State-resolved dynamics of oxygen atom recombination on polycrystalline Ag

Rotationally resolved, velocity distributions for desorbed O-2 molecules formed by O-atom recombination on the surface of a polycrystalline Ag surface are reported. Surface O atoms are generated by oxygen permeation through a 0.25-mm-thick Ag foil heated to 1020 K. Desorbing O-2 molecules are probed by (2+1) resonant multiphoton ionization via the C (3)Pi(g) (3ssigma), nu(')=2<--<--X (3)Sigma(g)(-), nu('')=0 transition and time-of-flight mass spectrometry. Measured velocity distributions are near Maxwell-Boltzmann and yield average translational energies which are significantly lower than the surface temperature ((E-t)/2k(B)similar to515 K) and essentially independent of rotational excitation. Comparison of the observed C-X (2,0) resonantly enhanced multiphoton ionization spectrum with spectral simulations suggests that the nu('')=0 rotational state distribution is more consistent with the surface temperature, but spectral congestion and apparent intensity perturbations prevent a more quantitative analysis. The calculated, sticking curves show a small barrier energy barrier (similar to10 meV) beyond which sticking decreases. These observations are consistent with low energy desorption and adsorption pathways involving a weakly bound molecular O-2 precursor. (C) 2004 American Institute of Physics.