Initial Trapping Probability Research Articles

The effect of hydrogen on the dynamics of the molecular adsorption of ethane on Pt(111)

Initial trapping probabilities for ethane adsorption on hydrogen-saturated Pt(111) were determined for incident translational energies from 9 to 38 kJ mol and incident angles from 0 to 60°. For energies less than 30 kJ mol the initial trapping probabilities are approximately 25% less than that for ethane adsorption on clean Pt(111). However, for energies greater than 30 kJ/mol, the initial trapping probabilities appear to asymptotically approach those of the clean Pt(111) surface. This observation cannot be explained by the one-dimensional modified hard cube model employing the 28 kJ mol binding energy of ethane on the hydrogen-saturated surface. The trapping probability for ethane on hydrogen-saturated Pt(111) scales monotonically with E T cos 1.1θ i, which is closer to “normal” energy scaling than either ethane on Pt(111) or ethane on ethane-saturated Pt(111). Stochastic trajectory simulations using a 28 kJ mol binding energy and the parameters for ethane adsorption on clean Pt(111) simulate the decrease in magnitude of the initial trapping probability but do not produce the change in energy scaling from that of the clean surface.

Numerical Analysis of Gas-Surface Interaction by Molecular Dynamics Method. 2nd Report. Scattering Dynamics of Xenon on Pt(111).

In the analysis of rarefied gas flows, much uncertainty exists regarding the boundary condition on the solid surface. This is mainly due to a lack of understanding of the micro scale dynamics of gas-surface interaction under various conditions, such as mass ratio, incident angle and wall temperature. In this paper, the behavior of a Xe gas molecule which collides with the clean Pt(111) surface of very thin film is computed by the molecular dynamics method. A clean surface of Pt(111) is formed in the thermal equilibrium state and then a Xe molecule starts to approach it. The calculated initial-trapping probabilities agree with the experimental data. The calculated results reveal that the average normal momentum of the gas molecule after the first collision has a good correlation with the incident normal momentum.

Adsorbate-assisted adsorption: Trapping dynamics of Xe on Pt(111) at nonzero coverages

The trapping dynamics of Xe on Pt(111) has been probed as a function of Xe coverage with supersonic molecular-beam techniques. Adsorption probabilities were directly measured at a surface temperature of 95 K at coverages ranging from zero to monolayer saturation at incident translational energies between 6 and 63 kJ/mol and incident angles between 0° and 60°. In apparent agreement with the predictions of the original Kisliuk model, the adsorption probability at the lowest incident translational energy (6 kJ/mol) remains almost constant with coverage up to near monolayer saturation. However, in contradiction to the original Kisliuk model, at higher incident translational energies, the trapping probability increases nearly linearly with xenon coverage up to near monolayer coverage. For example, the trapping probability increases from 0.06 to 0.42 for an incident translational energy of 63 kJ/mol at normal incidence as the coverage is increased from zero to saturation monolayer coverage. This behavior can be explained adequately by a model that incorporates enhanced trapping onto the monolayer compared to the clean surface, a property of the model that is confirmed directly by experiments presented herein. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing Xe surface coverage, proving that the degree to which parallel momentum participates in the adsorption process increases with adsorbate coverage. The initial trapping probability of Xe onto the monolayer is independent of incident angle indicating total-energy scaling. The above findings are qualitatively identical to our previous results for the molecular adsorption of ethane on the same surface, suggesting that these phenomena occur, in general, for weak molecular adsorption regardless of molecular shape and internal degrees of freedom, at least for small molecules.

Molecular beam studies of adsorption dynamics

We have investigated the trapping dynamics of C1-C3 alkanes and Xe on Pt(111) using supersonic molecular beams and a direct technique to measure trapping probabilities. We have extended a one-dimensional model based on classical mechanics to include trapping and have found semiquantitative agreement with experimental results for the dependence of the initial trapping probability on incident translational energy at normal incidence. Our measurements of the initial trapping probability as a function of incident translational energy at normal incidence are in agreement with previous mean translational energy measurements for Xe and CH4 desorbing near the surface normal, in accordance with detailed balance. However, the angular dependence of the initial trapping probability shows deviations from normal energy scaling, demonstrating the importance of parallel momentum in the trapping process and the inadequacy of one-dimensional models. The dependence of the initial trapping probability of Xe on incident translational energy and angle is quite well fit by three-dimensional stochastic classical trajectory calculations utilizing a Morse potential. Angular distributions of the scattered molecules indicate that the trapping probability is not a sensitive function of surface temperature. The trapping probability increases with surface coverage in quantitative agreement with a modified Kisliuk model which incorporates enhanced trapping onto the monolayer. We have also used the direct technique to study trapping onto a saturated monolayer state to investigate the dynamics of extrinsic precursor adsorption and find that the initial trapping probability onto the monolayer is higher than on the clean surface. The initial trapping probability onto the monolayer scales with total energy, indicating a highly corrugated interaction potential.

