Surface vibrations of large water clusters by He atom scattering

The low energy intermolecular vibrational modes of water clusters for the average size (n) over bar = 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.