TITLE: Intermolecular vibrations of large ammonia clusters from helium atom
             scattering

 AUTHORS: Titus A. Beuthanks-University "Babes-Bolyai", Faculty of Physics,
                   Department of Theoretical Physics, 3400 Cluj-Napoca, Romania

                   Christof Steinbach, Udo Buck-Max Planck Institut fuer
                   Stroemungsforschung, Bunsenstrasse 10, D-37073 Goettingen,
                   Germany

ABSTRACT:

The excitation of the low energy intermolecular modes of ammonia clusters by helium atom scattering has been calculated using classical trajectories. The energy transfer is investigated as a function of the scattering angle, the collision energy (95 and 50 meV), the cluster size (n = 18, 100, 1000), and the cluster temperature (Tc = 1 K, 30 - 50 K, and 105 K). It is observed that predominantly the mode at 7 meV and to a lesser extent also the one at 12 meV are excited. These are surface modes that mainly originate from the angular motion of three adjacent N atoms. The excitation is nearly independent of the cluster size and the probability for multi-phonon excitation steadily increases with increasing deflection angle.
This trend is even strengthened by increasing the collision energy. The role of the cluster temperature is to broaden the energy transfer distribution with increasing values. The calculations are compared with new measurements of the double differential cross sections of ammonia clusters of the average size \langle n \rangle = 92 
at two collision energies and 
\langle n \rangle = 1040 at one energy. 
While the general trends in the angular and energy dependence could be well reproduced, the correct cluster temperature was crucial in getting better agreement with the large energy transfer.