ADSORPTION AND DESORPTION DYNAMICS OF H-2 AND D-2 ON CU(111) - THE ROLE OF SURFACE-TEMPERATURE AND EVIDENCE FOR CORRUGATION OF THE DISSOCIATION BARRIER

We report the effect of surface temperature on the state resolved tran slational energy distributions for H-2 and D-2 recombinatively desorbe d from Cu(111). Sticking functions S(upsilon,J,E) can be obtained by a pplying detailed balance arguments and follow the familiar error funct ion form at high energy, consistent with previous permeation measureme nts [Rettner et al., J. Chem. Phys. 102, 4625 (1995)]. The widths of t he sticking functions are identical for both isotopes and are independ ent of rotational state. S(E) broadens rapidly with increasing surface temperature, with a low energy component which is slightly larger tha n represented by an error function form. This is similar to the behavi or seen on Ag(111) [Murphy et al., Phys. Rev. Lett. 78, 4458 (1997)] b ut on Cu(111) the low energy component remains a minor desorption chan nel. The broadening of S(E) can be explained in terms of a change in t he distribution of barriers caused by local thermal displacement of th e surface atoms, thermal activation of the surface producing sites whe re molecules can dissociate, or desorb, with a reduced translational a ctivation barrier. At low energy sticking increases rapidly with surfa ce temperature, with an activation energy of 0.54 and 0.60 eV for H-2 and D-2, respectively. These values are similar to the thermal activat ion energies calculated for translational excitation of H-2/D-2 and im ply that thermal excitation of the surface is just as efficient as tra nslational energy in promoting dissociation. The influence of surface temperature decreases with increasing translational energy as molecule s become able to dissociate even on the static Cu(111) surface. By com paring the energy distributions for desorption with existing angular d istributions we determine how the effective energy, E-e = E cos(n(E)) theta which contributes to adsorption-desorption, scales with translat ional energy. At translational energies near the threshold for stickin g n(E) approximate to 2, sticking scales with the normal component of the translational energy and is not influenced by motion parallel to t he surface. At lower energy n(E) drops towards zero, indicating that m otion parallel to the surface aids dissociation, consistent with disso ciation at a corrugated barrier. (C) 1998 American Institute of Physic s.