Inelastic processes in the scattering of metastable rare gas atoms at metal surfaces

Electron spectra of various metastable rare gas atoms systematically measured on a Pt(111) surface with Rb coverages ranging from submonolayers (3%) to multilayers are presented. The different decay channels of the excited particles are discussed in terms of resonant electron exchange processes between the substrate and the projectile in relation to the work function. It is shown that below a certain value of the work function a highly excited negative rare gas atom is formed which can undergo different de-excitation processes. A careful discussion of the branching ratios into the decay channels offers a natural explanation of the variations in the electron spectra induced by alkali metal adsorption. Additionally, an attempt is made to extract information about the alkali metal chemisorption state from the observed electron spectra.     

Interactions of rare gas cations with lighter rare gas atoms

High-level ab initio calculations are employed to generate potential energy curves for rare gas cations, RG⁺, interacting with neutral atoms of other rare gases, RG′, that are lighter (RG′ = Ne–Rn). The calculations employ the RCCSD(T) method, with doubly-augmented basis sets of quintuple-ζ quality. The interaction potential curves, with the full counterpoise correction applied, are calculated point-by-point. Spin-orbit coupling is applied analytically in an atom-based model. The potentials are used to calculate spectroscopic parameters, which are then compared to recent experimental work, and the very limited previous theoretical work. In addition, the potentials are used to calculate ion transport properties and the ion mobilities are compared to the few experimental data available.     

Photon emission from rare gas ion bombardment of metal surfaces

Metal surfaces (Mg, Cu, Zr, Mo) are bombarded with He⁺, Ne⁺ and Ar⁺ in the energy range of 400 eV to 8 keV. Radiation from scattered projectiles and sputtered target particles is observed between 200 and 700 nm. It is shown that most of the radiating particles originate from surface collisions. Auger neutralization, resonance tunneling and direct electron transitions are the important electronic processes involved.     

The damping function of the van der Waals attraction in the potential between rare gas atoms and metal surfaces

The physisorption potentials of the rare gases on noble metals recently reported by Chizmeshya and Zaremba [Surf. Sci. 268 (1992) 432] are recalculated with new damping parameters for the van der Waals attraction. The new damping parameters, which are applicable when the repulsive potential is not purely exponential, are consistent with the original analysis of Tang and Toennies. The uncertainties resulting from the two approximate sets of damping parameters used by Chizmeshya and Zaremba are removed, moreover a potential minimum is now predicted for Xe on the Au surfaces for which the previous approximations failed.     

Interactions of incident H atoms with metal surfaces

Atomic hydrogen is a highly reactive species of interest because of its role in a wide range of applications and technologies. Knowledge about the interactions of incident H atoms on metal surfaces is important for our understanding of many processes such as those occurring in plasma-enhanced catalysis and nuclear fusion in tokamak reactors. Herein we review some of the numerous experimental surface science studies that have focused on the interactions of H atoms that are incident on low-Miller index metal single-crystal surfaces. We briefly summarize the different incident H atom reaction mechanisms and several of the available methods to create H atoms in UHV environments before addressing the key thermodynamic and kinetic data available on metal and modified metal surfaces. Generally, H atoms are very reactive and exhibit high sticking coefficients even on metals where H₂ molecules do not dissociate under UHV conditions. This reactivity is often reduced by adsorbates on the surface, which also create new reaction pathways. Abstraction of surface-bound D(H) adatoms by incident H(D) atoms often occurs by an Eley-Rideal mechanism, while a hot atom mechanism produces structural effects in the abstraction rates and forms homonuclear products. Additionally, incident H atoms can often induce surface reconstructions and populate subsurface and bulk absorption sites. The absorbed H atoms recombine to desorb H₂ at lower temperature and can also exhibit higher subsequent reactivity with adsorbates than surface-bound H adatoms. Incident H atoms, either directly or via sorbed hydrogen species, hydrogenate adsorbed hydrocarbons, sulfur, alkali metals, oxygen, halogens, and other adatoms and small molecules. Thus, H atoms from the gas phase incident on surfaces and adsorbed layers create new reaction channels and products beyond those found from interactions of H₂ molecules. Detailed aspects of the dynamics and energy transfer associated with these interactions and the important applications of hydrogen in plasma processing of semiconductors are beyond the scope of this review.     

Nature of the short-range interaction between noble gas atoms and metal surfaces

I propose that an interpretation of the interaction of noble gas atoms with metal surfaces as predominantly physisorbing provides the best explanation for the systematics of their binding energies and surface dipoles, as well as for the tendency of noble gas atoms to bind in low coordinated sites. In the present context physisorption is defined as a process driven by the overlap of the electrostatic atomic potentials of the interacting species. PACS 68.43.Fg; 68.47.De     

Interaction of antihydrogen with ordinary atoms and solid surfaces

The characteristic features of cold atom-antiatom collisions and antiatom-surface interactions are discussed and illustrated by the results for hydrogen-antihydrogen scattering and for quantum reflection of ultracold antihydrogen from a metallic surface. We discuss in some detail the case of spin-exchange in ultracold $\bar{H}-H$ collisions, exposing the interplay of Coulombic, strong and dispersive forces, and demonstrating the sensitivity of the spin-exchange cross sections to hypothetical violations of Charge-Parity-Time (CPT) symmetry.     

Interaction of atomic hydrogen with metal surfaces

Received: 23 March 1998/Accepted: 21 August 1998     

Asymptotic potential coefficients for rare gas and alkali atoms and simple molecules interacting with metallic surfaces

Values of the coefficients C₃ and C₅ which describe the asymptotic potential energy between an atom or molecule and a surface, and the constants C_s1 and C_s2 which characterize the surface mediation of the long-range atom(molecule)-atom(molecule) interaction, are calculated for rare gas and alkali or simple molecules, such as NO, H₂, and H₂O, adsorbed on noble and transition metals. These calculated results are obtained using analytic representations of the atomic polarizability and a numerical treatment of the substrate dielectric response obtained from measured optical constants.     

