ISOTOPE EFFECTS IN THE DISSOCIATION DYNAMICS OF ENERGETIC HYDROGEN SCATTERED FROM CU(111) UNDER GLANCING ANGLES OF INCIDENCE

We have scattered fast beams of H-2 and D2 and their positive ions from Cu (111) under glancing angles of incidence. By directly measuring the relative velocity nu of the dissociation fragments we obtain the kinetic energy released in the internuclear coordinate as a function of the final orientation of the internuclear axis. These distributions exhibit a strong isotope effect for the case of neutral beam scattering, and no isotope effect for incident ion beam scattering. We propose a dissociation mechanism for neutral molecules which assumes multiple electronic excitation-relaxation cycles during scattering, with the detailed dynamics determined by the propagation times per cycle on each of the participating states. We attribute the observed isotope effect to a velocity induced change in the electron-hole pair excitation density, which determines the propagation time of the system on the lower state. We propose that the dissociation dynamics of H-2+ and D2+ ions at Cu(111) is determined both by this mechanism (for charge transfer to the X 1SIGMA(g)+ state) and direct dissociation via the b 3SIGMA(u)+ state, with a branching ratio of the order of unity.