Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with room-temperature methane gas over Ir(111) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, these calculations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to the results of a previous study of methane dissociation on Ir(110), although the reactivity of thermalized methane is approximately an order of magnitude higher on the (110) surface of iridium.

We present the results from a molecular beam study of the initial adsorption probability (S0) of O2 on Si(100)-2×1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trapping-mediated mechanism is dominant, exhibiting a strong increase in S0 with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are well-described by a simple precursor model, which gives a difference in activation barrier heights of (Ed−Ec)=28 meV, and a ratio of preexponentials νd/νc=24.2. Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in S0 with kinetic energy indicates that a direct chemisorption mechanism is active, with the observed energy scaling proportional to cos θi. There is also an unusual increase in S0 with surface temperature, with only a weak increase below 600 K, and a stronger increase above 600 K. The direct mechanism trends are discussed in terms of a possible molecular ion intermediate with thermally activated charge transfer. The molecular beam measurements are also used in calculating the reactivity of a thermalized gas with a clean surface. The precursor model is combined with a two-region fit of the direct adsorption data to predict chemisorption probabilities as a function of the incident conditions. These functions are then weighted by a Maxwell-Boltzmann distribution of incident angles and energies to calculate the adsorption probability for a thermal gas. These calculations indicate that the predominant mechanism depends strongly on temperature, with trapping-mediated chemisorption accounting for all of the adsorption at low temperatures, and direct adsorption slowly taking over at higher temperatures.

Abstract.A compact two-gas sensor based on quartz enhanced photoacoustic spectroscopy (QEPAS) was developed for trace methane and ammonia quantification in impure hydrogen. The sensor is equipped with a micro-resonator to confine the sound wave and enhance QEPAS signal.The normalized noise-equivalent absorption coefficients (1σ)

The trapping probability, or physical adsorption probability, of ethane on a clean Si(100)-(2×1) surface has been measured as a function of the incident translational energy and incident polar angle of the molecule at a surface temperature of 65 K. At all incident angles the trapping probability decreases as the translational energy of the incoming ethane molecule is increased from 0.05 to 1.3 eV. As the incident polar angle, with respect to the surface normal, is increased, the trapping probability decreases. This decrease in trapping probability with increasing polar angle contradicts the idea of normal energy scaling and has been seen in very few cases. Classical molecular dynamics calculations have been employed to study the cause of this unusual angular dependence. This simulation predicts trapping probabilities in good agreement with the experimental data. Analysis of the computed trajectories indicates that the initial site of impact within the unit cell, as well as energy exchange on initial impact with the surface, is important in determining the fate of an incident molecule. Normal momentum of the incident molecule is dissipated during the first impact much more efficiently than is parallel momentum. The simulations also indicate that the observed angular dependence can be explained in terms of parallel momentum accommodation. Large amounts of parallel momentum remaining after initial impact may be converted to normal momentum on subsequent impacts, causing molecules to scatter from the surface. Therefore, molecules that impact the surface at glancing angles and high translational kinetic energies are more likely to scatter from the surface than those at normal incidence or with lower translational kinetic energy.

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