Oxygen molecule dissociation on the Al(111) surface

The dissociative adsorption of the O2 on the Al(111) surface is studied using ab initio calculations based on density-functional theory. In the sticking probability experiments an activation barrier for the O2 dissociation exists, but our calculations predict a barrier only for one trajectory. Also our results do not support the model of charge transfer from the surface to the molecule as a bond breaking mechanism. Instead, the increasing hybridization between O2 orbitals and Al states, when the adsorbate approaches the slab, is significant for the dissociation.     

Dynamical aspects of precursor state kinetics

An examination of precursor adsorbed states from a dynamical viewpoint reveals that two commonly used models of precursor kinetics are not generally applicable. For the case of molecular chemisorption, simple, linear rate laws may not be valid unless the adsorbate-solid system meets certain dynamical criteria. On the other hand, for dissociatively chemisorbed species, a serious error can be made in estimating the effect of temperature on the adsorbate dissociation rate if one ignores, as is usually done, the temperature dependence of the initial sticking probability of the molecule.     

A molecular beam study of the adsorption dynamics of CO on Ru(0001), Cu(111) and a pseudomorphic Cu monolayer on Ru(0001)

We have investigated the sticking coefficient of CO on Ru(0001), a pseudomorphic Cu monolayer on Ru(0001), and a fully relaxed Cu(111) multilayer as function of kinetic energy, surface coverage, and surface temperature. At a low kinetic energy of 0.09 eV, the initial sticking coefficients, S₀, on these surfaces are determined to be 0.92, 0.96 and 0.87, respectively. In all cases, a decrease of S₀ with increasing beam energy was observed, yielding values of 0.58, 0.14 and 0.07, respectively, at a kinetic energy of 2.0 eV. For all three surfaces the coverage dependent sticking coefficients, S(Θ), display very characteristic behavior at low kinetic energies: S(Θ) remains more or less constant up to coverages close to saturation, indicative of precursor adsorption kinetics. However, characteristic minima at intermediate coverages are observed, which are correlated to the formation of well ordered adsorbate phases. For high kinetic energies we observe a transition towards a linear decrease of S(Θ) for Ru(0001). In contrast, for the pseudomorphic Cu monolayer and for Cu(111) we find an increase in the sticking coefficients at low coverages, followed by a decrease close to saturation. This behavior is attributed to adsorbate assisted sticking, that is, to a higher sticking coefficient on adsorbate covered regions than on the bare surface. The comparison between the pseudomorphic monolayer and Cu(111) reveals that the CO bond strength to the former is larger by 40%. The initial sticking coefficients for both surfaces are very similar at low kinetic energies; at high kinetic energies, S₀ for the pseudomorphic Cu monolayer is, however, larger by a factor of two.     

Preparation and properties of binary metal monolayers: I. Pb and Bi on Cu(100)

LEED and AES have been used to monitor the coadsorption of lead and bismuth vapour on the (100) face of copper. Breaks in the plots of substrate and adsorbate Auger signals as a function of deposition time, correlated with the appearance of various LEED patterns, permit a calibration of the spectrometer. For bismuth adsorbed alone a break before the completion of the dense monolayer is ascribed to a sudden change in the sticking probability; after the monolayer a second layer starts to form. The binary monolayers form with a maximum compactness compatible with the structures of the pure monolayers. The LEED patterns for these binary layers show that separate “domains” of each adsorbate are formed, each having the dense monolayer structure. Adsorption of lead causes a dispersed low-coverage structure of bismuth to condense into islands. For lead-rich layers the “surface melting” behaviour of the lead structure is influenced by the presence of bismuth.     

Desorption from a two-phase adsorbate: Zero or fractional order

We discuss the desorption kinetics from a two-phase adsorbate in terms of a simplified rate equation derived from an approach based on the general methods of nonequilibrium thermodynamics. We point out that within the coexistence region one should only expect zero-order desorption if (i) the adsorbate remains in quasi-equilibrium during the desorption process and (ii) if all sticking coefficients are equal.     

