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 ener gy selected ammonia clusters, (NH3)(n)NH4+, n = 1-11, impacting a sili con wafer coated with p-type diamond film are reported. The transition from the recoil of the intact parent cluster to the complete fragment ation into monomers has been studied as a function of the energy of im pact. Two experimental methods are used to place an upper bound on the time duration of the shattering resulting in < 120 ps and < 80 ps, re spectively. The experimental results are discussed in terms of the ana lytical theory of the shattering transition and compared also to compu tational output obtained by molecular dynamics. The simulations show t hat while shattering is practically instantaneous upon impact, evapora tion of the impact heated clusters is much slower and there is a clear separation of time scares between the two processes. The shattering o f a cluster, when the constituents move apart in concert, is also of i nterest for cluster impact induced chemistry. (C) 1998 Elsevier Scienc e B.V.