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 gas temperature, using seeded molecular beams. Sticking probabilities are dependent on the D-2 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 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 similar to the behavior on copper. Sticking parameters E(0), the translational energy required for S-0 to reach half of its maximum value, are determined with good precision (+/-25 meV) for levels upsilon=3 (328 meV) and upsilon=4 (170 meV) with width parameters w=54 and 63 meV, respectively, 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+/-100 and 510+/-70 meV, respectively. Parameters for the vibrational ground state (upsilon=0) could net be obtained, since sticking of this state is negligible at translational energies less than 500 meV. No dissociation could be observed at thermal energies E(i) less than or equal to 70 meV either on a flat or a defective surface. (C) 1997 American Institute of Physics.