Xe adsorption on Ag(111): Experiment

Precise structural and thermodynamic studies of Xe adsorbed on Ag(111) are made using low-energy diffraction. Spacings are measured relative to those of bulk adsorbed film. Effective equilibrium experiments are done using a directed gas beam and cryopumping. The density of only that gas in the coherently diffracting area is measured by the attenuation of the Ag(01) beam. With decreasing temperature at fixed gas flux one observes the sequential of 2D gas, monolayer, bilayer and bulk films. The effective pressure of the impinging gas is determined from the known bulk vapor pressure so bypassing absolute pressure measurements. The phase diagram, lattice spacings in the film at equilibrium and at zero pressure, latent heats of adsorption and isosteric heats are determined.     

Two-dimensional gas phases of Ar,Kr, AND Xe on Ag(111)

Observations and calculations are reported for the 2D gas phase of Ar, Kr, and Xe adsorbed on the (111) face of Ag. The measurement of the coverage of 2D gas is sensitive to the gas adsorbed on the coherently diffracting regions of the Ag; appreciable contributions to the measured coverage from gas adsorbed at extrinsic sites can be excluded. The gas density at the condensation of the solid monolayer is remarkably high, of the order of 15% of the 2D solid density. Statistical mechanical calculations, using Monte Carlo simulations, show that the unusually dense 2D gas is stabilized against condensation because substrate-mediated interactions raise the energy of the 2D solid relative to the value which would be calculated using the bare 3D pair potentials. The simulations show that the gas is very nonideal, with large specific heat and large density fluctuations. The only comparable states of a 3D gas occur near the gas-liquid critical point.     

Image theory of lateral interactions applied to desorption rate of Xe from W and Ag

The image theory of lateral interactions between rare gas atoms given by Mahanty and March [J. Phys. C9 (1976) 2905] is shown to be consistent with the mean features of desorption rate measurements for Xe on W(111) and W(110). Some results are also presented for both Xe and Ar on Ag(111).     

Pressure induced friction collapse of rare gas boundary layers sliding over metal surfaces

In this Letter we show that friction of anticorrugating systems can be dramatically decreased by applying an external load. The counterintuitive behavior that deviates from the macroscopic Amonton law is dictated by quantum mechanical effects that induce a transformation from anticorrugation to corrugation in the near-surface region. We describe the load-driven modifications occurring in the potential energy surface of different rare gas-metal adsorbate systems, namely, Ar, Kr, Xe on Cu(111), and Xe on Ag(111), and we calculate the consequent friction drop for the commensurate Xe/Cu system by means of combined ab initio and classical molecular dynamics simulations.     

Electronic transitions of Ar,Xe, N2, CO physisorbed on Ag(111) and Al(111)

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N₂, CO) on Ag(111) and Al(111) surfaces at ～20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes.In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases.The FWHM of the electronic excitations of CO and N₂ adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ～1eV.The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates.Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).     

The scattering of He from adsorbed layers of gases on Ag(110)

The adsorption of gases on Ag(110) has been studied using inelastic He atom scattering. Vibrational spectra have been obtained for Kr, Xe, C₂H₆, C₂H₄, CH₄, CF₄, CHF₃, CO₂ and H₂O. Spectra have also been obtained for multilayers of Xe (2 layers) and C₂H₆ (3 and 4 layers) where the energy changes move to lower values. The scattering from Kr and Xe can be shown to be dispersionless as has been previously found for these adsorbates on Cu(100) and Cu(110). The energy changes for Kr and Xe are smaller than on Cu surfaces and attempts were made to account for this based on an Einstein model of the adsorbed atoms in the surface holding potential.     

Electron stimulated desorption of Xe,Kr, and Ar

Electron stimulated desorption cross sections have been measured for Xe, Kr, and Ar overlayers on the Ag(111) surface. The Xe cross section is less than 10^?4 Ų; the Kr cross section is strongly temperature dependent, rising from 0.1 Ų at 10 K to 0.18 Ų at 50 K; the Ar cross section is 4 Ų and temperature independent. These results are rationalized using a model of the stimulated desorption similar to that proposed by Antoniewicz [Phys. Rev. B21 (1980) 3811], in which an atom is ionized by the incident beam, accelerates towards its image and is neutralized, desorbing only if the kinetic energy gained is greater than the neutral atom binding energy at the neutralization position. Fitting these data requires an exceedingly rapid dependence of the neutralization on distance for these slow ions. Rather than the effect of the mass on the ion velocity, the most important effect in determining the diverse behavior of the different gases is that the equilibrium position for the heavier gases is farther up the overlap repulsive potential and so in a region of more rapid neutralization. The model identifies several contributions to the isotope effect, predicting it to be temperature dependent. The results are extremely sensitive to the anharmonicity of the holding potential.     

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.     

