He-atom scattering as a probe of cluster vibrational dynamics

In a crossed molecular beam arrangement helium atoms are scattered from argon, ammonia, and water clusters which are generated in the size range of n = 25 to 4600 by adiabatic expansion through conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of better than 1 meV. For Ar the peak of the transferred energy is interpreted as compressional mode of a dense solid sphere. The positions of the maximum decrease as a function of the increasing average cluster size and converge to the value of the surface phonons of solid argon. The molecular clusters behave differently; the excitation of librational modes and of more than one peak in the transferred energy are observed.