Sticking of rare gases: the effect of lateral interactions

A theory of sticking, based on the kinetic lattice gas that accounts for intrinsic and extrinsic precursors and incorporates the effects of lateral interactions, is used to develop a model for the sticking of rare gases on metals and to derive analytically the model of Zeppenfeld et al. [Surf. Sci. 318 (1994) L1187] that explains their data as due to the formation and coalescence of condensed islands in the adsorbate.     

Clustering of chemisorbed H(D) atoms on the graphite (0001) surface due to preferential sticking

We present scanning tunneling microscopy experiments and density functional theory calculations which reveal a unique mechanism for the formation of hydrogen adsorbate clusters on graphite surfaces. Our results show that diffusion of hydrogen atoms is largely inactive and that clustering is a consequence of preferential sticking into specific adsorbate structures. These surprising findings are caused by reduced or even vanishing adsorption barriers for hydrogen in the vicinity of already adsorbed H atoms on the surface and point to a possible novel route to interstellar H2 formation.     

Adsorption-desorption kinetics of CO from clean and sulfur covered Ru(001)

The total uptake of CO, its adsorption kinetics and its desorption kinetics from clean and partially sulfur covered surfaces of the basal plane of ruthenium have been investigated. The method of desorption rate isotherms applied to the CO flash desorption spectra from these surfaces was used to evaluate the coverage dependence of the binding energy of CO as well as the effect of various levels of sulfur on this binding energy. Below a total surface concentration of 1 adsorbate atom per 3 surface Ru atoms, the binding energy and sticking probability of CO on the clean and sulfur covered surfaces are the same. Above this concentration of total adsorbates, the adsorption kinetics is the same on all surfaces studied, the binding energy decreases linearly with CO coverage while the magnitude of the decrease increases with sulfur coverage. The total uptake of CO depends on the amount of preadsorbed sulfur. At low coverages of sulfur, total CO uptake is effected by the excluded volume of sulfur. At higher coverages of sulfur (approaching $\text{1}\text{2}$ the maximum sulfur concentration on the clean surface) the site requirements of sulfur limits the amount of CO that can adsorb on the remaining surface, to the quantity of 1 adsorbate atom per 2 Ru atoms.     

On the kinetics of oxygen adsorption on a Pt(111) surface

The kinetics of O₂ adsorption on a clean Pt(111) surface were investigated in the temperature range 214–400°C. The oxygen coverage was measured by CO titration as well as Auger electron spectroscopy both of which show the same dependence on O₂ exposure. The initial sticking coefficient on clean Pt(111) is 0.08–0.10 and decreases exponentially with increasing oxygen coverage. For θ > 0.23 a (2 × 2)-O LEED pattern was observed. The highest oxygen coverage obtained was approximately 0.45. A theoretical model was proposed which correlates the coverage dependence of the sticking coefficient with adsorbate interactions in the chemisorbed state. These interactions cause a coverage dependent activation energy of adsorption assuming the existence of a precursor state. Experiments dealing with the effect of carbon contamination on the sticking coefficient showed that the initial sticking coefficient decreases with increasing carbon coverage.     

Auger electron spectroscopy of adsorbed metal monolayers: Lead on low-index and stepped copper surfaces

Lead was deposited onto single crystals of copper (orientations (110), (100), (711) and (511) in ultrahigh vacuum. For all substrate orientations there form dense monolayers characteristic of the Stranski-Krastanov growth mode. During growth of the monolayer (at constant vapour flux) the Auger signals of the adsorbate and the substrate do not vary with exact linearity as a function of deposition time. The signal-versus-time plots show changes of slope that are associated with changes in the adsorbed layer structures as observed by LEED. More gradual variations occur near the completion of the dense monolayer. All these changes are ascribed to variations in the sticking probability. Adsorbed layers on the stepped surfaces tend to be disordered, possibly due to high mobility. The results are discussed in terms of the possibilities and difficulties for quantitative Auger electron spectroscopy.     

