Initial Adsorption Probability Research Articles

Influence of Kinetic Energy for Initial Adsorption Probability of O2 Molecules on Si(001) Surfaces and SiO Desorption Yield.

Influence of Kinetic Energy for Initial Adsorption Probability of O2 Molecules on Si(001) Surfaces and SiO Desorption Yield.

The dynamics of ethylene adsorption on Pt(111) into di-σ and π-bonded states

The dynamics of ethylene adsorption on Pt(111) into both the di-σ- and π-bonded states were investigated at 95 and 40 K, respectively, using supersonic molecular beam techniques. The angular dependence of ethylene adsorption into both states is similar to the angular dependence for ethane adsorption, which has a much weaker bond to the surface in its final state. In contrast to ethane, high adsorption probabilities for ethylene prevail to high incident kinetic energies, suggesting that the strong interaction of ethylene with the surface influences adsorption. The initial adsorption probability of ethylene is approximately independent of surface temperature between 40 and 450 K, suggesting that there is no reversible, thermalized intrinsic precursor to adsorption. At 40 K, the adsorption probability increases with coverage (in the π-bonded state). However, at 95 K, the adsorption probability of ethylene remains constant with increasing self-coverage (in the di-σ- bonded state) for trajectories incident with low parallel momentum, but decreases with coverage at high parallel momentum. High parallel momentum may contribute to an increased overall scattering probability from a “hot” extrinsic precursor, resulting in a decreased net adsorption probability at higher ethylene coverage in the rigidly bound di-σ state.

The extrinsic precursor kinetics of molecular methane adsorption onto Pt(1 1 1)-p(2×2)-O, - n-butylidyne, and -isobutylidyne

The kinetics of methane trapping on Pt(1 1 1)-p(2×2)-O, - n-butylidyne, and -isobutylidyne were investigated using supersonic molecular beam techniques at surface temperatures between 30 and 84 K. The adsorption behavior was described by a model in which adsorption was forced through an extrinsic precursor. The temperature dependence of the initial adsorption probability suggests that the difference in activation energies for desorption and migration from the extrinsic precursor, E ′ d− E m ′, depends on the geometry and structure of the adsorbate covering the surface. The activation energy for migration of the extrinsic precursor to a binding site is greatest on the n-butylidyne-covered surface, followed by the isobutylidyne- and the oxygen-covered surfaces, respectively. The high activation energy for reconversion to the extrinsic precursor observed for the extrinsic precursor is consistent with the corrugated gas-surface potential measured in trapping dynamics experiments.

Intrinsic and extrinsic precursors to adsorption: Coverage and temperature dependence of Kr adsorption on Pt(111)

The kinetics of krypton adsorption on Pt(111) were investigated using supersonic molecular beam techniques. Krypton adsorbs at defects via an intrinsic precursor below a surface temperature of 85 K. The difference in activation energies for desorption and migration of a Kr atom on the terrace seeking a defect site is 10.7 kJ/mol, indicating that at 80 K, a Kr atom makes about 107 site hops before desorbing or finding a binding site. Below 60 K stable adsorption occurs on terraces, where the initial adsorption probability is independent of surface temperature. The activation energy for zero-order desorption from Pt(111) terraces is 12.9 kJ/mol; the activation energy for Kr migration on the terraces is then calculated to be ⩽2.2 kJ/mol. Krypton adsorption proceeds at nonzero coverages via an extrinsic precursor. The adsorption probability of Kr increases with self-coverage, and is described by the modified Kisliuk model [H. C. Kang, C. B. Mullins, and W. H. Weinberg, J. Chem. Phys. 92, 1397 (1990); C. R. Arumainayagam, M. C. McMaster, and R. J. Madix, J. Phys. Chem. 95, 2461 (1991)]. The Kr overlayer on terraces compresses between 60 and 42 K, then forms a second state before forming stable multilayers at 34 K.

Adsorption dynamics of CO on Cu(1 1 0): a molecular beam study

Molecular beam measurements of initial adsorption probabilities, S 0, as well as the coverage dependence of the adsorption probability, S( Θ), of CO on Cu(1 1 0) are presented. The influence of kinetic energy, polar impact angle, α i, azimuthal orientation ([1 0 0] and [ 1 1 ̄ 0 ]), and adsorption temperature, T s, on the adsorption dynamics have been studied. With regard to microkinetical models used to describe processes in heterogeneous catalysis, the dependence of the adsorption probability on the density of defects has been investigated. The surface has been characterized by measuring He atom angular distributions, He atom reflectivity curves, and LEED. The shape of the S( Θ) curves are consistent with a precursor mediated adsorption mechanism. For low temperatures T s⩽90 K, the adsorption dynamics are dominated by adsorbate assisted adsorption. Despite the pronounced difference in the surface corrugation along the different high-symmetry directions no significant differences in S 0 and S( Θ) were observed for the different azimuths studied.

