VIBRATIONAL-STATE DEPENDENCE OF D-2 DISSOCIATION ON AG(111)

Initial sticking probabilities for D-2 dissociative chemisorption at a Ag(111) surface have been measured for translational energies in the range E(i)=220-500 meV, as a function of incident angle theta(i) and g as temperature, using seeded molecular beams. Sticking probabilities a re dependent on the D-2 internal state distribution and scale with the normal component of the translational energy. The data has been fit b y assuming that dissociation is independent of molecular rotation, bei ng the sum of contributions from molecules in different vibrational st ates upsilon with a sticking function S-0(E(i),theta(i),upsilon)=A/2{1 +tanh[E(i) cos(2) theta(i)-E(0)(upsilon)]/w(upsilon)}, in a manner sim ilar to the behavior on copper. Sticking parameters E(0), the translat ional energy required for S-0 to reach half of its maximum value, are determined with good precision (+/-25 meV) for levels upsilon=3 (328 m eV) and upsilon=4 (170 meV) with width parameters w=54 and 63 meV, res pectively, while the barriers for levels upsilon=1 and 2 are close to the upper limit of the sticking data and E(0) is estimated as 700+/-10 0 and 510+/-70 meV, respectively. Parameters for the vibrational groun d state (upsilon=0) could net be obtained, since sticking of this stat e is negligible at translational energies less than 500 meV. No dissoc iation could be observed at thermal energies E(i) less than or equal t o 70 meV either on a flat or a defective surface. (C) 1997 American In stitute of Physics.