Dynamics of molecular adsorption of ethane with platinum(111): a supersonic molecular beam study

The molecular interaction of ethane with Pt(111) was investigated by using supersonic molecular beam techiques and temperature-programmed desorption (TPD). Ethane adsorbs molecularly on the terrace sites of Pt(111) at 95 K with a saturation coverage of ca. 0.3 monolayer and desorbs at ca. 132 K during TPD. Isothermal desorption experiments suggest that ethane desorbs from the second layer with first-order desorption kinetics. Dissociation of molecularly adsorbed ethane is negligible upon subsequent heating. The trapping probability of ethane on the clean surface at normal incidence decreases from approximately 0.91 to 0.13 as the incident translational energy, E{sub T}, increases from 6 to 40 kJ/mol at a surface temeprature of 95 K in apparent semiquantitative agreement with a modified hard-cube model. However, over a range of incident angles, {theta}{sub i}, the initial trapping probability scales with E{sub T}cos{sup 0.6} {theta}{sub i} rather than with E{sub t} cos{sup 2}{theta}{sub i}, demonstrating the participation of momentum parallel to the surface in the trapping process and the necessity of more sophisticated theories, such as three-dimensional trajetory calculations, to adequately describe trapping over a wide range of incident translational energies and incident angles.

The dynamics of precursor adsorption: ethane on Pt(111)

We have studied ethane trapping into a second-layer state on Pt(111) using supersonic molecular beam techniques to investigate the dynamics of extrinsic precursor adsorption. Initial trapping probabilities of ethane on an ethane covered Pt(111) surface were measured directly as a function of incident translational energy and incident angle at a surface temperature of 95 K. At all incident translational energies and angles the initial trapping probability into the second-layer state is higher than on a clean surface. In addition the initial trapping probability into the second layer decreases less with incident translational energy than the initial trapping probability onto the clean surface. In contrast to previous findings for non-dissociative weak adsorption on clean surfaces showing the initial trapping probability to increase with incident angle, the initial trapping probability into the second layer is independent of incident angle indicating “total” energy scaling. A dynamical corrugation of the adsorbed layer is postulated to rationalize this strong deviation from the “normal” energy scaling implicit in one-dimensional theories of trapping.

Dynamics of the interaction of ethane with Ir(110)-(1×2)

Experimentally determined values of the initial adsorption probability of ethane on Ir(110)-(1×2) are presented which probe the dynamics of the interaction. The data were obtained from supersonic molecular beam measurements with an incident kinetic energy Ei ranging between 1.2 and 24 kcal/mol, surface temperatures TS between 77 and 550 K, and incident angle θi between 0° and 45°. Experimentally determined values of the initial trapping probability ζ0 of ethane into a physically adsorbed state at TS=77 K as a function of Ei and θi and experimentally determined values of the initial probability of dissociative chemisorption S0 as a function of Ei,θi, and TS are presented. The value of ζ0 is found to decrease with increasing Ei consistent with the fact that an increasingly larger fraction of the incident kinetic energy must be dissipated in order for the molecule to physically adsorb. The initial trapping probability has a relatively weak dependence on θi such that the value of ζ0 is found empirically to scale as Ei cos0.5 θi. Two distinct mechanisms of dissociative chemisorption on the bare surface are revealed. At low Ei a temperature-dependent trapping-mediated chemisorption mechanism dominates, while at relatively high Ei a temperature-independent direct mechanism dominates. For Ei less than 13.4 kcal/mol, the value of S0 decreases rapidly with increasing TS, consistent with a trapping-mediated mechanism. For a surface temperature of 154 K, S0 decreases with increasing Ei for 1.2≤Ei≤13.4 kcal/mol, in a manner similar to that for the molecular trapping probability. The data in the low Ei regime also support quantitatively a kinetic model consistent with a trapping-mediated chemisorption mechanism. The difference in the activation energies for desorption and chemisorption from the physically adsorbed, trapped state Ed−Ec is 2.2±0.2 kcal/mol. In the trapping-mediated chemisorption regime, the value of S0 is found to be rather insensitive to incident angle, scaling with Ei cos0.5 θi just as for trapping of molecular ethane into a physically adsorbed state. For a normal energy Ei cos2 θi greater than 8 kcal/mol, chemisorption via a direct mechanism becomes significant and increases with increasing Ei. Values of S0 in the direct chemisorption regime scale with normal energy and are independent of TS over the range from 350 to 1350 K.

