DISSOCIATION DYNAMICS OF FAST NEUTRAL H2 BEAMS INCIDENT AT GLANCING ANGLES TO CU(111)

We have applied the technique of translational spectroscopy to a study of the dissociation dynamics of fast (0.2-0.5upsilon(F)) neutral H-2 beams incident at normal energies from 0.2-8 eV to a Cu(111) surface. Translational energy losses of the order of 1-10 eV/fs in the close coupling region are observed. We only see indirect evidence for surviving molecules at the lowest beam velocities. ''Mechanical'' excitation processes directly related to the intrinsic crystal corrugation do not appear to be responsible for dissociation. We see no clear signature of known features of the H-2/Cu(111) adiabatic ground state potential energy surface in the scattered atom distributions. The spectra are in fact very similar to those obtained under identical conditions using ionized H-2+ beams in the accompanying paper [Surf. Sci. 316 (1994) 63]. The spectral features and their dependencies on incident beam energy, normal energy and centre-of-mass scattering angle are, as for H-2+ scattering, well reproduced by a simulation which assumes that the H-Cu interaction is purely repulsive, and that the H-H interaction is either purely repulsive or even attractive. We propose that the state or states with this effective topography are in fact excited electronic states of the H-2/Cu system, and that repeated transitions to and relaxation of these states occurs during the dissociative scattering trajectory. Classical trajectory simulations of this potential surface hopping mechanism suggest that the mean propagation time on the upper state per cycle is at least as large, if not larger, than the mean propagation time per cycle on the ground state.