Collisional energy loss in cluster surface impact: Experimental, model, and simulation studies of some relevant factors

Measurements of the collisional energy transfer of size and energy-selected ammonia cluster ions (NH3)(n)H+, n = 1 - 10, impacting a silicon wafer coated with p-type diamond film are reported. The transfer from translational energy of the incident cluster ions to kinetic energy of intact scattered cluster ions has been studied as a function of impact energy, surface composition, and size of the impinging cluster cations. For low impact energies (< 2.5 eV/molecule), cluster ions scattered off the target surface lost most of their initial kinetic energy, while for higher impact energies the elasticity of the cluster-surface collision is surprisingly high: Typically 75% of the impact kinetic energy is retained by the scattered parent clusters. Larger cluster ions are scattered less elastically and a large fraction of them shatter to small(est) fragments. The molecular dynamics simulations examine the two energy disposal regimes, deep inelasticity and shattering. Deep inelastic scattering occurs already below the lowest impact energies probed by the experiment. At higher collision energies, the energy loss continues to increase but a point is reached where most clusters shatter. Those few clusters that rebound intact have lost a disproportionately low fraction of their initial energy. The simulations also explore the cluster size effects, the role of the attraction to the surface, and the importance of the anisotropic forces between the molecules in the cluster. The experimental results and the simulations are discussed using the hard cube model with special reference to collective effects. (C) 1998 American Institute of Physics.