Trapping dynamics of xenon on Pt(111)

The dynamics of Xe trapping on Pt(111) was studied using supersonic atomic beam techniques. Initial trapping probabilities ( S 0) were measured directly as a function of incident translational energy ( EinT) and angle of incidence (θ i) at a surface temperature ( Tins) 95 K. The initial trapping probability decreases smoothly with increasing E T cosθ i;, rather than E T cos 2θ i, suggesting participation of parallel momentum in the trapping process. Accordingly, the measured initial trapping probability falls off more slowly with increasing incident translational energy than predicted by one-dimensional theories. This finding is in near agreement with previous mean translational energy measurements for Xe desorbing near the Pt(111) surface normal, assuming detailed balance applies. Three-dimensional stochastic classical trajectory calculations presented herein also exhibit the importance of tangential momentum in trapping and satisfactorily reproduce the experimental initial trapping probabilities.

Dynamics of molecular CH 4 adsorption on Pt(111)

The dynamics of molecular methane adsorption on Pt(111) were probed with supersonic molecular beam techniques. Initial trapping probabilities were directly measured between 0.94 and 0.16 for incident total translational energies between 3.4 and 20.2 kJ/mol and angles of incidence (with respect to the surface normal) between 0° and 45° at a surface temperature ( T s) of 100 K. The incident methane molecules were rotationally and vibrationally cold. The initial trapping probability decreases with increasing incident translational energy ( E T) and decreasing angle of incidence (θ i) and varies smoothly with incident normal energy ( E n = E T cos 2 θ i ), indicating a low corrugation of the molecule-surf ace interaction potential. The dependence of the initial trapping probability on incident normal translational energy agrees quantitatively with both a modified hard cube model and Leuthäusser's theory at incident normal translational energies below 8 kJ/mol. At higher incident normal translational energies the observed initial trapping probabilities are higher than the values predicted by both models. Energy loss mechanisms other than surface phonon excitations may partially account for this discrepancy. A rapid decrease in the apparent adsorption probability as the surface temperature approaches 140 K is caused by the competitive influence of desorption. The temperature at which the apparent adsorption probability goes to zero agrees well with the desorption temperature measured independently by temperature programmed desorption. In accordance with the aforementioned models, the measured in-plane angular distributions suggest that the trapping probability is relatively independent of surface temperature in the range of 160 to 500 K. The relatively low intensity of methane found near the surface normal in the angular distributions may be partially explained by a wider than cosine angular distribution for the trapped-desorbed channel, which is consistent with the observation that the trapping probability increases with angle of incidence. Comparison of our initial trapping probability versus normal translational energy data to previous mean translational energy measurements of methane molecules desorbing from Pt(111) at the surface normal suggests that detailed balance applies for the non-equilibrium situation involving a collimated monoenergetic molecular beam of methane incident on a Pt(111) surface.

A Simple Classical Model for Trapping in Gas–Surface Interactions

A simple model based on an assumed attractive square well and impulsive repulsive potential is developed to enable the calculation of initial trapping probabilities from thermal accommodation coefficient data. The gas species impinge on a smooth solid surface and are considered to be trapped if they transfer enough energy to the solid to fall into the interatomic potential well. The amount of energy transfer is determined by empirical values of the thermal accommodation coefficient. The model predicts the correct trends with surface and gas temperature when compared with rare gas scattering data and gives values accurate within a factor of 2 or better. Agreement between the model calculations and the sticking coefficient for deuterium adsorption on platinum is also found.

Scattering of the Rare Gases (He, Ne, Ar, Kr, and Xe) from Platinum (111) Surfaces

The scattering of He, Ne, Ar, Kr, and Xe from Pt (111) surfaces characterized by LEED has been measured for surface temperatures between 100°C and 500°C using effusive thermal energy beams of 23°C and 700°C. The LEED and helium scattering indicate that the surfaces are geometrically smooth, regular, and free from low molecular weight contaminants. Results are in qualitative agreement with some of the predictions of the “cube” scattering theories but at variance with the effect of incident angle and surface mass. Data agree qualitatively in these respects with the finite range interaction (FRI) models. Approximate mass balances between the total scattered flux and the incident flux permit estimates of the initial trapping probabilities. No diffraction of He was observed.