Supersonic Molecular Beam Techniques Research Articles

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 dynamics of krypton trapping on Pt(1 1 1) at 50 K: experiment and molecular dynamics simulation

The dynamics of krypton trapping on clean Pt(1 1 1) were investigated using supersonic molecular beam techniques at a surface temperature of 50 K. The initial trapping probability of Kr scales with normal incident energy (E T cos 2θ) , indicating a smooth gas–surface potential. The Kr trapping probability decreases from 0.8 to 0 as the incident normal energy is increased from 2 to 23 kJ/mol. Trapping on the krypton-saturated monolayer is greatly enhanced at all incident energies and angles compared to the clean surface, and exhibits near total energy scaling (E T cos 0.3θ) , indicating a corrugated gas–surface potential. Stochastic trajectory simulations [J. Chem. Phys. 105 (1996) 1609] using a Morse potential with parameters based on simulations of Ar trapping [J. Chem. Phys. 94 (1991) 1516] agree with the experimentally measured energy and angular dependence of the trapping probability of Kr.

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.

The Extrinsic Precursor Kinetics of Methane Adsorption onto Ethylidyne-Covered Pt(111)

The kinetics of methane trapping on ethylidyne-covered Pt(111) were investigated using supersonic molecular beam techniques at surface temperatures between the temperature at which methane desorbs ...

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.

酸素分子の並進運動エネルギー誘起Ｓｉ（００１）表面初期酸化過程の放射光光電子分光法による“その場”観察

The effects of translational kinetic energy of incident O{sub 2} molecules for initial Si(001) oxidation at room temperature have been investigated by using a supersonic molecular beam technique and photoemission spectroscopy. The oxygen saturation coverage on Si (001) surface increased with increasing the translational kinetic energy of O{sub 2}. Two threshold energies, corresponding to the Si dimer backbond oxidation and the second layer (sub-surface) Si atoms backbond oxidation were observed at 1.0 eV and 2.6 eV, respectively. We clarified the relationship between the Si oxidation structure (Si{sup x+}; x =1,2,3,4) in Si-2p photoemission spectra and the translational kinetic energy of O{sub 2} in the initial Si(001) oxidation. (author)

Commissioning of surface chemistry end-station in BL23SU of SPring-8

An experimental apparatus for surface reaction research is now under commissioning as an end-station of a soft X-ray beamline, BL23SU, in the SPring-8. A supersonic molecular beam (SSMB) technique is applied to control the translational-energy of incident molecules. Owing to keeping low pressure in vacuum chambers during molecular beam operation by differential-pumping systems, the suppression of gas diffusion into the beamline was sufficiently achieved. The translational-energy control and photoemission spectroscopic performance were checked by preliminary experiments concerning the silicon oxidation using a conventional X-ray source. The photoemission intensity from chemisorbed oxygen atoms on an Si(1 0 0) surface was enhanced with increasing the translational-energy of incident oxygen molecules up to 3 eV.

Molecular beam studies on inelastic collision processes of methane molecules at a Pt(1 1 1) surface

Inelastic collision processes of CH 4 at a Pt(1 1 1) surface have been analyzed by means of super-sonic molecular beam technique. Obtained results of the angular intensity distribution of the reflected CH 4 with the incident kinetic energy ranging from 190 to 350 meV are found to qualitatively agree with the prediction from the classical collision model. Surface temperature dependence of the speed distributions of the reflected CH 4 qualitatively follows the prediction at the surface temperature below 500 K, but deviates from the prediction as the surface temperature increases under the irradiation of CH 4 with higher kinetic energies, where decomposition of CH 4 takes place. The discrepancy of data from the model can be understood by introducing into the model decreased effective surface mass and increased surface “roughness”.

