Supersonic Molecular Beam Techniques Research Articles

Adsorption dynamics of monoenergetic iodine monobromide (IBr) on the Si(111)-7×7 surface

The adsorption of monoenergetic IBr molecules on the Si(111)-7×7 surface has been studied using scanning tunneling microscopy, mass spectrometry, Auger electron spectroscopy, and supersonic molecular beam techniques. The adsorption proceeds predominantly via the direct abstractive adsorption mechanism and preferentially occurs at the center Si adatoms. The IBr abstraction probabilities at the incident energies of 0.15 and 0.82 eV have been determined to be 0.90±0.03 and 0.77±0.03, respectively. The minor dissociative adsorption channel of IBr can be enhanced at the expense of the abstractive adsorption channels by raising the incident energy. Most importantly, no atomic selectivity for iodine or bromine was observed. A reaction mechanism involving two types of transition states, Si⋯I⋯Br(s) and Si⋯Br⋯I(s), has been proposed to interpret the experimental observations. The attractive interaction between the nearly symmetric highest occupied molecular orbitals (HOMO, π* antibond) of IBr and the partially-filled Si adatom dangling bonds governs the surface site selectivity and the atomic selectivity of IBr adsorption on Si(111). Comparison with the adsorption of ICl on the surface has also been made to clarify the role of the asymmetric molecular bonding in adsorption dynamics.

Collision induced desorption and dissociation of O2 chemisorbed on Ag(001)

We have investigated desorption and dissociation of O2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms by high resolution electron energy loss spectroscopy and supersonic molecular beam technique. The cross section for both processes increases rapidly both as a function of angle of incidence and of total impact energy of the inert gas atom. While the increase with energy is expected, the increase with the angle is somewhat surprising and is sensibly larger than observed for previously investigated systems. The cross section for desorption decreases moreover with coverage. In the limit of high impact energy and high coverage its value is always larger than the one for dissociation. The branching ratio between the two processes depends thereby on energy and angle of incidence of the inert gas atom. Atomic oxygen is not removed under any impact condition, because of its larger binding energy. In order to explain the experimental results, molecular dynamics simulations have been performed using a simple model including multiple scattering. We find that the angular dependence of the cross section is determined by surface corrugation and by multiple scattering which suppresses desorption at normal incidence while the energetic threshold is determined by energy loss to the substrate.

The effect of strain on gas–surface reactivity in group-IV heteroepitaxial systems

Supersonic molecular beam techniques have been employed to examine the dissociative adsorption of GeH 4 and Si 2H 6 on strained Si 1− x Ge x (0≤ x≤1) epitaxial overlayers deposited on Si(100). Low-energy ion scattering spectrometry (LEISS) has been utilized to quantify in situ the topmost Ge surface composition of the Si 1− x Ge x films. Through the measurements of reaction probability we find that these ultrathin strained semiconducting overlayers, both alloy and pure component (i.e., Ge/Si(100)), exhibit enhanced gas–surface reactivity when compared to their bulk (relaxed) counterparts, reflecting the critical role played by strain in these epitaxial overlayers.

Adsorption of oxygen on Pd(111): Precursor kinetics and coverage-dependent sticking

The adsorption of oxygen on Pd(111) was studied using supersonic molecular beam techniques. The dependences of the initial sticking coefficient (zero coverage) on O2 translational energy (0.083–0.69 eV) and substrate temperature (100–650 K) indicate a precursor dissociation mechanism involving a chemisorbed and a physisorbed precursor state. At low translational energies, ⩽0.13 eV, the chemisorbed (molecular) precursor state is populated either directly or via trapping into the physisorbed precursor state, while the direct molecular chemisorption channel dominates at higher translational energies. No indication of direct dissociation was observed. The oxygen coverage dependence of the sticking coefficient shows that extrinsic precursors assist the sticking at low substrate temperatures. The saturation coverage is independent of O2 translational energy, supporting the proposed precursor dissociation 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.

