This paper reports detailed molecular beam measurements of the dissociative chemisorption probability for methane on a Pt(111) surface. We find large increases in the dissociative chemisorption probability S0 with increases in Ei cos2 θi (the normal component of translational energy), Ev (the vibrational energy of the incident methane), and Ts (surface thermal energy). The comparable activation of the reaction by addition of any of these three forms of energy cannot be accounted for by any single model for C–H bond activation proposed to date. A large kinetic isotope effect is also observed, with S0 decreasing significantly for CD4 relative to CH4.
This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O2 on a Pt(111) surface as a function of initial energy (Ei), angle of incidence (θi), and surface temperature (Ts). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi-direct dissociation can be observed, depending upon the initial conditions. The quasi-direct process is revealed by a step increase in the sticking with Ei. This feature scales intermediately between Ei and the normal component En, and is weakly dependent on Ts. The precursor mediated sticking is well described by standard precursor kinetic models. At low Ei and Ts, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with Ei than anticipated from simple models and scales intermediately between Ei and En. The sticking results are discussed in terms of likely dynamic processes occurring on a potential energy surface which contains an intermediate molecularly adsorbed species formed by charge transfer from the metal to the O2.
Angular variations in the kinetic energy of scattered species are found to provide a useful probe of the transition between gas-surface scattering regimes, complementing angular flux distributions. As incidence energies exceed a few eV, these change from being consistent with scattering from an extended target to being more typical of scattering from individual atoms. Results are presented for the Xe/Pt(l 11) system and are supported by detailed trajectory calculations.PACS numbers: 79.20.Rf An understanding of the dynamics of energy transfer at the gas-surface interface is required for detailed modeling of many different chemical and physical phenomena associated with this interface. These range from the trapping and sticking of atoms and molecules at relatively low en- ergies [1], to sputtering, plasma etching, and implantation at hyperthermal energies [2], Such knowledge is also of value in the design of spacecraft [3] and thermonuclear fusion reactors [4]. Molecular-beam scattering techniques offer a powerful tool for probing such interactionsand have been employed to examine many different systems. However, most studies have concerned angular distributions of scattered species for relatively low incidence energies, providing only a limited picture of the scattering dynamics. Since angular distributions reflect both the static corrugation of the gas-surface potential and differential momentum transfer parallel and perpendicular to the surface, velocity measurements are required for unambiguous interpretation. While high-energy collisions have been recognized as qualitatively distinct from those at low energy for many years [5-12], there is little experimental data that directly relate to the transition between these regimes.In this Letter we report results for the scattering of Xe from Pt(lll) which clearly show that variations in the energies of scattered species with scattering angle can be used to characterize the degree of penetration. At low incidence energies, ZT, < 1 eV, we find that the energies after scattering, £/, decrease with increasing scattering angle Of in a manner approximately consistent with parallel momentum conservation. At high energies, E,-> 5 eV, the opposite trend is observed, with E/ increasing with increasing final angle, in a manner consistent with scattering from one or more individual surface atoms. These qualitative conclusions are supported by detailed trajectory calculations.The molecular-beam surface scattering apparatus and the experimental techniques appropriate to this study have been described elsewhere [12][13][14]. The mounting of the Pt(l 11) crystal is such that thescattering plane intercepts the (111) face close to the [121] azimuth. Contamination levels are below our Auger detection limits (1%), sharp LEED patterns are obtained, and He scattering gives a specular peak width indistinguishable from the in-
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