The transition from recoil to shattering in cluster-surface impact: An experimental and computational study

Size and kinetic energy distributions of the products of size and energy selected ammonia clusters, (NH3)(n)NH4+, n = 1-11, impacting a silicon wafer coated with p-type diamond film are reported. The transition from the recoil of the intact parent cluster to the complete fragmentation into monomers has been studied as a function of the energy of impact. Two experimental methods are used to place an upper bound on the time duration of the shattering resulting in < 120 ps and < 80 ps, respectively. The experimental results are discussed in terms of the analytical theory of the shattering transition and compared also to computational output obtained by molecular dynamics. The simulations show that while shattering is practically instantaneous upon impact, evaporation of the impact heated clusters is much slower and there is a clear separation of time scares between the two processes. The shattering of a cluster, when the constituents move apart in concert, is also of interest for cluster impact induced chemistry. (C) 1998 Elsevier Science B.V.