Xe Overlayers Research Articles

Intermolecular Coulomb Decay at Weakly Coupled Heterogeneous Interfaces

Surface ejection of H(+)(H(2)O)(n=1-8) from low energy electron irradiated water clusters adsorbed on graphite and graphite with overlayers of Ar, Kr or Xe results from intermolecular Coulomb decay (ICD) at the mixed interface. Inner valence holes in water (2a(1)(-1)), Ar (3s(-1)), Kr (4s(-1)), and Xe (5s(-1)) correlate with the cluster appearance thresholds and initiate ICD. Proton transfer occurs during or immediately after ICD and the resultant Coulomb explosion leads to H(+)(H(2)O)(n=1-8) desorption with kinetic energies that vary with initiating state, final state, and interatomic or molecular distances.

Selective Ablation of Xe from Silicon Surfaces: Molecular Dynamics Simulations and Experimental Laser Patterning

The mechanism of laser-induced removal of Xe overlayers from a Si substrate has been investigated employing MD simulations and evaluated by buffer layer assisted laser patterning experiments. Two distinct regimes of overlayer removal are identified in the simulations of a uniform heating of the Si substrate by a 5 ns laser pulse: The intensive evaporation from the surface of the Xe overlayer and the detachment of the entire Xe overlayer driven by explosive boiling in the vicinity of the hot substrate. Simulations of selective heating of only a fraction of the silicon substrate suggest that the lateral heat transfer and bonding to the unheated, colder regions of the Xe overlayer is very efficient and suppresses the separation of a fraction of the overlayer from the substrate. Interaction with surrounding cold Xe is responsible for significant increase in the substrate temperature required for achieving the spatially selective ablation of the overlayer. The predictions of the MD simulations are found to be in a qualitative agreement with the results of experimental measurements of the threshold laser power required for the removal of Xe overlayers of different thickness and the shapes of metallic stripes generated by buffer-assisted laser patterning.

Determination of the electron’s solvation site on D2O/Cu(111) using Xe overlayers and femtosecond photoelectron spectroscopy

We investigate the binding site of solvated electrons in amorphous D(2)O clusters and D(2)O wetting layers adsorbed on Cu(111) by means of two-photon photoelectron (2PPE) spectroscopy. On the basis of different interactions of bulk- or surface-bound solvated electrons with rare gas atoms, titration experiments using Xe overlayers reveal the location of the electron solvation sites. In the case of flat clusters with a height of 2-4 bilayers adsorbed on Cu(111), solvated electrons are found to reside at the ice-vacuum interface, whereas a bulk character is found for solvated electrons in wetting layers. Furthermore, time-resolved experiments are performed to determine the origin of the transition between these different solvation sites with increasing D(2)O coverage. We employ an empirical model calculation to analyse the rate of electron transfer back to the substrate and the energetic stabilization of the solvated electrons, which allows further insight into the binding site for clusters. We find that the solvated electrons reside at the edges of the clusters. Therefore, we attribute the transition from surface- to bulk-solvation to the coalescence of the clusters to a closed ice film occurring at a nominal coverage of 2-3 BL, while the distance of the binding sites to the metal-ice interface is maintained.

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.

Structure and dynamics of excited electronic states at the adsorbate/metal interface: C6F6/Cu(111).

Excited state electron transfer at the adsorbate/metal interface represents a key step in molecular electronic devices. The dynamics of such processes are governed by ultrafast energy relaxation which can be probed directly by time-resolved two-photon photoemission (2PPE). Using 2PPE spectroscopy we investigate the energetics and lifetimes of the unoccupied electronic states of C6F6 adsorbed on Cu(111) as a model system for electron transfer at organic/metal interfaces. With increasing C6F6 layer thickness we find a pronounced decrease in the energetic position of the lowest unoccupied state, which is accompanied by a strong increase in its lifetime as well as a decrease in the effective electron mass. The frequently employed dielectric continuum model which describes delocalized (quantum well) states within adsorbate layers does not give a consistent explanation of these findings. By adsorption of Xe overlayers onto C6F6/Cu(111) we can show that, even for one monolayer of C6F6, the excited state must be localized predominantly inside the C6F6 layer and thus originates from a molecular state (presumably an antibonding sigma* orbital). With increasing coverage this state becomes more delocalized within the adsorbate layer, which reduces the coupling to the metal substrate and thus enhances the excited state lifetime.

