Vibrational state dependence of D2 dissociation on Ag(111)

Abstract

Initial sticking probabilities for D2 dissociative chemisorption at a Ag(111) surface have been measured for translational energies in the range Ei=220–500 meV, as a function of incident angle θi and gas temperature, using seeded molecular beams. Sticking probabilities are dependent on the D2 internal state distribution and scale with the normal component of the translational energy. The data has been fit by assuming that dissociation is independent of molecular rotation, being the sum of contributions from molecules in different vibrational states v with a sticking function S0(Ei,θi,v)=A/2{1+tanh[Ei cos2 θi−E0(v)]/w(v)}, in a manner similar to the behavior on copper. Sticking parameters E0, the translational energy required for S0 to reach half of its maximum value, are determined with good precision (±25 meV) for levels v=3 (328 meV) and v=4 (170 meV) with width parameters w=54 and 63 meV, respectively, while the barriers for levels v=1 and 2 are close to the upper limit of the sticking data and E0 is estimated as 700±100 and 510±70 meV, respectively. Parameters for the vibrational ground state (v=0) could not be obtained, since sticking of this state is negligible at translational energies less than 500 meV. No dissociation could be observed at thermal energies Ei⩽70 meV either on a flat or a defective surface.

Deuterium dissociation on ordered Sn/Pt(111) surface alloys

“…Thus if S 0 (E Ќ ) is measured at a constant nozzle temperature by increasing i , the values obtained will be larger than obtained at the same normal energy by dropping the nozzle temperature, since the latter decreases the population of vibrationally excited molecules in the beam. This is the behavior observed on Cu, 13,25 Ag, 14 and the (2ϫ1) Cu 85 Pd 15 (110) 26 surfaces, where vibrationally excited states dominate sticking and S 0 (E Ќ ) curves obtained for a constant nozzle temperature, by increasing i , drop slowly by comparison with the normal incidence curve. Sticking probabilities on the ) alloy were measured as a function of incidence angle at various fixed nozzle temperatures, in order to investigate the role of vibrational energy and the energy scaling relationship.…”

“…However, this function does not give satisfactory fits to the data from the ) alloy, which shows an exponential increase in S(E) followed by a linear increase in S(E) at higher energy, and cannot be described by any single values of E 0 and w. Despite the large barrier D 2 dissociation on the ) alloy shows no evidence for vibrational enhancement, in contrast to other activated dissociative chemisorption systems such as H 2 /D 2 on Cu, 13,25 Cu 85 Pd 15 (110)(2 ϫ1), 26 NiAl͑110͒, 16 and Ag͑111͒, 14 as well as dissociation of CH 4 ͑Ref. 33͒ and N 2 ͑Ref.…”

“…Noble metals on the other hand have filled d-bands and are characterized by large activation barriers to H 2 /D 2 dissociation, 8 while sticking shows a threshold behavior, becoming efficient over a relatively narrow range of a͒ FAX: 0151 708 0662; electronic mail: andrewh@ssci.liv.ac.uk energy. 13,14 Similarly, Al surfaces have no d electron density of states at the Fermi level and have an extremely high barrier to H 2 dissociation. 15 Substitution of a metal with no dband holes into a transition metal surface may alter the reactivity of the surface towards H 2 dissociation by modifying the local surface electronic structure as well as by blocking adsorption sites.…”

“…Dissociation of H 2 /D 2 on transition metals 8 such as Ni, 9 Pt, 10 and Fe 11,12 is facile but becomes increasingly activated for metals such as Cu 13 and Ag 14 where the d band lies well below the Fermi level. For Pt͑111͒ and Fe͑110͒ sticking occurs at thermal energies and a steady increase in sticking probability with translational energy is observed, reflecting dissociation over a distribution of barriers corresponding to different collision geometries, molecular orientations, and sites in the surface unit cell.…”

Role of Surface Thermal Motion in the Dissociative Chemisorption and Recombinative Desorption ofD2on Ag(111)

“…Sticking has been modeled using "S " shaped vibrational state dependent sticking functions of the form S͑y͒ A͑y͒͞2͕1 2 tanh͓E i 2 E 0 ͑y͔͒͞w͑y͖͒, where the dissociation probability S͑y͒ is determined by three parameters, A͑y͒ the limiting sticking probability at high energy, E 0 ͑y͒ the median barrier height, and w͑y͒ the effective width of the barrier distribution. For D 2 dissociation, parameters E 0 328 and 170 meV were obtained for y 3 and 4, with estimates for y 1 and 2 of ͑700 6 100͒ and ͑510 6 70͒ meV, respectively, while dissociation of ground state D 2 ͑y 0͒ could not be observed even at 800 meV [5].An intense beam of D atoms was generated using a microwave powered beam source and collimated before striking a Ag(111) crystal at 45 ± . The crystal was cleaned and characterized using standard techniques and reproduced the desorption kinetics reported previously [3].…”