Surface vibrations of argon clusters by helium atom scattering

In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are generated in the size range of n=23 to n=4600 by adiabatic expansion with 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. The angular dependence allows to distinguish between single and multiphonon excitations. The measured peaks of the averaged transferred energy in the single phonon regime decrease with increasing average cluster size and converges to the values of the surface phonons of solid argon. This behavior is explained by treating the data as collective breathing vibrations of a solid sphere.