Angular and velocity distributions of small cluster fragments in neutral (NH3)(n) scattering off LiF(100)

The scattering of neutral ammonia clusters off a LiF(100) surface is studie d. A supersonic expansion of ammonia and ammonia seeded in Kr and He produc es clusters of various sizes but uniform kinetic energies of 176, 57 and 28 5 meV per monomer molecule, respectively. The mass distribution of scattere d particles is measured in a reflecting time-of-flight mass spectrometer us ing single-photon photoionization with vacuum ultraviolet (VUV) laser radia tion at lambda = 118 nm (h nu = 10.49 eV). While in the incident beam (NH3) (n) clusters up to n = 65 can be detected, the mass spectrum of the scatter ed particles is dominated by NH3+, and only a few small clusters are found. Angular distributions of these fragments show that the maximum of the scat tered intensity shifts to greater detection angles for increasing fragment mass. Velocity distributions of released monomers are measured for all thre e cluster beams and found to be independent of the impact velocity of the c lusters. The velocity distributions of scattered fragments larger than NH3 show a decreasing width with increasing fragment size and a most probable velocity slightly higher than that of the monomer molecules. (C) 1998 Elsev ier Science B.V. All rights reserved.