Adsorption dynamics of CO on the polar surfaces of ZnO

Measurements of initial adsorption probabilities, S0, as well as the coverage dependence of the adsorption probability, S(ΘCO), of CO on Zn–ZnO [ZnO(0001)] and O–ZnO [ZnO(0001̄)] are presented. The samples have been characterized by He atom scattering, He atom reflectivity measurements, LEED, and XPS. Samples with different densities of defects were examined, either by investigating different samples with identical surface termination (for O–ZnO) or by inducing defects by ion sputtering at low temperatures (for Zn–ZnO). The influence of kinetic energy and impact angle (for Zn–ZnO) as well as adsorption temperature on the adsorption dynamics have been studied. For both polar surfaces the shape of the coverage dependent adsorption probability curves are consistent with a precursor mediated adsorption mechanism. Adsorbate assisted adsorption dominates the adsorption dynamics for high impact energies and low adsorption temperatures, especially for Zn–ZnO. The He atom reflectivity measurements point to the influence of an intrinsic precursor state. In contrast to the Zn–ZnO surface, for O–ZnO a weak thermal activation of the CO adsorption was observed. Total energy scaling is obeyed for Zn–ZnO. The heat of adsorption for CO on both polar faces varies between 7 kcal/mol (low coverage) and 5 kcal/mol (high coverage). A comparison of He atom reflectivity with S(ΘCO) curves demonstrates that CO initially populates defect sites on both surfaces. For O–ZnO an increase in S0 with decreasing density of defects was observed, whereas for the Zn-terminated surface S0 was independent of the defect density within the range of parameters studied.

Adsorption probabilities of CO on O–ZnO: A molecular beam study

We present measurements of initial adsorption probabilities, S0, as well as its coverage dependence, S(ΘCO), of CO on oxygen terminated ZnO(0001). The impact energies of the CO (48 meV<Ei<750 meV) and surfaces temperature (75 K<TS< 220 K) have been varied. The shape of the coverage dependent adsorption probability curves indicates the presence of precursor mediated adsorption. The heat of adsorption has been determined to (7–2 ΘCO) kcal/mol by assuming a pre-exponential factor of 1×1013 l/s.

Chemisorption of isobutane and neopentane on Ir(110)

Molecular beam techniques have been employed to investigate the chemisorption of isobutane and neopentane on Ir(110) at surface temperatures, T s, from 85 to 1000 K. The beam translational energies, E i, varied from 3.1 to 31 kcal/mol for isobutane, and 1.8 to 48 kcal/mol for neopentane. For T s<100 K, isobutane formed molecular multilayers, as did neopentane for T s<110 K. The initial adsorption probabilities, P a, remained nearly independent of surface temperature between 100 and 300 K for isobutane, and 110 and 350 K for neopentane for different beam impact energies. For T s=100 K, isobutane adsorbs molecularly with an intrinsic trapping probability, ξ, equal to 0.96 at E i=3.1 kcal/mol while neopentane adsorbs molecularly with ξ=0.94 at E i=1.8 kcal/mol and T s=110 K. In contrast, for isobutane with T s between 300 and 1000 K, and for neopentane with T s between 350 and 1000 K, the initial adsorption probabilities decrease with increasing T s. For both molecules, the adsorption probabilities show characteristics of both trapping-mediated and direct dissociative chemisorption which dominate the kinetics for low and high impact energies, respectively. A kinetic model is proposed, which includes CH bond cleavage reaction pathways.