A pseudopotential study of molecular spectroscopy in rare gas matrices: absorption of NO in argon

We present a pseudopotential method to study the absorption spectroscopy of NO in an argon matrix modeled by a large albeit finite cluster. The excited states of NO are described with the virtual orbitals of a NO⁺ Hartree-Fock calculation plus a core-polarization operator to account for the electron-NO⁺ correlation. The argon atoms of the matrix are replaced by pseudopotentials for the repulsive contributions and core-polarization operators to account for matrix polarization and correlation with the excited electron. The model is shown to account for the matrix-induced transition shifts and also for the cut-off of the Rydberg series for n >3 reported in absorption experiments from the ground state. Received: 6 March 1998 / Revised: 1st June 1998 / Accepted: 16 June 1998     

Interaction of carbon monoxide with transition metal surfaces

XPS studies of the interaction of carbon monoxide with surfaces of Fe, Co and Ni indicate that at 300 K, the disproportionation reaction is prominent up to exposures of 10³ L giving rise to high surface concentrations of carbon. At higher exposures and higher temperatures, dissociation of carbon monoxide accompanied by the formation of surface oxide layers becomes more prominent. In the case of copper, disproportionation is prominent up to 10⁴ L even at 500 K followed by dissociation at higher exposures. These results are also supported by Auger spectroscopic studies.     

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.     

Lattice dynamics of lithium: A pseudopotential approach

The phonon dispersion of lithium is reported using the model potential approach. One unknown parameter of the pseudopotential was adjusted to fit the observed vibration frequency at the zone boundary in the [110] direction. The agreement between theory and experiment is fair, in particular, reproducing the crossing of the longitudinal and the transverse branches along the [100] direction.     

Electron emission from crystal surfaces of condensed aromatics under the impact of metastable rare gas atoms

Energy distributions of electrons ejected from polycrystalline surfaces of naphthacene, perylene and coronene by the impact of metastable He ^*, Ne ^* and Ar ^* atoms have been measured. Two types of peaks, which are similar to the “non-moving” and “moving” structures in photoelectron spectra, are observed in each spectrum. The non-moving structures (ca. 1 eV) for perylene and coronene are similar to those in the photoelectron spectra, whereas the relative intensities of the two non-moving structures for naphthacene (0.6 and 1.7 eV) are remarkably different from the corresponding structures in the photoelectron spectrum. The peak positions (but not necessarily their intensities and widths) for the moving structures for all the samples (> 2 eV) agree with those of the corresponding photoelectron spectra. The origin of these moving structures is ascribed to Penning ionization on the solid surface.     

Excitons in rare gas atoms and solids

Excitons in rare gas are described using the integral equation approach and an appropriate semi-empirical simplification of the short-range terms. In the solids, polarization effects are taken into account using a semi-continuum model in analogy with F-centre problem. Reasonable agreement with experiments is obtained for exciton binding energy shifts passing from atoms to crystals.     

Radial Sternheimer shielding-antishielding for rare gas atoms

The radial dependence of Sternheimer quadrupole shielding-antishielding factor is reported for He, Ne, Ar and Kr atoms at the coupled Hartree-Fock level of accuracy. The results would be useful in the interpretation of the nuclear quadrupole interactions measured at the rare gas nuclei in different chemical environment such as the various van der Waals molecules.     

Differential cross section surfaces for low energy scattering of electrons and positrons from rare gas atoms

The differential cross sections for scattering of electrons and positrons from He, Ne, Ar, Kr, and Xe at projectile energies below the inelastic thresholds are calculated using a model potential approach in which the interaction between the projectile and the target atom is partitioned into static, exchange (for electrons), and correlation-polarization parts. Two different forms of the parameter-free correlation-polarization potential are suggested; in both cases the correlation-polarization potential is determined by smoothly matching the asymptotic form of the polarization potential (1/r ⁴) to the correlation potential at the outermost orbital radius of the target atom. The results of angular distributions are presented in the form of contours of constant differential cross sections as well as in the form of differential cross section surfaces in three-dimensional plots. Both of these presentations display the locations of the principal maxima and minima of the differential cross sections as well as the critical points in a very useful manner.     

Phonon dispersion in thorium: A pseudopotential approach

A one parameter model pseudopotential is used in investigating the phonon dispersion in thorium. A very good agreement has been obtained between theory and experiment, especially, in transverse branches. This is attributed to the removal of the discontinuity in the pseudopotential in r-space.     

Interaction of the uracil dipole-bound electron with noble gas atoms

Very diffuse, but localized, electrons trapped in dipole—bound states of polar polyatomic molecules may provide excellent targets for testing electron—molecule interactions. Ab initio calculations are used to investigate systems where a dipole—bound electron attached to a uracil molecule is interacting with noble gas atoms (He and Ne) and forming very weakly bound adducts. In these adducts, the noble gas atoms are separated from the uracil molecule by considerable distances, and the excess electron is suspended between the uracil molecule and the noble gas atom. Calculations are performed to determine the vertical electron detachment energies of these systems and to determine what happens when the excess electrons are removed from them.     

Multiple ionization of rare gas atoms irradiated with intense VUV radiation

The interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and approximately 10(13) W/cm(2) multiple charged ions up to Xe6+ (Ar4+) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of approximately 10(12)-10(13) W/cm(2) the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5-30 times lower power densities than at 193 and 564 nm wavelength, respectively.