Multiphonon He atom scattering from Xe overlayers on Cu(111) and Cu(001) surfaces

We have studied the scattering of He atoms from ordered monolayers of Xe atoms adsorbed on Cu(111) and Cu(001) surfaces. Both Xe overlayers, the commensurate (√3×√3)R30° on Cu(111) at substrate temperature above 47 K and the incommensurate one on Cu(001), exhibit weak diffraction. On both surfaces the Xe overlayers sustain Einstein-like, vertically polarized Xe vibrations which can be multiply excited and annihilated even at low He atom incident energies and substrate temperatures. In spite of the anharmonic Xe-substrate potentials, the energies of multiphonon excitations are not found to exhibit any noticeable anharmonic shifts in either system. The results of the measurements are compared to theoretical energy and lateral momentum resolved scattering distributions, which were calculated by using the recently developed formalism for treating multiphonon scattering in the collision regimes, in which both the motion of the scattered particle and surface vibrations must be treated quantum mechanically. We can interpret the multiphonon scattering spectra and obtain good agreement with experiments by assuming linear He atom–phonon coupling, delocalized phonons in Xe adlayers, and by employing the one-phonon interaction parameters determined from the He–Xe interaction potentials. Effects of the quantum recoil on the motion of the scattered He atom, which give rise to a difference between phonon emission and absorption probabilities, and thereby non-Poissonian scattering distributions, were assessed.

Quantum stochastic approach for molecule/surface scattering. II. Adsorption resonances of He atoms on Xe overlayers

He scattering from Xe monolayers adsorbed onto Ag substrates is studied using a quantum stochastic method. Both the He atom and the surface and bulk modes are treated quantum mechanically using a mean-field partitioning of the quantum Liouville equation. The resulting equations of motion permit the evaluation of the reduced density matrix of the He atom subsystem by averaging over a series of ‘‘quantum trajectories.’’ The final reduced density matrix of the atomic subsystem, when projected onto the asymptotic states, provides an accurate estimate of energy transfer processes between the atom and the surface. Our calculations indicate that inelastic energy transfer is enhanced near adsorption resonances due primarily to the increased lifetime of the atom near the surfaces as evidenced by marked deviations from the Debye–Waller thermal attenuation rates.

Surface assisted xenon-xenon bonding?

An idea for forming noble gas atom-noble gas atom bonds on a surface is explored theoretically, for Xe on Pd. The notion is that interaction with the surface should withdraw electrons from the top of the forming Xe 5p band, thus allowing Xe-Xe bonding, truly compressed structures. Molecular orbital studies of Xe overlayers and Xe 2 pairs on Pd(100) and Pd(111) are not encouraging; the Xe-Pd interaction appears insufficient to produce the desired effect

Helium diffraction from overlayers physisorbed on a self-assembled organic monolayer

Low-energy helium diffraction has been used to study the structure of overlayers of Xe, Kr, H 2 O, CH 3 OH, and n-C 6 H 14 physisorbed on a monolayer of CH 3 (CH 2 ) 21 SH self-assembled on a Au(111) film supported by mica. Xe and Kr have a close-packed structure with lattice constants 4.45±0.05 and 4.07±0.03A, respectively, which are slightly larger than the interatomic distances in the corresponding bulk phases. No surface order is detected with H 2 O and CH 3 OH overlayers, which is consistent with the fact that these adsorbates do not wet this organic surface

Erratum: Two- and three-body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite [J. Chem. Phys. 9 1, 6477 (1989)

Erratum: Two- and three-body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite [J. Chem. Phys. <b>9</b> <b>1</b>, 6477 (1989)

Two- and three-body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite

In order to address the problem of three-body interactions in gas–surface scattering, we considered the collision of a He atom with the (0001) surface of graphite coated by a monolayer of Xe. To eliminate the uncertainties connected with errors in the two-body He–Xe interaction, we determined the latter by crossed-beam differential collision cross-section measurements performed at two energies (67.2 and 22.35 meV). These scattering data together with room-temperature bulk diffusion data are then fitted with a Hartree–Fock–dispersion–type function to yield an interaction potential that explains most of the properties of this system within the experimental errors and represents an improvement on previously published He–Xe potentials. Helium diffraction measurements are then carried out from the Xe overlayer and the dependence of the specular intensity from the angle of incidence is carefully determined. Further, a He–surface potential is constructed by adding together the following terms: (1) the He–Xe pairwise sum, (2) the long-range He–(0001)C interaction, (3) the three-body contribution generated by the Axilrod–Teller–Muto term, (4) the so-called surface-mediated three-body interaction He–Xe–(0001)C first considered by A. D. McLachlan [Mol. Phys. 7, 381 (1964)], and finally (5) a small correction which is meant to take into account the nonstationary nature of the surface. Using this potential, well-converged close-coupling scattering calculations are carried out, and their results compared with the data. In general, good agreement is obtained. The agreement can, however, be improved by (a) an increase of about 30% in the contribution of three-body forces, (b) the lowering of the He–graphite long-range attraction coefficient by about 15%, or (c) a reduction of the two-body interaction well depth of 1.6% (the experimental error) together with any combination of the factors under (a) and (b) reduced by an adequate amount. Elimination of the contribution of the graphite surface by studying Xe multilayers is hindered by the uncertainties in the ‘‘thermal correction’’ [point (5) above] which, due to the multilayer increased ‘‘softness,’’ becomes an appreciable source of uncertainty.

