, while the structured low-temperature, solid-like spectrum differs appreciably from that of ice. Accompanying calculations suggest that the low-temperature spectra originate from two cube-like structures. The high-temperature spectra appear to be above an isomeric transition to more open amorphous arrangements of fused three-to five-membered rings.

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...

Elastic and rotationally inelastic cross sections have been measured for He+H2O scattering at two collision energies, 66.3 and 99.0 meV, using the crossed molecular beam technique. The inelastic events are detected by time-of-flight analysis of the scattered He atoms. The data are converted to elastic differential cross sections and inelastic angular-dependent energy loss spectra in the center-of-mass system. They are compared with averaged, full close-coupling calculations of state-to-state cross sections for rotational excitation based on a newly calculated ab initio potential using symmetry-adapted perturbation theory. The agreement with the elastic differential cross sections is excellent. The energy loss spectra are reproduced satisfactorily and among the largest differential cross sections that contributed to the measurements are excitations around all three possible axes for ΔJ=1 but a preference of the excitation around the in-plane C axis for ΔJ=2 transitions.

The low energy intermolecular vibrational modes of water clusters have been measured by high resolution inelastic helium atom scattering. The water clusters are generated in adiabatic expansions through conical nozzles at the mean sizes n̄=22, 80, and 194. By accompanying semiclassical and classical calculations for n=90 the excited mode near 5.1 meV has been identified as O⋅⋅O⋅⋅O bending motion between adjacent hydrogen bonds, involving 3-coordinated water molecules on the amorphous cluster surface. The energy of this mode increases slightly from 4.3 to 5.5 meV with increasing cluster size from n̄=22 to 194 and approaches the results for the dispersionless surface phonons of ice. The cluster temperature determined from the deexcitation is between 69 and 101 K.