Bonding analysis for NgMOH (Ng=Ar,Kr and Xe; M=Cu and Ag)

The structures and stabilities of a new class of species, noble-gas–coinage-metal hydroxides NgMOH (Ng =?Ar, Kr and Xe; M =?Cu and Ag), are investigated at the MP2 theoretical level. All species are found to be in Cs symmetry with an approximate linear Ng–M–O moiety. The noble-gas–coinage-metal bond lengths are in the range of the respective covalent and van der Waals limits, showing a different degree of approach to the former along the series Ar–Kr–Xe for Ng–Cu and Ng–Ag bonds, respectively. The dissociation energies of noble-gas–coinage-metal bonds are relatively large as compared to the van der Waals complexes. Besides the charge-induced dipole contribution, other effects –?higher-order charge-induction energies, dispersion interaction, etc., should be considered to explain the noble-gas–coinage-metal bonding mechanism. The present results suggest that the title species are stable enough to be prepared experimentally.     

A simple tight-binding theory with spin-orbit coupling for the analysis of two-dimensional band structures of adsorbates

A Slater—Koster-type simplified tight-binding theory is applied to the analysis of electronic states of ordered adlayers on single-crystal surfaces. The effect of spin—orbit coupling is introduced by using Pauli spinors. The Hamiltonian matrix including the spin—orbit terms takes a simple form, which enables us to reproduce and explain relatively easily angle-resolved UPS data for such systems. Examples of application to Te on Al(111) and Xe, Kr, Ar on Pd(100) are given.     

A scanning tunneling microscopy study of the adsorption of Xe on Pt(111) up to one monolayer

The adsorption of Xe on Pt(111) has been investigated from the arrival of the very first atoms up to completion of the monolayer using a variable-temperature Scanning Tunneling Microscope (STM). Surprisingly, in the initial stages of the adsorption Xe preferentially binds to a low coordination site, theupper edge of the platinum steps. The strong binding to these sites leads to a local repulsive interaction with further Xe atoms. Therefore, the Xe atoms located at the upper edge of the steps do not serve as nuclei for 2D Xe islands, which, instead, form on the terraces and at thelower edges of the platinum steps. Only during completion of the monolayer do these islands make contact with the atoms adsorbed at the beginning in the upper-edge positions. The full monolayer exhibits the Hexagonal Incommensurate Rotated (HIR) phase already known from earlier helium-diffraction experiments.     

Plasma line emission during single-bubble cavitation

Emission lines from transitions between high-energy states of noble-gas atoms (Ne, Ar, Kr, and Xe) and ions (Ar(+), Kr(+), and Xe(+)) formed and excited during single-bubble cavitation in sulfuric acid are reported. The excited states responsible for these emission lines range 8.3 eV (for Xe) to 37.1 eV (for Ar(+)) above the respective ground states. Observation of emission lines allows for identification of intracavity species responsible for light emission; the populated energy levels indicate the plasma generated during cavitation is comprised of highly energetic particles.     

Anisotropic intermolecular forces. I. Rare gas—hydrogen chloride systems

Anisotropic potential energy surfaces for Ne.HCl, Kr.HCl and Xe.HCl are obtained by simultaneous least squares fitting to molecular beam spectra and rotational line broadening cross sections. A revised potential surface for Ar.HCl is also presented. The potentials are all very similar in shape, with the absolute minimum at the linear rare gas-HCl geometry in each case. The absolute well depths and well depth anisotropies increase steadily as the size of the rare gas atom increases.

The potentials should be reliable in the region of the absolute minimum and on the repulsive wall of the potential. The molecular beam spectra for Ne.HCl can be fitted only by a potential with a significant secondary minimum at the linear Ne.Cl-H geometry, but the existing data for the more anisotropic Ar, Kr and Xe systems are not sensitive to the presence of this potential feature. The potential surfaces for these systems have accordingly been constrained to have a similar secondary minimum near the linear rare gas-ClH geometry. Experiments which would provide further information on the intermolecular potentials are suggested and predictions of the results are made.     

UV photoemission from physisorbed atoms and molecules: Electronic binding energies of valence levels in mono- and multilayers

Angle-resolved UV photoemission spectra were measured for Ar, Kr, Xe, CO, O₂ and N₂ adsorbed on a Ni(110) surface at 20 K. The different gases were adsorbed also on the Ni(110) surface which had been precovered by mono- and multilayers of the same gases. Upon physisorbing one of these species onto the bare and precovered Ni surface, binding energy shifts up to 3 eV were found. These shifts will be explained by work function changes of the substrate onto which the gas is physisorbed. It will be shown that for the investigated gases the binding energy referred to the vacuum level is an atomic or molecular property which is independent of the substrate, to a first approximation. By physisorption of a known gas the work function of any substrate can be evaluated by UPS. The density of valence states for bulk Ar, Kr and Xe will be discussed. There is evidence that the conduction band can be seen in the secondary electron background of the UP spectra.     

