Kinetics, mechanism, and dynamics of the gas-phase H(D) atom reaction with adsorbed D(H) atom on Pt(111)

We have investigated the kinetics of the abstraction reaction H(D)+D-ad(H-ad)/Pt(111) at 100 K and saturation coverage (theta(sat) = 0.95 +/- 0.06 ML) using an H(D) atom beam, in which the angle-integrated and angle-resolved product desorption rates were simultaneously monitored with two mass spectrometers. HD molecules are formed by the abstraction reaction as well as by the secondary hot atom (s-HA) reactions, D-s*(H-s*) + H-ad(D-ad)--> HD, where D-s*(H-s*) is a collisionally excited surface D(H) atom. The two reaction components of HD show quite distinct angular distributions; while the former component is sharply forward-peaked to be represented by cos(12)(theta(f)-3 degrees), the latter component preferentially desorbs at large desorption angles centered at theta(f)similar to 45 degrees. The two HD formation reactions also exhibit distinct kinetics, which could be separately identified by properly selecting the desorption angle. Concurrent desorption of D-2(H-2) formed by a homonuclear s-HA reaction was also observed with a relatively large yield amounting to 37%(31%) of the initially adsorbed D(H) atoms. The angular distribution of D-2 is very similar to that of HD formed by the s-HA reactions. Varying the beam incidence angle has no effect on the reaction rate constants and the product branching ratio. From a kinetic analysis, we estimate a cross section sigma(abst) = 1.30 +/- 0.07(1.49 +/- 0.11) Angstrom(2) for HD formation by abstraction in H(D)-on-D-ad(H-ad) reaction. For D-2(H-2) formation, an effective cross section for generating reactive D-s*(H-s*) atom is estimated as sigma* = 1.87 +/- 0.08(1.61 +/- 0.24) Angstrom(2). These values can be translated into the probabilities P-abst = 0.19(0.21), P-ex = 0.27(0.23), P-ads = 0.73(0.67), and P-scatt = 0.08(0.12) for abstraction reaction, s-HA generation, adsorption, and scattering of an incident H(D) atom, respectively. The isotope effects are small and the corresponding cross sections differ at most by 15%. The mechanism-dependent product angular distributions are discussed in terms of the different reaction dynamics from the view points of the surface potential corrugation experienced by the energetic hydrogen atoms (incident, primary, and secondary hot atoms) and the extent of the parallel momentum conservation in their reactions. (C) 2000 American Institute of Physics. [S0021-9606(00)70531-9].