Surface vibrations of large water clusters by helium atom scattering

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 (n) over bar=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 (n) over bar=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. (C) 2000 American Institute of Physics. [S0021-9606(00)01624-X].