Zero temperature calculations of the states of a monoatomic two-dimensional solid adsorbed on an fcc (110) surface

Calculations of the energy of centered rectangular monolayer lattices in the periodic potential of an fcc(110) solid surface are reported. The parameters are chosen to model the systems Xe/Ag(110), Kr/Cu(110) and Xe/Cu(110). The critical values of Fourier amplitudes of the adatom-substrate potential required in order to stabilize uniaxial registry lattices of the adlayers are estimated and the variation of the adlayer energy with uniaxial compression is calculated. The calculation of the modulation of the adlayer by the rigid periodic substrate potential includes effects of anharmonicity of the adlayer response and of the dispersion relation of adlayer vibrations. The critical Fourier amplitudes to stabilize the registry structures can be estimated with 20% accuracy by using a comparison of the lateral potential energies of the registry lattice and of the intrinsic two-dimensional close-packed triangular lattice.     

Current characteristics of barrier discharges in inert gases (Ar,Kr, Xe)

The current characteristics of barrier discharges in inert gases (Xe, Kr, and Ar) are compared. It is shown that the shape of the current curve (the number of peaks and their amplitudes) depends on the kind of gas (breakdown voltage) and the frequency of the sinusoidal supply voltage; as the frequency of the voltage decreases, the number of breakdowns tends to increase. It is found that, under the experimental conditions (P = 300 Torr, d = 4 mm), the amplitude of the discharge current in Xe is two to three times higher, its time is tenfold shorter, and the diameter of the discharge channel is five to ten times smaller compared to other inert gases. These differences can be explained by a considerable difference of the transfer coefficients in Xe from those in Ar and Kr.     

Inelastic atom scattering from layers of atoms and molecules on Cu(110)

The vibrational properties of adsorbed layers have been studied using inelastic He atom scattering. The substrate was Cu(110) viewed along the [001] and [11̄0] azimuth. The adsorbates included, Kr, Xe, CH₄, CD₄, C₂H₆, CO, CO₂ and O₂ and covered the region of both physisorption and chemisorption. Despite a range of binding energies and mass ratios the vibrational frequencies showed no dependence on the momentum change, i.e. scattering was dispersionless, although intensity changes occurred in the inelastic peaks. There seemed to be no dependence of the adsorbate spectra on coverage below a monolayer. The peaks of CH₄ and C₂H₆ appeared broader than those of Kr and Xe suggesting molecular motions. Further, CH₄ and C₂H₆ gave very similar spectra. Both energy gain and loss events were observed in the inelastic spectrum. For those adsorbates which gave multilayer adsorption (Xe, C₂H₆) changes in the spectra were observed as the second layer developed. In the latter category also, mixed layers (Xe on C₂H₆, Xe on CO and C₂H₆ on Xe) were studied.     

Calculation of scanning tunnelling microscopy images for Kr/graphite system

The scanning-tunnelling-microscopy (STM) images of Kr atoms adsorbed on a monolayer graphite sheet (Kr/graphite system) are calculated using the first-principle total-energy electronic structure calculations within the density functional theory in the local density approximation. The results obtained agree well with the observations. It is found that the optimal site of the adsorbed Kr atom is at the top of the centre of the carbon hexagon, and its equilibrium distance from monolayer graphite surface is about 0.335nm. It is shown that the hybridization of C 2p electronic states (π-electronic states) and Kr 4p and 5s electronic states is the main origin of the Fermi-level local density of state.     

Population processes in high-pressure inert-gas-mixture lasers

Working-level-population processes are analyzed on the basis of detailed investigations of the amplitude-time structure of the laser and spontaneous emission following a pulsed electric discharge in the mixtures He + R (R = Ar, Kr, Xe), Ar + Xe. Account is taken in the analysis of excitation by electrons (direct and stepwise) and of population as a result of relaxation processes (collisions of second kind with electrons; radiative cascades, recombination processes; collisions with the atoms of the working and buffer gases; excitation transfer from helium molecules). It is concluded that under optimum efficiency conditions inversion is produced in the lasers considered as a result of direct electron collision with the working atoms (Ar, Kr, Xe), which are in the ground state.Translated from Lazernye Sistemy, pp. 15–34, 1982.     

Gain and lasing of optically pumped metastable rare gas atoms

Optically pumped alkali vapor lasers are currently being developed in several laboratories. The objective is to construct high-powered lasers that also exhibit excellent beam quality. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np(5)(n+1)s (3)P(2) configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. In principle, optically pumped lasers could be constructed using excitation of the np(5)(n+1)p←np(5)(n+1)s transitions. We have demonstrated this potential by observing gain and lasing for optically pumped Ar(*), Kr(*) and Xe(*). Three-level lasing schemes were used, with He or Ar as the collisional energy transfer agent that established the population inversion. These laser systems have the advantage of using inert reagents that are gases at room temperature.