Initial adsorption of O2 on Si(1 0 0): Non-adiabaticity originating both from a discrete and a continuous set of electronic excitations

The initial adsorption of O₂ on Si(1 0 0) is investigated by density-functional theory calculations. The potential energy surface shows strong corrugations which can be interpreted as precursor states, however, there are also large areas where adsorption proceeds without a barrier. Furthermore, the initial sticking probability as a function of translational energy using first-principles molecular dynamics is calculated. The result is in disagreement with measurements of sticking probability which vary from high-low-high values as the translational energy of the oxygen molecules increase. A simple non-adiabatic model is put-forth that explains not only the measured sticking probability, but also have a novel interpretation of the increased sticking probability owing to tensile stress. The model deals with non-adiabatic effects originating both from a discrete and continuous set of electronic excitations. The implications are general and can be applied to other systems.     

Adsorption and desorption of hydrogen on bare and Xe-covered Cu(111)

Using polarization-modulated ellipsometry to monitor adsorbate coverage in-situ, we studied the activated adsorption of filament-heated molecular hydrogen on Cu(111) and subsequent isothermal desorption of hydrogen adatoms. The adsorption is characterized by a zeroth-order kinetic with a constant sticking probability of S₀=0.0062 up to θ=0.25, followed by a Langmuir kinetic until the saturation coverage θ_s=0.5 is reached. The desorption follows a second-order kinetic with an activation energy of 0.63 eV and a pre-exponential factor of 1×10⁹ /s. A pre-adsorbed monolayer of Xe atoms on Cu(111), with a desorption activation energy of 0.25 eV and a pre-exponential factor of 8×10¹⁴ /s, efficiently blocks the subsequent adsorption of hot molecular hydrogen, making physisorbedXe useful as templates for spatial patterning of hydrogen adatom density on Cu(111). PACS 68.43.Jk; 78.68.+m; 81.15.-z; 82.40.Np     

Li+ neutralization as a probe of the local electronic potential in Li+-alkali covered metal surface collisions

A theoretical study of Li⁺ neutralization by collision on an alkali covered Al surface is presented. The neutralization probability is computed for Li atoms back scattered from Al sites and from alkali sites on the surface. The calculations are performed with a model representation using lithium as a representative for alkali adsorption. The results reproduce the very large difference in neutralization probability for Li scattered from substrate and adsorbate sites experimentally observed by Weare and Yarmoff [Surf. Sci. 348 (1996) 359] and thus confirm the importance of local effects in atom-surface charge transfer. For scattering from adsorbate sites, the neutralization is shown to be associated with a very large excitation probability of the scattered Li atom.     

What determines the sticking probability of water molecules on ice?

We present both experimental and theoretical studies of the sticking of water molecules on ice. The sticking probability is unity over a wide range in energy (0.5 eV-1.5 eV) when the molecules are incident along the surface normal, but drops as the angle increases at high incident energy. This is explained in terms of the strong orientational dependence of the interaction of the molecule with the surface and the time required for the reorientation of the molecule. The sticking probability is found to scale with the component of the incident velocity in the plane of the surface, unlike the commonly assumed normal or total energy scaling.     

Dependence of the sticking probability on initial molecular orientation: NO on Ni(100)

The sticking probability of NO at Ni(100) was examined using a beam of orientated NO molecules. It is found to be higher for N-end collisions. The asymmetry of the sticking probability has been measured to be a linear function of the molecular degree of orientation. It was determined to be A = 0.7 ± 0.1 and nearly independent of coverage when normalized to the degree of orientation. The orientational dependence of the sticking probability as a function of coverage shows that the adsorption of the molecules cannot be described by a precursor model that neglects direct chemisorption.     