Trapping of Ar on well ordered Ar, Kr, and Xe overlayers on Pt(1 1 1) at 30 K

The dynamics of Ar trapping on Ar, Kr, and Xe covered Pt(1 1 1) were investigated using supersonic molecular beam techniques at a surface temperature of 30 K. The initial trapping probability of Ar on the clean surface decreases from 0.8 to 0 as the normal incident energy is increased from 1.75 to 28 kJ mol −1, and scales with normal incident energy (E T cos 2θ) , indicating a smooth gas–surface potential [A.F. Carisson R.J. Madix, submitted for publication]. In contrast, the trapping probability on the Ar, Kr and Xe saturated surfaces decreases from 0.9 to only 0.3 in the same energy range and is nearly independent of the angle of incidence. The relative trapping probability on the Ar and Xe covered surfaces is in general accord with the relative energy exchange expected from the simple Baule model, but the trapping probability on the Kr covered surface appears further enhanced by potential corrugation. Experimental results for Ar trapping on Pt(1 1 1)–Ar are in qualitative agreement with a recoil effect predicted by Head-Gordon and Tully for Ar trapping on Ar covered Ru(0 0 1) [Surf. Sci. 268 (1992) 113]. At high incident energies on the gas covered Pt(1 1 1) surface, the adsorption probability is nearly independent of angle of incidence between 0° and 30°, but it then increases rapidly between 30° and 60°. Similar effects are observed for Ar trapping on Pt(1 1 1) saturated by Kr and Xe. The dependence of the trapping probability on coverage is quantitatively described by the modified Kisliuk model [J. Chem. Phys. 92 (1990) 1397; J. Phys. Chem. 95 (1991) 2461], which allows for an extrinsic precursor to adsorption as well as a direct channel. The probability of Ar trapping increases with Ar uptake to different degrees on the Ar, Kr and Xe covered surfaces.

The effects of alkylidyne and atomic oxygen overlayers on the dynamics of adsorption of methane on Pt(111)

The dynamics of the trapping of methane on p(2×2)-oxygen, -ethylidyne, -butylidyne and -isobutylidyne overlayers on Pt(111) at 50 K was investigated using supersonic molecular-beam techniques. Since each of these adsorbates forms the same structure, trapping dynamics could be studied as a function of the internal structure of the adsorbate. Trapping probabilities on these surfaces at a fixed incident energy and angle increase in the order oxygen<ethylidyne<isobutylidyne<butylidyne, and each of the adsorbate-covered surfaces enhances trapping compared with the clean surface. On the butylidyne-covered surface the initial molecular trapping probability decreases from 0.88 to 0.38 as the incident energy is increased from 8 to 24 kJ mol −1, compared with a decrease in trapping probability from 0.3 to zero over the same energy range on the clean surface. The angular dependence of the trapping probability indicates that the overlayers lead to an increase in the static corrugation in gas–surface potential. The generally higher values of the trapping probabilities on the alkylidyne-covered surfaces and the weaker angular dependence they exhibit suggest that they have a more corrugated potential than the clean surface. The further increase in trapping probabilities for methane on Pt(111) covered by the analogous alkanes suggests that the additional frustrated translations of the alkanes may also enhance trapping into the extrinsic precursor. The change in the adsorption probability with methane coverage on the oxygen-covered surface is described by the modified Kisliuk model [J. Phys. Chem. 95 (1991) 2461; J. Chem. Phys. 92 (1990) 1397]. The trapping probability increases with methane coverage, indicating that trapping into the extrinsic precursor state is more efficient in the presence of methane.

On the initial steps in the decomposition of energetic materials from excited electronic states

Decomposition studies of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX-C3H6N6O6, see Fig. 1) isolated in the gas phase and cooled in a supersonic expansion are reported for the excited electronic state near 225 nm. The RDX is handled safely and effectively through matrix-assisted laser desorption (MALD) of a thin film of RDX/R6G laser dye (1:1) adsorbed on an aluminum oxide coating on an aluminum drum. The aluminum oxide coating is generated by plasma electrolytic oxidation of aluminum. The combination of MALD and supersonic molecular beam techniques generates intact and cold RDX molecules isolated in the gas phase. Two basic conclusions are reached in this study. (1) Photodissociation of RDX at ≈225 nm generates NO as an initial product. (2) Nascent NO thus generated is vibrationally hot (Tvib∼1800 K) and rotationally cold (Trot∼20 K).