Molecularly chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beam and electron energy-loss spectroscopy study

High translational energy adsorption of oxygen on the (111) surface of platinum was examined with electron energy loss spectroscopy (EELS) and molecular beam techniques. EEL spectra indicate that over an incident energy range of 0.2–1.37 eV and on a Pt(111) surface held at 77 K, oxygen adsorbs in an associative chemisorbed state—yielding to the dissociated state only after sufficient substrate heating. Simple direct dissociation appears negligible for all incident kinetic energies studied. At near-zero surface coverages, exclusive population of the peroxolike molecular precursor is observed for adsorption at these high translational energies, while both superoxolike and peroxolike forms are detected for low energy adsorption (0.055 eV). This peculiarity represents evidence that translational energy is effective in differentially populating reaction intermediates and provides better quantification of potential energy barriers to dissociation. We estimate the activation barrier for dissociation from the peroxolike precursor to be approximately 0.29 eV. Initial adsorption probability measurements over a wide range of surface temperatures and high incident kinetic energies corroborate a molecular chemisorption mediated mechanism.

Oxygen adsorption on Si(100)-2×1 via trapping-mediated and direct mechanisms

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.

Direct verification of a high-translational-energy molecular precursor to oxygen dissociation on Pd(111)

Adsorption of high-translational-energy oxygen on Pd(111) was examined with electron energy-loss spectroscopy (EELS) and molecular beam techniques. EEL spectra indicate that a direct molecular chemisorption mechanism holds at high kinetic energies (up to 1.27 eV) on a 77 K substrate. The initially intact peroxo-like species proceeds to dissociation or desorption only after sufficient heating – direct dissociation from the gas phase appears to be negligible. We estimate the thermal activation barrier for conversion from the peroxo-like species to be approximately 0.32 eV. Initial adsorption probability measurements in this high-kinetic-energy regime support a molecular chemisorption-initiated mechanism.

Coverage dependence of neopentane trapping dynamics on Pt(111)

Adsorption probabilities for neopentane on Pt(111) were measured directly using supersonic molecular-beam techniques at coverages ranging from zero to monolayer saturation, incident translational energies between 18 and 110 kJ mol −1 and incident angles between 0° and 60° at a surface temperature of 105 K. The adsorption probability was found to increase with coverage up to near monolayer saturation at all incident translational energies and incident angles. The coverage dependence of the adsorption probability predicted by a modified Kisliuk model with enhanced trapping into the second layer exhibits good quantitative agreement with the experimental values. The angular dependence of the adsorption probability decreases with increasing coverage, suggesting that the effective corrugation of the gas–surface interaction potential increases with the adsorbate coverage. The initial adsorption probability into the second layer onto the covered surface decreases from 0.95 to 0.75 with increasing energy over the energy range studied, and exhibits total energy scaling. A comparison with second-layer trapping data of simpler molecules onto covered Pt(111) indicates that the structural complexity of adsorbed neopentane molecules facilitates collisional energy transfer during adsorption.

Effect of preadsorbed oxygen on the adsorption of CO on Ir(110)

We report measurements of the initial adsorption probability S 0 as well as its coverage dependence S( θ co) of CO on clean and oxygen-precovered Ir(110) for adsorption temperatures T s below the onset of CO 2 formation. The impact energy (1.3 < E 0 < 24 kcal mol −), the oxygen precoverage θ o, and the surface temperature (85 < T s < 200 K) were varied. The initial adsorption probability for CO on the clean surface decreases linearly with increasing impact energy E 0, and is independent of T s. The shape of the S( θ co, E 0) curves points to non-activated precursor-mediated adsorption kinetics for low impact energies; for E 0 ≥ 20 kcal mol −1, direct adsorption events also contribute to the overall adsorption probability. For high impact energies ( E 0 > 3 kcal mol −1), S 0( θ o) decreases almost linearly with increasing oxygen precoverage. However, for low impact energies ( E 0 < 3 kcal mol −1), S 0( θ o) remains constant at 0.92 ± 0.03 independent of θ o. The results suggest a physical site blocking mechanism. No significant temperature dependence of S 0( θ o) was observed below the onset of CO oxidation.

Adsorption mechanisms of translationally-energetic O 2 and N 2: direct dissociation versus direct molecular chemisorption

A direct dissociation mechanism has been traditionally assigned to molecular beam data that exhibit an increase in the initial adsorption probability with increasing kinetic energy. Yet, recent experiments of nitrogen and oxygen adsorption provide support for an alternative high kinetic energy pathway in which incident energy assists in surmounting barriers to molecular chemisorption on a surface as the first step to dissociation. Moreover, systems for which the experimental evidence supports such a mechanism also demonstrate that molecularly chemisorbed intermediates can be spectroscopically observed at low temperatures and coverages from exposure to a gas in thermal equilibrium at room temperature. Likewise, such observations have not been measured for systems which are consistent with direct dissociation. A consideration of this trend regarding the existence of molecularly chemisorbed states and the implications for the dominant, dissociative chemisorption pathway at high kinetic energy is presented for a number of gas surface systems.