Gas—surface reactivity in mixed-crystal systems: the reaction of GeH 4 and Ge 2H 6 on Si surfaces

Supersonic molecular beam techniques have been employed to examine the dissociative adsorption of GeH 4 and Ge 2H 6 on the Si(100) and Si(111) surfaces. At sufficiently high incident kinetic energies (⩾ 1 eV), both species on both surfaces react via a direct dissociation mechanism. At sufficiently low incident kinetic energies (⩽ 0.6 eV) and substrate temperatures, however, dissociation of Ge 2H 6 on the Si(100) surface proceeds primarily through a trapping, precursor-mediated dissociation channel. Study of this “mixed-crystal” system, i.e., Ge nH 2n + 2 on Si, has permitted important comparisons to two related systems: the reactions of these same gas phase species (GeH 4 and Ge 2H 6) on Ge surfaces, and the reactions of silanes (SiH 4 and Si 2H 6) on these same Si surfaces. We find that the Si surfaces are much more reactive (as much as a factor of 10) than their Ge counterparts, whereas, under similar reaction conditions, and on the same Si surface, the Ge hydrides are moderately more reactive (≈ factor of 2) than the Si hydrides. The reactivity of ultrathin (≈ 2 monolayers) Ge( a) epitaxial layers has also been examined. On Si(100), these strained Ge epitaxial layers exhibit a reactivity that is intermediate between those observed on clean Si(100) and on clean (bulk) Ge(100) surfaces. This example illustrates the role that strain can play in gas-surface reactivity, heretofore, an almost unexamined subject. On the other hand, on Si(111), the Ge epitaxial layer is less reactive than both clean elemental surfaces, and this behavior is attributed to intrinsically low reactivity exhibited by the Ge-induced (5 × 5) reconstruction known to form on this surface.

Laser induced gas-surface reaction of Cl 2 supersonic molecular beam with a nickel surface

The supersonic molecular beam technique is used to study the dynamics of laser-induced reaction of Cl 2 with a Ni surface irradiated by 355 nm and 1064 nm laser photons. The mass and velocity distribution of reaction products are measured by time-resolved mass spectrometry. Results show that the genuine products generated by the irradiation at 355 nm are NiCl and NiCl 2, while only NiCl 2 is identified at 1064 nm. The reaction yield can be enhanced by increasing the translational energy of incident Cl 2 molecules and the laser fluence. The possible mechanisms for the laser-induced Cl 2Ni surface reaction at different laser wavelengths are discussed.

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<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.

Energy and angle dependence of the initial sticking coefficient of O 2 on Ag(001)

We investigated the dynamics of the adsorption of O 2 on Ag(001) with the supersonic molecular beam technique combined with EEL spectroscopy, TDS and the reflection detector technique. The initial sticking coefficient S 0 was measured as a function of the impact energy and of the angle of incidence of the molecules at crystal temperatures of 100 and 300 K, corresponding to molecular and dissociative adsorption, respectively. Both regimes are characterized by the same distribution of energy barriers. This indicates that, in analogy to the case of our previous investigation of O 2Ag(110), the main reaction path involves activated adsorption into the molecular well, followed eventually by thermally induced dissociation for T s > 150 K. Again in analogy to O 2Ag(110), the physisorbed state plays no role in the chemisorption process.

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.

Kinetics and dynamics of the trapping-mediated dissociative chemisorption of oxygen on Ru(001)

Supersonic molecular beam techniques were used to determine the initial dissociative chemisorption probability, S0, of O2 on the Ru(001) surface as a function of incident kinetic energy, Ei, angle of incidence, θi, and surface temperature, Ts. For Ts between 77 and 900 K, two kinetic energy regimes are observed as Ei increases from 30 to 700 meV. In this article we discuss the mechanism for chemisorption in the low kinetic energy regime. S0 decreases with Ei for incident energies less than ∼70 meV, and there is little dependence of S0 on θi in this low energy range. Additionally, S0 decreases rapidly as Ts is increased in this regime, and we believe that a simple trapping-mediated mechanism dominates adsorption. The low Ei data are fit to a simple one-dimensional potential energy surface model that includes the molecule initially trapping into a physical adsorption well and then surmounting a barrier into the dissociatively chemisorbed state. Analysis of an Arrhenius construction yields a difference in activation energies for desorption and dissociation from the physically adsorbed state that is approximately 28 meV below the vacuum zero, for both Ei =30 and 53 meV.