Dynamics of ammonia decomposition on Ru(0001)

Using supersonic molecular beam techniques we have investigated the dissociative adsorption of NH3 on a Ru(0001) surface. At high incident energies, the dissociation increases substantially due to a direct breaking of the N–H bond on impact with the surface. For low incident translational energies, the dissociation depends on surface temperature Ts in an unusual manner, peaking sharply around 400 K. Increasing the surface defect density by low-fluence Ar+ sputtering strongly enhances the dissociation probability while preserving the overall Ts-dependence. We interpret the low incident energy behavior as due to a mechanism in which a molecular precursor must undergo diffusion to defects before dissociating. At the lowest surface temperatures, dissociation is limited by the diffusion of the reaction products away from the defects in order to reactivate them. A kinetic model based on this mechanism is developed which is in good agreement with all experimental observations.

Trapping-mediated chemisorption of disilane on Si(100)-2×1

Disilane adsorption probabilities have been measured on Si(100)-2×1 over a wide range of incident kinetic energies, incident angles, and surface temperatures using supersonic molecular beam techniques. The trapping-mediated chemisorption mechanism is shown to be the dominant adsorption pathway under the conditions investigated. The first step in such a mechanism, namely trapping into the physical adsorption well, has been studied directly via measurements at a surface temperature of 77 K. As expected, the trapping probability drops with increasing kinetic energy, but nearly 50% of incident molecules trap at 1 eV incident energy, indicating that trapping is quite efficient over a wide range of translational energies. Chemisorption probability values measured at higher surface temperatures are fit to a simple trapping-mediated chemisorption model that can be used to predict adsorption probabilities over a wide range of conditions. Measurements of the chemisorption probability at 500 K are independent of incident angle at kinetic energies of 0.75 eV and below. However, trapping probabilities measured at 77 K are shown to decrease with increasing angle of incidence at kinetic energies of 0.6 eV and above. This unusual effect is discussed in terms of molecular scattering during parallel momentum accommodation. In order to investigate the effect of surface hydrogen formed as a result of disilane decomposition, adsorption probabilities were measured as a function of monohydride coverage as well. On a monohydride-saturated surface the trapping probability is found to be lower than on a bare surface, most likely due to a decreased disilane physical adsorption binding energy compared to the bare surface. Also, the trapping probability varies linearly with hydrogen coverage between bare-surface and monohydride-saturated values. On the other hand, the hydrogen coverage dependence of the chemisorption probability is found to follow a simple second-order kinetic scheme based on chemisorption occurring at two vacant surface sites.

The trapping of methane on ordered structures of CO on Pt(111)

The dynamics of methane trapping on CO-covered Pt(111) in low coverage, c(√3×5)rect, and c(4×2) structures was investigated using supersonic molecular beam techniques at a surface temperature of 50 K; at this temperature methane was stably adsorbed on the clean (A. F. Carlsson and R. J. Madix, to be published) surface, but not in multilayers (A. F. Carlsson and R. J. Madix, to be published), and thus trapped amidst adsorbed CO molecules. Molecular trapping was enhanced to greater degrees with increasing CO coverage, and the methane uptake decreased with increasing CO coverage, as would be expected. The trapping probability further increased as methane covered the Pt(111)–CO surface; the modified Kisliuk model [J. Chem. Phys. 92, 1397 (1990); J. Phys. Chem. 95, 2461 (1991)] describes the coverage-dependent trapping probability. Methane adsorption may occur directly on the surface, or via two entrance channels into an extrinsic precursor, where the trapping probability is higher. The angular dependence of methane trapping on the CO-covered surface suggests an increasing corrugation in the gas–surface potential as the CO coverage increases; the corrugation may also contribute to the enhancement of the overall trapping probability by facilitating interconversion of perpendicular to parallel momentum during the gas–surface collision.

