Incident Translational Energy Research Articles

Molecular propane adsorption dynamics on Pt(111)

Supersonic molecular beam techniques and temperature programmed desorption (TPD) were used to study the adsorption dynamics of propane onto clean and propane-covered Pt(111). The propane sticking probability was measured directly as a function of incident translational energy, E i, incident angle, θ i, and propane coverage, θ cov, at a surface temperature of 95 K. Under these experimental conditions, propane adsorbs molecularly onto Pt(111) terrace sites. Non-normal energy scaling is observed at all propane coverages indicating the importance of parallel momentum in the adsorption process. At all incident translational energies and angles studied, the sticking probability on a propane covered surface, S(θ cov), increases with increasing propane coverage. Upon saturation of the Pt(111) terrace sites, spontaneous desorption is observed in the direct adsorption probability experiments.

Dissociative adsorption of Si2H6 on silicon at hyperthermal energies: The influence of surface structure

The reactions of Si2H6 with the (100) and (111) surfaces of silicon have been investigated employing supersonic molecular beam scattering techniques. Incident translational energy has been found to influence strongly the probability of dissociative adsorption (SR) on both surfaces. The reaction on the Si(111) surface is distinct from that observed on Si(100) concerning the dependence of SR on the substrate temperature and the incident angle. In particular, the reaction on the (111) surface shows sensitivity to the surface phase transformation (7×7)→(1×1) that occurs above 825 °C. Both the dangling bond density and the effective corrugation of the surface are important in determining the reaction probability.

Dynamics of the dissociative adsorption of disilane on Si(100): Energy scaling and the effect of corrugation

The reaction of Si2H6 with the Si(100) surface has been examined via supersonic molecular beam scattering techniques. It is found that the reaction probability is most sensitive to the incident translational energy, varying nearly linearly with increasing energy for 〈Etr〉≳1 eV. The effect of incident angle θi is described by a model that accounts explicitly for surface corrugation and assumes that the reaction probability varies with 〈Etr〉 cos2 θloc, where θloc is the incident angle with respect to the local surface normal.

Dynamics of dissociative chemisorption: Cl2/Si(111)-(2 x 1).

We report the first simulation of a surface chemical reaction performed within the ab initio molecular dynamics approach. A set of trajectories with different initial conditions has been generated for single ${\mathrm{Cl}}_{2}$ molecules impinging on the Si(111)-2\ifmmode\times\else\texttimes\fi{}1 surface with incident translational energy of 1 eV. We observe a high probability of dissociation, triggered by active sites on the \ensuremath{\pi}-bonded chains, and accompanied by a large surface response and local rehybridization effects.

Energy and momentum distributions versus incident energy in the scattering of CO from Ag(111)

The effect of changing the incident translational energy on the exit translational energy, rotational state population distributions, and angular momentum alignment of monoenergetic, rotationally cold CO scattered from Ag(111) at normal incidence was measured. As the incident energy is raised, a dramatic increase in the fraction of energy transferred to the surface, a shift of the rotational rainbow to higher rotational energy, and a significant increase in the amount of angular momentum alignment in the scattered molecules were observed. In addition, the alignment versus J distribution exhibits positive curvature for CO at all incident energies. The results are consistent with (1) the effect of the surface phonons and/or physisorption well on the final energy of the scattered molecules being small except at the lowest incident energy; (2) the large ∂V/∂θ being the primary source of the greater rotational excitation seen for CO compared to N2 scattered from Ag(111); and (3) the corrugated attractive portion of the CO/Ag(111) potential being primarily responsible for the modest alignment of CO at low incident energy.

Alkane dissociation dynamics on Pt(110)–(1×2)