Kinetics and dynamics of the initial adsorption of nitric oxide on Ir(111)

The interaction of nitric oxide (NO) with an Ir(111) surface has been studied with supersonic molecular beam techniques and electron energy loss spectroscopy. Initial adsorption probability S0, measurements as a function of incident kinetic energy Ei, surface temperature Ts, and angle of incidence θi reveal that separate mechanisms govern adsorption at low and high kinetic energy. This distinction is reflected in measurements of the initial molecular adsorption probability where a decrease in the value of S0 with increasing Ts (between 77 and 300 K) is observed at low kinetic energy (Ei&amp;lt;0.45 eV), but no surface temperature dependence is detected at high kinetic energy in this temperature range. We present a model describing both the molecular and dissociative chemisorption of NO on Ir(111). At low kinetic energy, NO adsorbs initially as a physically adsorbed species. From this state, desorption to the gas phase or conversion to a molecularly chemisorbed state on the surface are competing processes which depend on surface temperature. The molecularly chemisorbed state is the precursor to dissociation for elevated surface temperatures. At high kinetic energy, NO adsorption occurs directly into the molecularly chemisorbed well, with the probability of trapping as a physically adsorbed species near zero and with undetectable direct dissociation. Indeed, after exposure of the Ir(111) surface at 77 K to a high kinetic energy (1.3 eV) beam, surface vibrational spectroscopy measurements show only features attributable to molecularly chemisorbed NO. The success of this model in describing our measurements is demonstrated by the separate calculation from low and high kinetic energy data of rate constants corresponding to forward and reverse conversion from the molecularly chemisorbed well. Additionally, we discuss attempts to promote dissociation on the surface with vibrational energy and with a combination of translational and surface thermal energy.

Surface microstructure effects: molecular ethane adsorption dynamics on Pt(110)-(1 × 2)

The molecular adsorption probability of ethane on clean Pt(110)-(1 × 2) at a surface temperature of 95 K was measured as a function of incident translational energy, E T, incident polar angle, θ i, and azimuthal angle, ø. At normal incidence the adsorption probability decreases with incident translational energy from near unity at an incident kinetic energy of 10 kJ/mol to 0.5 at 40 kJ/mol. For molecules incident with the tangential velocity component directed along the [11̄0] (smooth) direction, the initial adsorption probability increases with increasing θ i, scaling with E T cos 0.6 θ i; however, the adsorption probability decreases with θ i for molecular beams directed along the [100] (rough) direction, indicating the effects of corrugation. Stochastic trajectory simulations employing ethane-Pt Morse potential parameters previously developed from measurements of the adsorption probabilities of ethane on Pt(111) give quantitative predictions of the initial trapping probability of ethane on Pt(110)-(1 × 2) for both azimuthal angles at all energies and polar angles of incidence. The simulations suggest that interconversion of normal and parallel momenta due to the surface corrugation governs the adsorption process and serves as an effective mechanism which facilitates trapping. Excessive parallel momentum, however, can cause ethane to scatter on subsequent bounces because of the reconversion of large amounts of parallel energy into normal energy. Analysis of a large number of trajectories illustrated that collisions on the ridges of Pt(110)-(1 × 2) mitigate against trapping of ethane while collisions within the troughs facilitate trapping. Finally, the simulations show that the trapping probability is determined to within 12% by the fate of the first bounce.

The molecular adsorption of ethane on sulfur- and ethylidyne-covered surfaces of Pt(111)

Initial adsorption probabilities, α 0, were determined for ethane incident on sulfur- and ethylidyne-covered surfaces of Pt(111) at 95 K for incident translational energies from 10 to 44 kJ mol −1 and incident angles from 0 to 60°. At normal incidence and for all translational energies studied, α 0 on these surfaces was found to be enhanced relative to clean Pt(111). The initial adsorption probabilities on both adsorbate-covered surfaces were found to be independent of the angle of incidence, suggesting a highly corrugated static gas-surface potential. The interconversion of normal and parallel momenta due to this corrugation appears to be the mechanism which best explains these results. The notion of increased energy transfer from a match in mass of the collision partners (ethane/sulfur) is clearly not consistent with the data, since the adsorption probability of ethane on the sulfur-covered surface at glancing incidence (60°) is less than that on the clean surface at this same angle. Ethylidyne was found to increase α 0 more than sulfur for all incident conditions, apparently due to energy transfer to the internal degrees of freedom of ethylidyne, a mechanism which is not available to the sulfur adsorbate.