Dynamics of molecular chemisorption of N2 on the Ru(001) surface

The chemisorption of N2 on Ru(001) has been studied using supersonic molecular beam techniques. The initial molecular chemisorption probability, S0, was measured as a function of incident energy, Ei, from 0.021 to 0.90 eV and angle of incidence, θ, from 0° to 60° at a surface temperature of 77 K. At a normal angle of incidence, the value of S0 decreases with increasing kinetic energy in the low and intermediate energy regimes and is constant in a higher energy (0.4 eV<Ei<0.7 eV) regime. As the angle of incidence increases, the value of S0 decreases more rapidly with Ei at low energies. At θ=60°, S0 decreases with increasing kinetic energy, passes through a local minimum near Ei=0.2 eV, and then increases to the same constant value as in the normal incidence case. At Ei=0.90 eV, the most energetic beam used in this study, the value of S0 decreases slightly from this constant value. Similar measured trends in the angular and energy dependence, particularly at intermediate energies, have not been reported previously for a molecular chemisorption system, and we qualitatively describe this behavior by combining a trapping-mediated mechanism (dominant at low incident energies) and a direct molecular chemisorption mechanism (dominant at high incident energies). We also report measurements of the initial dissociative chemisorption probability of N2 on Ru(001) and conclude that this probability at Ei=1.25 eV and normal incidence does not exceed 1×10−3 at a surface temperature of 600 K.

The adsorption of PH3 on the Si(111)-(7 × 7) surface: an example of autocatalytic dissociative chemisorption

The dissociative chemisorption of phosphine, PH 3 , on the Si(111)-(7 × 7) surface has been examined employing supersonic molecular beam techniques. The initial probability of reaction, S R,0 , has been found to be sensitive to substrate temperature, T s , where S R,0 increases sharply by approximately a factor of 4–5 as T s is increased above 800°C, which corresponds well with the (7 × 7) ↔ “(1 × 1)” phase transition. The reaction probability, S R , measured as a function of dose for PH 3 reacting on Si(111)-(7 × 7) at T s < 800°C, exhibits a dramatic increase as the surface is exposed to the PH 3 molecular beam. This unique autocatalytic behavior is consistent with a mechanism in which submonolayer coverages of P(a) are capable of lifting the (7 × 7) reconstruction thus giving rise to a more reactive “(1 × 1)-like” phase. The reaction probability of Si 2 H 6 on Si(111)-(7 × 7) is also observed to pass through a maximum with increasing P(a) coverages, and can be explained by considering similar changes in surface structure and reactivity.

Effect of PH3 on the dissociative chemisorption of SiH4 and Si2H6 on Si(100): Implications on the growth of in situ doped Si thin films

The effect of PH3on the dissociative chemisorption of SiH4 and Si2H6 on the Si(100) surface has been investigated with supersonic molecular beam techniques. Adsorbed phosphorus atoms, formed as a result of the reaction of PH3on the Si surface, inhibit the dissociative chemisorption of both SiH4 and Si2H6 through a short-range steric mechanism. The probability of dissociative adsorption of both silicon hydrides is found to be proportional to the quantity 1-θP2, where θP is the fractional coverage of adsorbed phosphorous atoms. These results have been employed to formulate a predictive model for the kinetics of Si thin-film growth in the presence of PH3(g).

Initial probability of dissociative chemisorption of oxygen on iridium(110)

The dissociative chemisorption of oxygen on Ir(110) has been investigated using supersonic molecular beam techniques. The initial probability of dissociative chemisorption (in the limit of zero surface coverage) as a function of incident kinetic energy between 1 and 28 kcal/mol and surface temperature from 85 to 1000 K is reported. For beam kinetic energies less than approximately 4 kcal/mol, the measured values of the initial probability of dissociative chemisorption are explained by a trapping-mediated adsorption mechanism. In this adsorption regime initial probabilities of dissociative chemisorption decrease with both increasing beam energy and surface temperature. The trapping probability of oxygen into the physically adsorbed state on Ir(110) as a function of incident beam energy is presented. For beam kinetic energies greater than ∼4 kcal/mol, a direct chemisorption mechanism dominates. In the direct adsorption regime, initial probabilities of dissociative chemisorption increase with increasing beam energy, and they are dependent on surface temperature, with the dependence decreasing with increasing surface temperature. This behavior is attributed to direct chemisorption into a molecularly chemisorbed state, from which there is a thermally activated kinetic competition between desorption and dissociation. A pseudo-steady-state kinetic model including physically adsorbed oxygen, molecularly chemisorbed oxygen, and atomically chemisorbed oxygen is applied to find that the activation barrier to desorption from the physically adsorbed molecular state is 1.6±0.1 kcal/mol higher than the barrier to conversion to the molecularly chemisorbed state. The activation barrier for desorption from the molecularly chemisorbed state is 1.5±0.15 kcal/mol greater than the barrier to dissociation from this state.