Supersonic molecular beam techniques were used to study the reactive adsorption dynamics of methane and ethane on Pt(110)–(1×2). The initial dissociative sticking probability, S0, was measured as a function of surface temperature, incident translational energy, incident total vibrational energy, and incident polar angle at two azimuthal orientations. Under all experimental conditions, both alkanes dissociated via direct collisional activation. Over the range of translational energies studied here neither S0(CH4) nor S0(C2H6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl deformation, methyl rocking, C–C stretching, and torsional vibrational modes do not play a significant role in the direct dissociation of either alkane on Pt(110)–(1×2) under these experimental conditions. The C–H stretching modes were not sufficiently populated to determine the extent of their participation. Methane and ethane displayed almost identical initial reaction probabilities at a fixed incident translational energy and polar angle, similar to our findings for methane and ethane dissociation on Pt(111). However, the reactivity of both species was about a factor of 2 lower on Pt(110)–(1×2) than observed on Pt(111) at a fixed incident translational energy and polar angle. When the crystal was positioned such that the tangential velocity component of the beam was incident along the atomic rows (the [11̄0] direction) the dissociation of both alkanes exhibited normal energy scaling. When the azimuthal orientation was rotated 90° such that the tangential velocity component of the beam was directed perpendicular to the close-packed rows (the [001] direction), the initial dissociation probabilities of both alkanes appeared to scale with Ei cos0.5 θi. This is the first reported observation of non-normal energy scaling for direct alkane activation and is attributed to the corrugation of the surface microstructure.

The interaction of CO with Ni(111): Rainbows and rotational trapping

Angularly resolved rotational state distributions of CO scattered and desorbed from a clean, single-crystal Ni(111) surface were measured using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered from the surface displayed highly non-Boltzmann rotational distributions that varied with incident translational energy and detection angle, but not with surface temperature. A rotational rainbow was seen in the scattering distribution and interpreted as arising from the interaction of the weakly attractive O end of the CO molecule with the Ni(111) surface. Up to total rotational-to-translational energy conversion was seen at incident translational energies of 0.26–0.45 eV. This energetic cutoff was the result of rotational trapping and was caused by the strongly attractive interaction of the C end of the molecule with the surface. The rotational state distributions of molecules desorbed from the Ni(111) surface were well fit by Boltzmann distributions each with a temperature which is 0.82±0.08 of the surface temperature.

The dynamics of H2 dissociation on Cu and Ni surfaces. Mixed quantum-classical studies with all degrees of freedom

The dissociative adsorption of hydrogen on metals is examined using models which contain all six molecular degrees of freedom. Fully classical studies are implemented, as well as a mixed approach in which three degrees of freedom are treated quantum mechanically, and three classically. Probabilities for dissociation and rovibrational excitation are computed as a function of incident translational energy for both H2 and D2 on a reactive Ni surface and a less reactive Cu surface. Two sudden approximations are tested, in which either the center of mass translation parallel to the surface or the azimuthal orientation of the molecule are frozen. The quantum and classical results are compared for the above cases.

Dynamics of cluster scattering from surfaces

Classical trajectory calculations of van der Waals cluster scattering from a rigid surface are presented and compared with recent experimental results on argon cluster scattering from graphite by Châtelet (1992). The cluster fragmentation at surface impact is studied for different incident translational energies and can at moderate energies be described as evaporation of small particles from the parent cluster. The experimentally observed dependence of angular distributions on cluster fragment size is reproduced by the model, as well as the appearance of two components in the experimental angular distributions.

Angular and vibrational effects in the sticking and scattering of H2

The results of quantum mechanical simulations of H2 dissociation on metal surfaces are presented using an extension of the familiar two-dimensional ‘‘elbow’’ potential. By including corrugation parallel to the surface, it has been possible to examine the effects on the angular and energy distributions of dissociative adsorption and scattering. Additionally, trends obtained by moving the activation barrier from entrance to exit channel have been studied. To effect a closer analogy with experiment, seeding of the incident beam has been simulated by Boltzmann weighting dissociation probabilities. It is particularly important to include the experimental spread of the incident translational energy in calculations. It is found that for hydrogen dissociation on Cu and Fe, dissociative adsorption results can only be reconciled with a late barrier, while for Ni and Pd it appears to be early. For the scattered fraction, the late barrier gives rise to a significant enhancement in the diffraction of vibrationally excited molecules. This is explained in terms of the corrugation of the vibrationally adiabatic potential energy surfaces.

