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

The scattering of neutral ammonia clusters off a LiF(100) surface is studied. A supersonic expansion of ammonia and ammonia seeded in Kr and He produces clusters of various sizes but uniform kinetic energies of 176, 57 and 285 meV per monomer molecule, respectively. The mass distribution of scattered particles is measured in a reflecting time-of-flight mass spectrometer using single-photon photoionization with vacuum ultraviolet (VUV) laser radiation at λ=118 nm (hν=10.49 eV). While in the incident beam (NH3)n clusters up to n=65 can be detected, the mass spectrum of the scattered particles is dominated by NH3+, and only a few small clusters are found. Angular distributions of these fragments show that the maximum of the scattered intensity shifts to greater detection angles for increasing fragment mass. Velocity distributions of released monomers are measured for all three cluster beams and found to be independent of the impact velocity of the clusters. 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.