Kr and Xe monolayers adsorbed on MgO powder studied by x-ray diffraction

Abstract The structure of Xe overlayers on MgO powder in the temperature range 20 ⪯ T ⪯ 130 K at coverages from 0.6 to 2.0 monolayers has been investigated by means of X-ray diffraction. In addition, Kr on MgO has been investigated at a coverage of 0.8 monolayers. The coverage was calibrated by adsorption isotherm measurements. In both cases the structure of the adsorbate is triangular and hence incommensurate to the underlying square symmetry of the MgO(100) surface. The existence of a two-dimensional triple point temperature and regions of solid-gas and solid-liquid coexistence are deduced from the Xe diffraction patterns. In both adsorbates the long range order is disturbed due to the frustration arising from the difference between the adsorbate and substrate symmetry.

Kr and Xe monolayers adsorbed on MgO powder studied by X-ray diffraction

The structure of Xe overlayers on MgO powder in the temperature range 20 ⪯ T ⪯ 130 K at coverages from 0.6 to 2.0 monolayers has been investigated by means of X-ray diffraction. In addition, Kr on MgO has been investigated at a coverage of 0.8 monolayers. The coverage was calibrated by adsorption isotherm measurements. In both cases the structure of the adsorbate is triangular and hence incommensurate to the underlying square symmetry of the MgO(100) surface. The existence of a two-dimensional triple point temperature and regions of solid-gas and solid-liquid coexistence are deduced from the Xe diffraction patterns. In both adsorbates the long range order is disturbed due to the frustration arising from the difference between the adsorbate and substrate symmetry.

Elastic helium scattering studies of ordered overlayers of Ar, Kr, and Xe physisorbed on Ag(111)

We describe experiments that measured the angle resolved intensity of He (Ei=18 and 66 meV) elastically scattering from the surfaces of rare gas overlayers physisorbed on Ag(111). These studies were done on a layer-by-layer basis for 1, 2, 3, and ∼25 ordered overlayers of Ar, Kr, and Xe. Two types of experiments are described. The first is diffraction, where the scattered He intensity was measured as a function of the detector angle, with the incident polar and azimuthal angles held constant. In the second type of experiment, selective adsorption, we measured the specular intensity as a function of incident angle. The purpose of these experiments was to examine the He–surface potential, to assess the relative contributions that various He–rare gas pair potentials, nonadditive multibody terms, and He–substrate interactions make to the systems studied. The experiments are compared with the results of accurate close-coupling calculations, in order to quantitatively perform these assessments. The comparisons between the selective adsorption data and scattering calculations demonstrate the extreme sensitivity that such measurements have to the He–surface potential. In particular, observable changes in the calculated selective adsorption spectra appear when different He–rare gas potentials are tested, or when various nonadditive terms are included in the potential. The results suggest that further refinements in the He-heavy–rare gas pair potentials may be in order.

Inelastic helium scattering studies of the vibrational spectroscopy and dynamics of ordered Ar, Kr, and Xe multilayers physisorbed on Ag(111)

This is the second of three papers discussing our studies of overlayers of Ar, Kr, and Xe physisorbed on Ag(111). All of these rare gases form ordered structures, which are azimuthally aligned but translationally incommensurate with the Ag substrate. In this paper, we discuss experiments in which we utilized the angle resolved time of flight of inelastically scattered 18 MeV He atoms to examine the surface dynamics of multilayer rare gas films along the Γ̄–M̄ direction. This was done on a layer-by-layer basis for 2, 3, and ≥20 layers for each of the rare gases. Unlike the monolayers, the vibrational modes observed for the multilayers show dispersion across the surface Brillouin zone, the amount of dispersion increasing with the number of adsorbed layers. These results reveal in detail how the surface dynamical properties of a thin film evolve towards those of a thick crystal as a function of increasing dimension. Lattice dynamics calculations, which utilize realistic gas phase pair potentials, reproduce the experimentally observed phonon dispersion relations quite well. We also examine the inelastic scattering probabilities and linewidths of the transitions. One of the more notable results is that the inelastic scattering probabilities vary by at most a factor of 2–3 across the entire surface Brillouin zone. Isothermal desorption measurements for the Xe overlayers are also discussed. Like the monolayer, the bilayer and trilayer exhibit nearly zeroth order desorption until ∼90% of the top layer has desorbed, where the desorption kinetics become first order.