The DLCA-RLCA transition arising in 2D-aggregation: simulations and mean field theory

A sticking probability model based on the average cluster lifetime is employed for deducing a kernel capable to describe the kinetics of computer simulated irreversible aggregation processes in two dimensions. The deduced kernel describes not only the time evolution of the cluster size distribution for diffusion limited aggregation (DLCA) and reaction limited aggregation (RLCA) but also for the entire transition region between both regimes. The model predicts a crossover to diffusion limited cluster aggregation for all sticking probabilities at long aggregation times. The time needed for reaching the DLCA limit increases for decreasing sticking probability. Received 16 April 2001 and Received in final form 24 May 2001     

用反应48Ca+254Es合成119号元素的可能性研究

利用最近发展起来的两步模型分析重离子核反应48Ca+254Es 的熔合过程。在这个模型中,熔合过程被分为前后独立的两个过程,即粘连过程和形成过程。本研究组计算了对应的粘连概率和形成概率,并结合适用于蒸发过程的统计蒸发模型,计算了该反应合成119 号超重元素的剩余截面,即当Elab = 250:2 MeV时,得到最大剩余截面为0.7 pb。Two-step model is adopted to analyze the fusion process of heavy-ion reaction 48Ca+254 Es. Based on this model, the fusion process is divided into two consecutive steps, i.e., the sticking step and the formation step, and corresponding sticking probability and formation probability are calculated. Combining the statistical evaporation model for the evaporation stage, the maximum evaporation residue cross section for reaction 48Ca+254 Es is 0.7 pb at 4n, Elab =250.2 MeV.     

O2 interaction with Pt{100}-hex-R0.7°: scattering, sticking and saturating

The interaction of oxygen with the Pt{100}-hex-R0.7° surface has been studied using supersonic molecular beams at incident translational energies from 0.06 to 0.9 eV and surface temperatures from 300 to 600 K. Scattering measurements show the existence of both intrinsic and extrinsic precursor states, and the trapping probability into these states is high at low incident energies. However, sticking probability measurements on the clean surface indicate that O₂ dissociative adsorption on Pt{100}-hex-R0.7° is a direct activated process, in contrast to that on Pt{100}-(1 × 1) or Pt{111}. Strong temperature enhancement of the initial sticking probability has been observed and accounted for partly by a dynamical barrier model. The sticking probability varies strongly with oxygen coverage, which is explained through computer simulations of island formation. The formation of small islands is demonstrated by TEAS measurements. Thermal desorption measurements show that, at high incident energies above 0.5 eV, new states are populated and higher coverages, up to a full monolayer, are reached.     

Laser-induced vibrational excitation and desorption in the Cs/Cu(1 1 1) and Na/Cu(1 1 1) systems

The Cs/Cu(1 1 1) and Na/Cu(1 1 1) systems exhibit a transient excited electronic state localized on the adsorbate. Photo-excitation of this state triggers a motion of the alkali adsorbate away from the surface, leading to vibrational excitation of the adsorbate and possibly to desorption. A theoretical study of these photo-induced processes in the case of an exciting fs laser pulse is reported, based on a time-dependent approach of the adsorbate motion. The mean energy transfer from the laser photon energy to the adsorbate motion is shown to be weak, about 1% of the photon energy. Correspondingly, the vibrational excitation to high lying levels is very weak as well as the desorption process. The initial electronic state of the photo-induced process belongs to a continuum and vibrational excitation and desorption are found to vary rapidly with the energy of the initial electronic state. Initial vibrational excitation of the alkali adsorbate is also found to efficiently favour the desorption process, leading to a drastic variation of the desorption probability with the vibrational temperature of the adsorbate. The present results for the two systems are discussed and compared, in connection with available experimental data on these systems and on similar ones.     

Isomeric effects with di-iodobenzene (C6H4I2) on adsorption on graphite

Differences are seen in the adsorption of 1,2-di-iodobenzene, 1,3-di-iodobenzene, and 1,4-di-iodobenzene on graphite, as a function of exposure, using core level photoemission. The isomer 1,3-di-iodobenzene exhibits significant differences from 1,2-di-iodobenzene, and 1,4-di-iodobenzene in apparent sticking coefficients and core level binding energies. 1,3-Di-iodobenzene adsorb on graphite at 110 K in a strongly Stranski-Krastanov or Volmer-Weber (island) growth mode. The implication is that, even for small molecules adsorption, the adsorbate dipole in the plane of surface and the choice of isomer may matter.