I. Harrison

A statistical model for activated dissociative adsorption: Application to methane dissociation on Pt(111)

A statistical model of activated dissociative adsorption is developed using microcanonical, unimolecular rate theory. Dissociation is treated as occurring through energy randomizing collisions between incident molecules and local clusters of surface atoms. The predictions of the statistical model are found to be in remarkable accord with existent experimental data for methane dissociative adsorption and the thermal hydrogenation of methyl radicals on Pt(111). Perhaps surprisingly, the ‘‘over the barrer’’ statistical model adequately describes the known kinetics of these reactions without any explicit provision for quantum tunneling.

Photofragmentation dynamics of submonolayers of CH3Br adsorbed on Pt(111)

Angle‐resolved, photofragment translational energy distributions, Pϑ(ET)s, arising from 308 nm laser irradiation of CH3Br adsorbed on Pt(111) are presented. Methyl photofragments were observed with translational energies extending to 2 eV and with Pϑ(ET)s which varied sharply with angle of exit from the surface. The fragmentation dynamics were consistent with a mechanism of dissociative electron attachment of subvacuum level, photoexcited substrate electrons to adsorbed CH3Br. The CH3 Pϑ(ET)s and the angular variation of the CH3 yield gave evidence that submonolayers of CH3Br form islands on Pt(111).

Photoreaction dynamics of CO oxidation on Pt(111)

The dynamics of the surface aligned photoreaction between submonolayers of O2 and CO coadsorbed on a Pt(111) surface at 25 K and laser irradiated at 308 nm are explored by measurement of angle‐resolved translational energy distributions for the CO2 photoproduct.

Single hop diffusion of CO from bridge to top sites on Pt(111)

The isothermal kinetics of single hop diffusion of CO from bridge sites to top sites of Pt(111) have been examined by reflection absorption infrared spectroscopy over the temperature range from 37–42 K.

Photoinduced dissociative electron attachment of CH3Br on Pt(111): The role of the local work function

The photoinduced dissociative electron attachment of CH3Br on Pt(111) has been examined by measuring CH3 photofragment translational energy distributions from CH3Br adsorbed on Xe multilayers deposited upon CH3Br covered Pt(111). Subvacuum level, photoexcited substrate electrons produced by 308 nm laser irradiation were found to propagate through the Xe layer where attachment to CH3Br led to fragmentation. A simple model for surface dissociative electron attachment, DEA, was found to quantitatively predict the observed fragmentation dynamics for the CH3Br/Xe/CH3Br/Pt(111) system. The role of the local work function in surface DEA processes was explored by examining the dependence of the photofragment yield upon the thickness of the intermediate Xe layer and the fractional coverage of the Pt(111) bound first layer of CH3Br. Three dimensional variation of the local work function above the substrate was required to account for the observed DEA dynamics.

Photochemical hydrogenation of methyl radicals on Pt(111)

Submonolayers of methyl bromide and hydrogen were coadsorbed on a 25 K Pt(111) surface and laser irradiated at 308 nm. Photoinduced dissociative electron attachment of methyl bromide produced energetic methyl radicals that reacted with hydrogen to form methane. The photoreaction dynamics is discussed.

Methane dissociative chemisorption and detailed balance on Pt(111): Dynamical constraints and the modest influence of tunneling

A dynamically biased (d-) precursor mediated microcanonical trapping (PMMT) model of the activated dissociative chemisorption of methane on Pt(111) is applied to a wide range of dissociative sticking experiments, and, by detailed balance, to the methane product state distributions from the thermal associative desorption of adsorbed hydrogen with coadsorbed methyl radicals. Tunneling pathways were incorporated into the d-PMMT model to better replicate the translational energy distribution of the desorbing methane product from the laser induced thermal reaction of coadsorbed hydrogen and methyl radicals occurring near T(s) = 395 K. Although tunneling is predicted to be inconsequential to the thermal dissociative chemisorption of CH4 on Pt(111) at the high temperatures of catalytic interest, once the temperature drops to 395 K the tunneling fraction of the reactive thermal flux reaches 15%, and as temperatures drop below 275 K the tunneling fraction exceeds 50%. The d-PMMT model parameters of {E0 = 58.9 kJ/mol, s = 2, η(v) = 0.40} describe the apparent threshold energy for CH4/Pt(111) dissociative chemisorption, the number of surface oscillators involved in the precursor complex, and the efficacy of molecular vibrational energy to promote reaction, relative to translational energy directed along the surface normal. Molecular translations parallel to the surface and rotations are treated as spectator degrees of freedom. Transition state vibrational frequencies are derived from generalized gradient approximation-density functional theory electronic structure calculations. The d-PMMT model replicates the diverse range of experimental data available with good fidelity, including some new effusive molecular beam and ambient gas dissociative sticking measurements. Nevertheless, there are some indications that closer agreement between theory and experiments could be achieved if a surface efficacy less than one was introduced into the modeling as an additional dynamical constraint.

Communication: Angle-resolved thermal dissociative sticking of CH4 on Pt(111): Further indication that rotation is a spectator to the gas-surface reaction dynamics

Effusive molecular beam measurements of angle-resolved thermal dissociative sticking coefficients for CH(4) impinging on a Pt(111) surface, at a temperature of 700 K, are reported and compared to theoretical predictions. The reactivity falls off steeply as the molecular angle of incidence increases away from the surface normal. Successful modeling of the thermal dissociative sticking behavior, consistent with existent CH(4) supersonic molecular beam experiments involving rotationally cold molecules, required that rotation be treated as a spectator degree of freedom.

An effusive molecular beam technique for studies of polyatomic gas-surface reactivity and energy transfer.

An effusive molecular beam technique is described to measure alkane dissociative sticking coefficients, S(T(g), T(s); ϑ), on metal surfaces for which the impinging gas temperature, T(g), and surface temperature, T(s), can be independently varied, along with the angle of incidence, ϑ, of the impinging gas. Effusive beam experiments with T(g) = T(s) = T allow for determination of angle-resolved dissociative sticking coefficients, S(T; ϑ), which when averaged over the cos (ϑ)/π angular distribution appropriate to the impinging flux from a thermal ambient gas yield the thermal dissociative sticking coefficient, S(T). Nonequilibrium S(T(g), T(s); ϑ) measurements for which T(g) ≠ T(s) provide additional opportunities to characterize the transition state and gas-surface energy transfer at reactive energies. A resistively heated effusive molecular beam doser controls the T(g) of the impinging gas striking the surface. The flux of molecules striking the surface from the effusive beam is determined from knowledge of the dosing geometry, chamber pressure, and pumping speed. Separate experiments with a calibrated leak serve to fix the chamber pumping speed. Postdosing Auger electron spectroscopy is used to measure the carbon of the alkyl radical reaction product that is deposited on the surface as a result of alkane dissociative sticking. As implemented in a typical ultrahigh vacuum chamber for surface analysis, the technique has provided access to a dynamic range of roughly 6 orders of magnitude in the initial dissociative sticking coefficient for small alkanes on Pt(111).

Oxygen photochemistry on Pt(111)

The 308 nm laser induced photodesorption of physisorbed and chemisorbed O2 on Pt(111) has been examined. Photodesorption of physisorbed O2 occurred with 9 X 10-20 cm2 cross section, and exhibited a sharp approximately cos34