Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid water films show that this process is extremely efficient in a temperature range of 8 to 20 kelvin, temperatures relevant for H2 formation on dust grain surfaces in the interstellar medium (ISM). The fate of the 4.5 electron volt recombination energy is highly dependent on film morphology. These results suggest that grain morphology, rather than the detailed chemical nature of the grain surface, is most important in determining the energy content of the H2 as it is released from the grain into the ISM.

This paper reports the simultaneous internal state and translational energy resolved associative desorption flux of N 2 from Ru͑0001͒ using two different experimental approaches. Both experiments show that the nascent N 2 is formed with little vibrational excitation and that the total excitation in all N 2 degrees of freedom accounts for only 1 3 of the barrier energy. Roughly 2 3 of the energy necessary to surmount the barrier is lost to the surface in desorption. This behavior, as well as the unusual behavior noted previously in direct measurements of dissociative adsorption, both imply strong vibrational quenching in reactive trajectories passing over the high exit channel ͑vibrational͒ barrier. Adiabatic quasiclassical dynamical calculations based on the ab initio potential energy surface and various models of coupling to the lattice are not qualitatively consistent with N 2 vibrational damping to phonons. However, including a strong nonadiabatic coupling of the vibrational coordinate to electron-hole pairs in the dynamics does yield qualitative agreement between experiments and calculated dynamics, and we suggest this as indirect evidence for strong nonadiabatic coupling. We argue that the nonadiabatic coupling is strong in this case because of the high vibrational excitation necessary to pass over the high exit channel barrier in the reactive processes and the large charge transfer inherent in making or breaking bonds. We believe that the same factors will be important in most activated dissociations of bonded molecules on transition metal surfaces, e.g., for O 2 , NO, N 2 , and CO, and if this scenario is correct then nonadiabaticity should be important in the activated dissociation dynamics of these systems as well.

The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water (ASW) films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 (D2) to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.

New molecular beam experiments on the dissociation probability S 0 for N 2 on Ru͑0001͒ are presented. These are in general agreement with prior measurements and exhibit very unusual behavior; a very slow increase of S 0 with incident kinetic energy E and the fact that S 0 is still only ϳ10 Ϫ3 at incident energies considerably above the barrier. A simple dynamical model is developed to describe this unusual sticking behavior. The key aspect is that there is considerable energy loss ⌬ from E upon initial impact with the surface ͑principally to the lattice͒ and only EϪ⌬ is then available to surmount the activation barrier in the exit channel. Using experimentally measured values of ⌬ from scattering experiments gives good qualitative agreement of this model with the measured S 0. One implication of the strong energy loss is that there is an apparent violation of detailed balance when comparing only the reactive fluxes of activated adsorption and associative desorption.