Inelastic helium scattering studies of ordered Ar, Kr, and Xe monolayers physisorbed on Ag(111): Dispersion curves, scattering cross sections, and excitation line shapes

This paper is the first in a series covering our experiments examining the physical properties of ordered overlayers of Ar, Kr, and Xe physisorbed on Ag(111). In this paper, we examine the desorption kinetics of the Xe monolayer/Ag(111) system. Desorption is zeroth order until ∼90% of the monolayer has desorbed, then becomes first order. We also measured the inelastic scattering of an 18 meV He beam from unconstrained monolayers of (111) oriented Ar, Kr, and Xe. The transitions are mapped across the entire surface Brillouin zone from Γ̄ to M̄. The data are dispersionless, indicating that for the measured mode the adatoms are behaving as independent Einstein oscillators. Parametrized physisorption potentials for RG–Ag(111) [RG=Ar, Kr, Xe] are constructed using these results. Inelastic scattering probabilities and linewidths are also presented. The inelastic scattering probabilities vary by at most a factor of 3 across the entire surface Brillouin zone, and are reported as a function of incident angle, final wave vector, and surface temperature. Variations in the inelastic scattering probabilities are indicative of dynamical adatom–substrate coupling. Experimental techniques which turn these dynamical couplings on or off for the same phonon energy are discussed. Limited results for clean Ag(111) are also presented. It is hoped that these measurements, on such ideal systems as ordered rare gas monolayers, will provide further impetus for developing improved theoretical treatments of inelastic single phonon scattering.

Lattice dynamics of rare-gas overlayers on smooth surfaces.

We discuss the lattice dynamics of rare-gas overlayers on metal surfaces, with attention to monolayers, bilayers, and trilayers on the Ag(111) surface. For this system, the adsorbate overlayers are incommensurate with the metal substrate. Our interest is in the role of the interaction between the motions of the atoms in the adlayers and those of substrate atoms. In certain regions of the two-dimensional Brillouin zone, the adlayer phonon dispersion curves (calculated with the substrate atoms held in fixed position) overlap the bulk phonon bands of the semi-infinite substrate. A consequence is that when excited, the adlayer phonon may decay by emitting phonons into the substrate. They thus become leaky modes. Also, the adlayer modes may hybridize with the Rayleigh surface phonon on the substrate, to produce anomalous dispersion near those wave vectors where crossover occurs. For Ar, Kr, and Xe overlayers on models of Ag(111), we explore these phenomena through calculations of the phonon spectral density functions probed in inelastic He-surface scattering experiments.

Atom scattering from disordered surfaces: calculations for a model of a Xe + Ar mixed overlayer

Exact quantum-mechanical calculations are present for He scattering from one-dimensional models of disordered, mixed Xe + Ar overlayers. A time-dependent wavepacket approach is used with a recent technique for solving the Schrödinger equation. Results are given for several overlayers of different Xe : Ar concentration ratios. The dependence of scattering intensities on the disordered structures is discussed. The results provide a reference for testing approximations for scattering from disordered surfaces.

Atom scattering as a quantitative surface probe: Noble-gas monolayer and bilayer adsorbed on graphite

Excellent agreement is found between measured and calculated H-atom scattering by (strongly corrugated) Xe overlayers on graphite. Elastic scattering is calculated exactly using a surface potential generated by summing gas-phase H-Xe potentials. The diffraction peaks are carefully simulated by convolution over the velocity distribution, correcting for inelasticity and geometric effects, and including an inelastic background generated by a new method. Thresholds are found to affect strongly the resonance data, and the behavior of resonances at some crossings is different from that seen in weakly corrugated systems.

A coverage study of the self-diffusion and chemical diffusion constants for disordered overlayers of Xe on W(110)

The coverage dependence of the self-diffusion and chemical diffusion of submonolayer, disordered overlayers of Xe on W(110) at 150 K is found to be weak. The activation energies are all approximately 0.7 ± 0.2 kcal/mol for low to moderate ( θ = 0.2 to 0.6) coverages. The pre-exponential factors are on the order of 10 −3 to 10 −4 in the same range of coverages. In all cases, the chemical diffusion constant is found to be slightly larger than the self-diffusion constant. On the average, the velocities of different particles are positively correlated. The equal-time particle fluctuations are on the order of the average number of particles in a fixed probe area within the molecular dynamics cell. Consequently, for these disordered systems, we find the chemical diffusion constant to be comparable in magnitude to the self-diffusion constant.