The dynamics of argon and methane trapping on Pt(111) at 30 and 50 K: energy scaling and coverage dependence

The dynamics of argon and methane trapping on clean Pt(111) were investigated using supersonic molecular beam techniques at a surface temperatures of 30 and 50 K, well below the desorption temperatures of 46 and 67 K, respectively. The initial trapping probabilities for both Ar and methane scale with normal incident energy ( E T cos 2 θ), indicating a smooth gas–surface potential. The trapping probability for Ar decreases from 0.8 to zero as the incident normal energy is increased from 2 to 29 kJ mol −1. Below 10 kJ mol −1 incident energy, the results agree with previous experiments [C.B. Mullins et al., Chem. Phys. Lett. 163 (1989) 111] and molecular dynamics simulations [M. Head-Gordon et al., J. Chem. Phys. 94 (1991) 1516] which were conducted above the desorption temperature. The trapping probability for methane decreases from 0.7 to zero as the incident normal energy is increased from 3 to 20 kJ mol −1. Trapping on the argon- or methane-saturated surface is greatly enhanced compared to the clean surface at all incident energies and angles, and exhibits near total energy scaling ( E T cos 0.3 θ), indicating a corrugated gas–surface potential. The kinetics of trapping as a function of coverage are quantitatively described by the modified Kisliuk model [H.C. Kang et al., J. Chem. Phys. 92 (1990) 1397; C.R. Arumainayagam et al., J. Chem. Phys. 94 (1991) 1516], which allows for an extrinsic precursor to adsorption. The trapping probabilities of both argon and methane increase with coverage, indicating that trapping into an extrinsic precursor state is more efficient than trapping onto the bare Pt(111) surface.

Direct dissociative chemisorption of alkanes on Pt(111): Influence of molecular complexity

The direct dissociative chemisorption of ethane, propane, n-butane, isobutane, and neopentane on Pt(111) was investigated as a function of the initial translational energy, ET, polar angle of incidence, θi, initial vibrational temperature, and surface temperature using supersonic molecular beam techniques. For each alkane, the initial probability for direct dissociative chemisorption scales with the initial normal energy of the alkanes, En=ET cos2 θi, and is independent of both the surface temperature and initial vibrational energy of the alkanes under the experimental conditions employed. Above initial normal energies of approximately 125 kJ/mol, at constant En, the dissociation probability decreases with increasing chain length of the C2–C4 linear alkanes; however, the dissociation probability of neopentane is greater than that of isobutane, and both isobutane and neopentane are more reactive than n-butane. By assuming that cleavage of primary C–H bonds is the dominant reaction pathway for all of the alkanes investigated here, the trends in reactivity are best explained by considering the differences in the steric factors for primary C–H bond cleavage for these alkanes. Secondary C–H bond cleavage does appear to contribute to the reactivity of propane and n-butane but only at the highest energies examined. Additionally, the reaction probabilities of each of these alkanes were estimated using a statistical model recently proposed by Ukrainstev and Harrison [J. Chem. Phys. 101, 1564 (1994)]. Assuming cleavage of only primary C–H bonds, the trends in reactivity for ethane, propane, n-butane, and isobutane were qualitatively reproduced by the statistical model; however, except for ethane, which was used to obtain the necessary parameters for the theory, there was poor quantitative agreement, and the predictions for neopentane were significantly lower than the measured values. The model also predicts that the dissociation probability is enhanced by increasing the energy in all vibrational modes, which is inconsistent with the experimental results. Thus, we believe that direct alkane dissociation would be better described using a dynamical rather than statistical approach.