Low energy ion–molecule reaction dynamics: Complex and direct collisions of O− with NH3

Reactive and nonreactive collisions of O− with NH3 are studied at relative collision energies of 0.65 and 1.24 eV. We observed a significant contribution to the collision dynamics from nonreactive encounters between the reagents. In addition to elastic scattering, we observed a direct contribution to this nonreactive scattering with a very strong dependence of energy transfer on scattering angle. A third contribution to nonreactive scattering arose from O−⋅NH3 collision complexes that regenerate the reactants. In these collisions, ∼80% of the incident translational energy is transformed into vibrational–rotational excitation of the NH3 reagent. The kinetic energy distribution is in reasonable agreement with statistical phase space theory calculations. We also observed reactive collisions. The hydrogen atom transfer process to yield OH− is exothermic by 0.11 eV and exhibits direct dynamics at all collision energies. Proton transfer to form NH−2, endothermic by 0.9 eV, was studied as its deuterium analog and was observed only at the higher collision energy, and took place with very small cross section. The product kinetic energy distributions for the hydrogen atom transfer reaction approach a Gaussian form at the higher collision energy, and we ascribe that behavior to the impulsive nature of reactive collisions in which the ground state vibrational wave function of the N–H bond to be broken is reflected onto product translational energy states through the ‘‘corner’’ of the potential energy surface. Such a Franck–Condon picture of the reaction is a consequence of the highly skewed potential energy surface associated with the heavy–light–heavy mass combination.

Energy and momentum distributions and projections in the scattering of CO from Ag(111)

We have studied the cross correlations between the rotational state distributions, angular momentum alignment, and velocity distributions of monoenergetic, rotationally cold CO and N2 scattered at 0.75 eV from Ag(111). Measurements were made for both normal incidence and glancing incidence beams and for specular and off-specular detection. The comparison between N2 and CO is most dramatic. (1) For N2 the rotational state selected velocity distributions are very narrow while for CO the rotational state selected velocity distributions are wide. (2) For glancing incidence beams, N2 exhibits a higher degree of parallel momentum conservation than CO. (3) The velocity resolved rotational rainbows for N2 are more prominent for glancing incidence while the velocity resolved rotational rainbows for CO are more prominent for normal incidence. (4) There is 100% cartwheeling type alignment for N2 in medium and high exit rotational states while for CO the alignment is weak except at the very highest rotational states where it is still <100% cartwheeling. Our data can be interpreted as showing that the N2 molecules at these relatively high energies collide with a slightly corrugated surface and have nearly linear trajectories. Conversely, the CO scattering data are consistent with scattering from a more corrugated surface. The CO molecules have a permanent dipole moment, therefore the gradient for the CO–Ag(111) gas surface potential with respect to molecular orientation is larger. In addition, CO has a deeper physisorption well on Ag(111). Thus, the CO molecules probe deeper into the corrugated repulsive portion of the potential and have a more inelastic collision that results in greater rotational and phonon excitation but lower exit translational energy. The lower alignment for CO scattering into high J states is consistent with the CO molecules having curved exit trajectories and/or multiple collisions with the surface. For both N2 and CO, rotational excitation into high J states scales with the normal component of incident translational energy, but the phonons can be excited by both the parallel and normal components of the incident translational energy.

Energy transfer and vibrational effects in the dissociation and scattering of D2 from Cu (111)

ACTIVATED surface dissociation of adsorbed molecules is of basic importance to surface chemistry, but in only a few systems are details of the molecule–surface potential-energy surface well understood. Central to this problem are the processes of energy disposal and transfer during the molecule–surface collision and the manner in which molecular motions (translational, rotational and vibrational) are channelled into the dissociative coordinate. The dissociation of hydrogen and deuterium molecules on a copper surface provides a prototypical system for studying these processes. Here we describe measurements of the scattering of D2 from a Cu(111) surface as a function of incident translational energy, angle and vibrational state of the incoming molecules. Molecules in the ground and first excited vibrational levels show very different translational-energy thresholds for removal (0.6 and 0.35 eV respectively). At higher incident energies, efficient transfer of translational to vibrational energy occurs in those ground-state D2 molecules that survive the collision without dissociating. These data will allow a better test of the various potential-energy curves that have been proposed for this system in the dissociative region.

Reactive Scattering of Si2 H6 from the Si(100) Surface

ABSTRACTThe reaction of Si2H6 with the Si(100) surface has been examined via supersonic molecular beam scattering techniques. The effects of incident translational energy, incident angle, mean vibrational energy and surface temperature have been considered explicitly. It is found that the reaction probability is most sensitive tothe incident translational energy, varying nearly linearly with increasing energy. The effect of incident angle θi can be accounted for by an energy scaling law that is given approximately by <Etr>cosnθi, where n ∼ 1.