Influence of orientation on coadsorption dynamics: CO displacement from ac(2×2) precovered Ni(100) surface by free oriented NO

Collision experiments between two different molecules both oriented with their axis are presented: The kinetics of the CO desorption and NO sticking probability by oriented NO has been investigated using supersonic molecular beam and work function techniques. The work function measurements exhibit mainly molecular adsorption of NO on a CO precovered Ni(100) surface at T=300 K. The desorption and the sticking depend strongly on the initial orientation of the NO molecules in the gas phase. The sticking probability is higher for preferential N‐end collisions and the sticking asymmetry is constant, whereas the desorption asymmetry changes its sign from initially negative to positive values, depending on the exposure time or NO coverage. We explain the asymmetries by two different mechanisms, namely direct and indirect molecular exchange. N‐end collisions lead preferentially to NO sticking and NO induced CO desorption, whereas O‐end collisions enhance the direct CO displacement.

Beam investigations of D2 adsorption on Si(100): On the importance of lattice excitations in the reaction dynamics

The adsorption of D2 on Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D2 sticking coefficient S on surface temperature Ts and nozzle temperature Tn. The sticking coefficient increases gradually in the range 300≤Tn≤1040 K. The influence of increased v=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on Ts is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5±0.1)×10−5 at Ts=630 K. The decrease in the effective sticking coefficient beyond this Ts is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excitations play an important role in the adsorption process.

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.

Kinetic isotope effect in direct ethane dissociation on Pt(111)

Supersonic molecular beam techniques were used to study the dynamics of direct C 2H 6 and C 2D 6 dissociation on Pt(111). The initial dissociation probabilities for both isotopes, S 0(C 2H 6 and S 0(C 2D 6), increased with normal translational energy, E n, over the entire range of E n studied. A significant normal kinetic isotope effect was observed; the ratio S 0(C 2H 6/ S 0(C 2D 6) decreased from 2.7 to 1.6 as E n was increased from 80 to 118 kJ mol . A one-dimensional quantum mechanical tunneling model based on an Eckart potential barrier quantitatively accounts for these observations. After correcting for translational energy dissipation to the lattice, the extracted barrier heights, V 0, and widths, L, are 123 kJ mol , 1.1 Å and 130 kJ mol , 1.1 Å for C 2H 6, and C 2D 6, respectively. The larger barrier height for the direct dissociation of C 2D 6 relative to C 2H 6 can be attributed to differences in zero point energy for C-H(D) stretching motion. Neither. S 0(C 2H 6) nor S 0(C 2D 6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl rocking and deformation, C-C stretching and torsion do not promote direct ethane dissociation on Pt(111) under these experimental conditions.

Kinetic isotope effect in direct ethane dissociation on Pt(111)

Supersonic molecular beam techniques were used to study the dynamics of direct C2H6 and C2D6 dissociation on Pt(111). The initial dissociation probabilities for both isotopes, S0(C2H6 and S0(C2D6), increased with normal translational energy, En, over the entire range of En studied. A significant normal kinetic isotope effect was observed; the ratio S0(C2H6/S0(C2D6) decreased from 2.7 to 1.6 as En was increased from 80 to 118 kJmol. A one-dimensional quantum mechanical tunneling model based on an Eckart potential barrier quantitatively accounts for these observations. After correcting for translational energy dissipation to the lattice, the extracted barrier heights, V0, and widths, L, are 123 kJmol, 1.1 A and 130 kJmol, 1.1 A for C2H6, and C2D6, respectively. The larger barrier height for the direct dissociation of C2D6 relative to C2H6 can be attributed to differences in zero point energy for C-H(D) stretching motion. Neither. S0(C2H6) nor S0(C2D6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl rocking and deformation, C-C stretching and torsion do not promote direct ethane dissociation on Pt(111) under these experimental conditions.