Molecular ethane adsorption dynamics on oxygen-covered Pt(111)

The dynamics of ethane trapping on Pt(111)-p(2×2)-O were investigated by supersonic molecular beam techniques at a surface temperature of 100 K. The initial trapping probability was measured in the range of incident energy from 10 to 45 kJ/mol and incident angles from 0° to 60°. A broad angular distribution of scattered ethane and total energy scaling (ET cos 0.2θ) for ethane trapping indicated a corrugated gas–surface potential. Stochastic trajectory simulations employing a potential developed from the trapping of ethane on Pt(111) gives quantitative agreement of the measured initial trapping probabilities over entire ranges of incident energies and angles. Calculations of energy transfer for ethane after the first bounce on Pt(111) and Pt(111)-p(2×2)-O clearly indicate that interconversion of parallel and perpendicular momentum and energy transfer to lattice vibrations account primarily for the differences in trapping probabilities between ethane on the two surfaces. At glancing incidence trapping is not significantly reduced on the oxygen-covered Pt(111) because the parallel momentum appears to be transferred partially to phonons.

Molecular beam study on interaction dynamics in a reactive system: NO on bare Ru(0001)

The interaction dynamics of NO with the reactive Ru(0001) surface has been investigated with the use of supersonic molecular beam techniques. Helium scattering, King and Wells sticking measurements and time-of-flight (TOF) experiments are performed. The initial sticking coefficient is remarkably high with incident energy, varying from unity at thermal energies to ∽90% for 0.25⩽Ei⩽2.4 eV and does not seem to depend on surface temperature for 400⩽Ts⩽850 K. Counter intuitively, the non-sticking ∽10% of the incident flux has only about 12% translational energy transfer in the specular scattering direction. Molecular sticking is observed at low surface temperatures for above thermal incident energies, up to at least 0.45 eV.An attempt is made to distinguish between molecular precursor mediated pathways and direct dissociation. In order to account for the surprisingly constant initial sticking coefficients, the opening of a direct dissociative channel is proposed for beam energies exceeding the depth of the molecular chemisorption well. Several open problems are identified.

Trapping dynamics of isobutane, n-butane, and neopentane on Pt(111): Effects of molecular weight and structure

The molecular adsorption dynamics of isobutane, n-butane, and neopentane on Pt(111) was investigated using supersonic molecular beam techniques and stochastic trajectory calculations. Using the united atom approach, a single, pairwise-additive Morse Potential for methyl (methylene)–plantinum interactions quantitatively simulates the dependence of the initial trapping probability, α, on the initial translational energy, ET, and angle of incidence, θi, for each alkane. For both isobutane and n-butane, the dependence of α on ET and θi best scales with ET cos0.8θi, which is similar to that found previously for ethane and propane trapping on Pt(111). The initial trapping probability of neopentane exhibits a more pronounced dependence on θi, which scales according to ET cos1.3θi. The simulations suggest that the enhanced angular dependence of α for neopentane is related to its molecular weight. As the mass of the incident species is increased, momentum transfer to the surface becomes more efficient than the interconversion of incident parallel and normal momentum due to corrugation of the surface potential. The net effect is an increase in the trapping probability at glancing incidence compared to lighter molecules, and a resulting shift in the angular dependence of α towards normal energy scaling. The calculations also predict that collisional energy transfer to rotation is important in promoting adsorption. For each molecule, rotational excitation is determined to be the most effective energy transfer process that discriminates trapping from scattering. In addition, translational energy transfer to torsional vibration about the central C–C bond is highly efficient for n-butane, and greatly facilitates adsorption. Less excitation is predicted for C–C–C bending modes for all of these molecules.

Design of Surface Chemistry End-Station of BL23SU in SPring-8

An experimental apparatus for surface reaction research has been designed as an end-station of a soft x-ray beamline, BL23SU, in the SPring-8. The soft x-ray with intense flux density is provided by a variable-polarizing undulator and is monochromized by using a varied line-spacing plane grating with grazing-incidence. A supersonic molecular beam technique is applied to obtain translational-energy control of incident molecules. Owing to keeping low pressure in the surface reaction chamber during molecular beam operation, the analysis of translational-energy-induced surface reactions is achieved by simultaneous application of the photoelectron spectroscopy. The end-station will promote research activities on new atomic layer reactions induced by the kinetic energy of incident molecules as well as new photofragmentation of monolayer due to the inner-shell selective photoexcitation.