Vibrational excitation of ammonia clusters by helium atom scattering

In a crossed molecular beam arrangement helium atoms are scattered from ammonia clusters of the averaged sizes n=18, 745, and 1040 which are generated by isentropic expansions with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of less than 5 meV at a collision energy of 95 meV. The energy transfer increases with increasing deflection angle and extends to 65 meV. Intensity maxima are observed between 11 and 16 meV, around 27 meV, and at 33 meV. The one in the middle is attributed to the vibration of a specific cluster network, while the other two occur in the energy regime of the translational or librational modes of the solid with a preference for small or large clusters, respectively.