The reaction of atomic oxygen with Si(100) and Si(111): II. Adsorption, passive oxidation and the effect of coincident ion bombardment

The reactions of atomic oxygen with the (100) and (111) surfaces of silicon have been investigated by employing supersonic molecular beam techniques, X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering spectroscopy (ISS). Atomic oxygen adsorbs with unit probability on the clean silicon surface, independent of substrate temperature (110–800 K) and incident mean translational energy (3–16 kcal mol −1). Oxidation of clean silicon with an oxygen atom beam is characterized by wo stages: a “fast” stage that corresponds to oxygen chemisorption in the topmost 2–3 silicon layers; and a “slow” stage that corresponds to oxygen incorporation and oxide film growth. The chemisorption stage is described by first-order Langmuirian kinetics with an apparent saturation coverage of approximately 4 ML O(a), the oxide growth stage by lddirect” logarithmic kinetics, where d x/d t = α exp(− x/ L), where x is the oxide thickness. Observation of significant oxidation at substrate temperatures of 110 K suggests that oxie growth in the slow stage may occur bya field-assisted mechanism, where an internal electric field aids transport of oxygen to the underlying silicon substrate layers. XPS and ISS results support a two-dimensional layer-by-layer growth mechanism for oxidation at substrate temperatures below 900 K. At higher temperatures, T ≥ 1050 K, oxide growth is three-dimensional involving nucleation and growth of bulk-like oxide islands even for mean coverages as low as 3 ML O(a). ISS results lend support to the formation of “on-top” adsorbed oxygen atoms that cap silicon dangling bonds at the oxide/gas interface. Coincident bombardment of the silicon substrate with an Ar − ion beam leads to an enhanced rate of oxidation. The enhancement can be understood in terms of a model involving secondary implantation of adsorbed oxygen atoms, coupled with the simultaneous formation of reactive sites (e.g., dangling bonds, vacancies) for oxygen chemisorption. The effect of coincident ion bombardment is reduced at elevated substrate temperatures (∼ 800 K), since the resulting increased propensity for adlayer rearrangement leads to a decrease in the number of active sites for oxygen chemisorption.

Scattering of vibrationally excited H2(D2) from Cu(111)

The interaction of vibrationally excited H2(D2) with a Cu(111) surface has been studied using molecular beam scattering with state-selected detection of the incident and scattered molecules by resonance-enhanced multiphoton ionization. The ratio of the survival probability for H2( upsilon =1, J=1) in the specular beam to that for upsilon =0, J=1 is 0.67+or-0.1 for an incident translational energy of 0.34 eV at 45 degrees incidence. This is consistent with efficient dissociative chemisorption of the H2( upsilon =1) level, dissociation of the upsilon =0 level being negligible at this energy. Under the same conditions the survival probability for D2 is 0.8, again consistent with the sticking measurements which show a lower sticking probability for D2 than for H2.

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.

Scattering of vibrationally excited H 2 from Cu(111)

The interaction of hydrogen with a Cu(111) surface has been studied using a vibrationally hot molecular beam with state selected detection of the incident and scattered molecules by resonance-enhanced multiphoton ionisation. The ratio of the survival probability for H 2 ( v=1, J=1) in the specular beam to that for ( v=0, J=1) is 0.67±0.1 for an incident translational energy of 0.34 eV at 45° incidence. This is consistent with efficient dissociative chemisorption of the H 2 ( v=1) level, dissociation of the ( v=0) level being negligible at this energy.

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.

Evidence for precursor‐mediated Cl2 etching of GaAs{110}: Effect of surface temperature and incident translational energy on the reaction probability

Angle‐resolved supersonic molecular beam scattering and time‐of‐flight techniques are used to probe the dynamics of the Cl2/GaAs{110} thermal etching reaction by measuring the polar angle and velocity distributions of the unreacted Cl2 and of the primary etch product GaCl3. Measurements of the reaction probability versus incident Cl2 translational energy reveal that the reaction mechanism is precursor mediated and that the weakly bound molecular (Cl2)ads species is a key reaction intermediate. At high surface temperatures (∼550 K) the angle distribution of the GaCl3 product species is consistent with the absence of an activation barrier to desorption and from x‐ray photoelectron spectroscopy measurements the GaAs{110} surface appears to etch stoichiometrically.