Kinetics and dynamics of the initial dissociative chemisorption of oxygen on Ru(001)

We have used supersonic molecular beam techniques to measure the initial dissociative chemisorption probability S0 of O2 on Ru(001) as a function of incident kinetic energy Ei, surface temperature Ts, and angle of incidence θi. We observe different behavior in the adsorption dynamics in two separate kinetic energy regimes: the value of S0 decreases with incident energy in the low kinetic energy regime, and the value increases with incident energy in a higher kinetic energy regime. In the low energy regime, we observe a large inverse dependence of S0 on surface temperature which is consistent with a trapping-mediated mechanism. Moreover, adsorption in the low energy regime can be accurately modeled by a trapping-mediated mechanism, with a surface temperature independent trapping probability α into a physically adsorbed state followed by a temperature dependent kinetic competition between desorption and dissociation. The barrier to dissociation from the physically adsorbed state is ∼28 meV below the barrier to desorption from this state as determined by analysis of kinetic data. In the high kinetic energy regime, values of the initial adsorption probability scale with normal kinetic energy, and S0 approaches a value of unity for the highest incident energies studied. However, we report an unusual surface temperature dependence of S0 in the high energy regime that is inconsistent with a simple direct mechanism. Indeed, in this higher energy regime the value of S0 rises as the surface temperature is increased. We suggest a mechanism involving electron transfer from the ruthenium surface to account for this phenomena.

Intermediate states in the adsorption of NO on Ir(111): Substrate temperature effects

Measurements of the initial adsorption probability, S0, of nitric oxide (NO) on Ir(111) as a function of incident kinetic energy, Ei, and surface temperature, Ts, are presented. We observe a decrease in S0 with increasing kinetic energy, Ei, from 0.052 to 1.3 eV. At low kinetic energies, the initial molecular adsorption probability decreases with increasing surface temperature (ranging between 77 and 300 K in this study), while at high kinetic energies, this quantity is independent of surface temperature. We propose a trapping-mediated mechanism for adsorption at low kinetic energies. In this low energy regime, the surface temperature dependence reflects a kinetic competition between desorption from a physically adsorbed state and conversion to a more strongly bound, molecularly chemisorbed state. At high kinetic energies, we propose that adsorption initially occurs directly into the molecular chemisorption well. Indeed, electron energy loss spectroscopy measurements show no evidence for direct dissociation at a low Ts and indicate that the surface temperature must exceed ∼400 K for dissociative chemisorption to occur. The probability of dissociative chemisorption of NO on Ir(111) decreases with increasing temperature (in the range 500–900 K in this study) at all kinetic energies investigated. Here, we propose a model for low kinetic energies that includes both a physisorbed species and a molecularly chemisorbed species as precursors to dissociation. For the high energy regime, the trapping probability into the physically adsorbed state is assumed to be zero, and thus, we model the adsorption occurring directly as a molecularly chemisorbed intermediate with subsequent dissociation at elevated temperatures. The success of the model is demonstrated by the agreement of kinetic parameters determined separately for data employing a high kinetic energy beam and a low energy beam.

Atomic hydrogen adsorption on Si(100)-(2×1) with preadsorbed acetylene: Structure, bonding, and chemical reactions

The adsorption of atomic hydrogen on acetylene-saturated Si(100)-(2×1) was studied using high-resolution electron energy loss spectroscopy, temperature programmed desorption, Auger electron spectroscopy, and low-energy electron diffraction (LEED). With saturation coverage of preadsorbed acetylene, the initial adsorption probability of atomic hydrogen on Si is reduced to 40% of the initial adsorption probability of atomic hydrogen on clean Si(100)-(2×1), and the uptake of hydrogen to form the Si monohydride up to 0.65±0.05 of a monolayer is observed. Although the hydrogenation of acetylene is substantially slower than the chemisorption of hydrogen as the Si monohydride, molecular desorption of ethylene is observed in temperature programmed desorption following the hydrogenation of chemisorbed acetylene. With further postexposures of atomic hydrogen, the LEED pattern changes from (2×1) to (1×1) and the Si dihydride forms, confirming the conversion of ethylene to ethyl.