J. R. Manson

Helium atom differential cross sections for scattering from single adsorbed CO molecules on a Pt(111) surface

Differential cross sections have been measured for the scattering of helium atoms from isolated CO molecules on a Pt(111) surface. The cross sections reveal an oscillatory structure as a function of scattering angle extending to large momentum transfer on both sides of the specular peak. Using a hard hemisphere model to approximate the interaction potential, the data can be well reproduced by a hard core of radius about 2.5 A. This result is compared to the charge density profile of the adsorbed molecule and also to the gas phase interaction potential, and in both cases good agreement with the experimentally predicted classical turning points is found.

A comprehensive experimental study of the dynamical interaction of He atoms with Cu(001) surface phonons

We present new measurements of inelastic He atom scattering from the surface phonons of Cu(OO1) as a function of crystal temperature, incident energy, and parallel momentum transfer. A careful subtraction of the multiphonon intensity and other background contributions from the time-of-flight intensities reveals three distinct surface-localized vibrational modes which are ascribed to the Rayleigh phonon, the longitudinal bulk resonance, and a further acoustic bulk resonance at higher energy transfers. The longitudinal resonance couples very strongly to the scattering He atoms and, for a wide range of incident conditions, gives peaks which are more intense than those due to the Rayleigh mode. The energy and momentum dependence of these peak intensities are analyzed with the aid of a simple distorted wave Born approximation, and the different coupling parameters for the two modes are determined and compared with other available data. The incoherent diffuse elastic peak is shown to decrease as a function of parallel momentum transfer according to the theory of Fraunhofer scattering from a random array of point defects. The multiphonon background is shown to be in agreement with a quick scattering approximation. 0 I994 American Institute of Physics.

Elastic and inelastic scattering of neutral atoms by a corrugated hard wall

Abstract A general treatment of the elastic and inelastic scattering of neutral atoms by a corrugated hard wall potential is presented. The influence of the atomic thermal motion is divided into two parts, one corresponding to a translation of the whole profile and due to long wavelength phonons, and the other inducing a deformation of the potential shape corresponding predominately to short wavelength phonons. The Debye-Waller factor (for both the elastic and inelastic contributions) has the expected form and contains terms which correspond to a translation of the profile. However a correction term is introduced by the profile deformation. The one phonon exchange can occur with all the crystal phonons but it appears more important with those of long wavelength. The inelastic cross section is expected to be larger in the vicinity of diffracted peaks of large intensity. The only approximation is made in the determination of the source function where one supposes that a limited and small amount of energy is exchanged between the particle and the crystal.

Multiphonon atom-surface scattering

Abstract We consider the interaction of a neutral atom colliding with a solid surface and develop a general quantum mechanical theory describing the multiphonon inelastic transfer processes to arbitrarily high order in numbers of exchanged phonons. Starting from general principles of formal scattering theory the trajectory approximation to the scattering intensity is developed. From the trajectory approximation it is shown that a fast collision approximation is applicable when the multiphonon exchange is dominated by small frequency and long wavelength phonons. The semiclassical and classical limits are developed. Numerical calculations produce good agreement with a variety of experiments for conditions ranging from the quantum mechanical regime to nearly classical scattering.

Helium atom scattering from isolated CO molecules on copper(001)

Angular distributions have been measured for helium atoms scattering from isolated CO molecules chemisorbed on a Cu(001) surface as a function of incident beam energy between 9.4 and 100 meV and surface coverage from 1.3% ML to 9.3% ML. Up to five oscillations are clearly observed in the angular distributions. The parallel wave vector transfers of the peaks vary only slightly with incident energy and are independent of coverage up to 20% of a c(2×2) layer. New hard wall scattering calculations show that all of the distinct peaks observed can be explained by interference structures involving both Fraunhofer diffraction and illuminated face scattering from CO molecules with an approximate hard wall radius of 2.4 A with no evidence of the classical rainbows predicted in several recent theoretical studies.

Rare gas scattering from molten metals examined with classical scattering theory

Classical limit expressions of the differential reflection coefficient for atoms scattering from a surface are compared with recent experiments for the scattering of monoenergetic beams of Ne, Ar, and Xe with incident energies in the eV range from the molten metal surfaces Ga, In, and Bi. We find that single collision events usually make the greatest contribution to the backscattered intensity, double collision events make a significant but smaller contribution, and scattering of atoms that are completely trapped and subsequently thermally desorbed has a small probability. In the case of large mass incident projectiles and small mass target atoms we find some evidence for collective mass effects of the target. An analysis of the temperature dependence of the intensities shows that these surfaces act neither as a smooth continuous repulsive barrier nor as a collection of discrete scattering centers; rather they exhibit a behavior in between these two extremes.

The Dynamics of Cu(100) Surface Atoms at High Temperature

We present a comparison between theoretical calculation and experimental data for the thermal attenuation of helium diffraction peaks for a beam scattered from Cu(100). The calculations take into account 2-phonon exchange exactly and multiphonon exchange with a good approximation up to the highest temperatures. It is shown that there is good agreement up to T = 450 K with the model of crystal surface atom thermal displacements developed by Jayanthi et al. Above, there is a significant enhancement of these displacements which indicates either an enhancement of anharmonicity in the surface or the onset of a thermal structural roughening.

Angular distributions of Ar reflected from molten metal surfaces

Recent experimental measurements of the angular distributions of monoenergetic beams of Ar atoms, with incident energies of up to 1 eV scattered from a molten In surface, are compared with calculated intensities. The data are described by classical scattering theory and the agreement indicates that the shapes of the lobes are dominated by single collisions with the surface, and the interaction potential has a smooth repulsive barrier similar to that commonly used to describe rare-gas scattering from crystalline metals. The attractive adsorption well of the interaction potential is considered, and is shown to have significant effects on the angular distributions at low incident energy. The dependence of the angular distributions on surface temperature is also well described.

Photodesorption of Na atoms from rough Na surfaces

We investigate the desorption of Na atoms from large Na clusters deposited on dielectric surfaces. High-resolution translational energy distributions of the desorbing atoms are determined by three independent methods, two-photon laser-induced fluorescence, as well as single-photon and resonance-enhanced two-photon ionization techniques. Upon variation of surface temperature and for different substrates (mica vs lithium fluoride) clear non-Maxwellian time-of-flight distributions are observed with a cos2 θ angular dependence and most probable kinetic energies below that expected of atoms desorbing from a surface at thermal equilibrium. The half-width of the time-of-flight distribution decreases with increasing surface temperature. A quantitative description of the energy and angular distributions is presented in terms of a model which assumes that following the initial surface plasmon excitation neutral atoms are scattered by surface vibrations. Recent experiments providing time constants for the decay of t...

Simple Model for the Energy Accommodation Coefficient

Quantum mechanical expressions for the energy accommodation of a gas at a solid surface are developed using the general theory of inelastic surface scattering as developed by Manson and Celli. Calculations are presented for the accommodation of helium at a simplified model of a tungsten surface. The phonon spectrum at the surface is taken to be that of a semi‐infinite isotropic continuum, and contributions to the accommodation are obtained from bulk, mixed, and Rayleigh phonon modes. For temperatures not too small the calculations agree qualitatively with experiment, and exhibit the property that the accommodation is nearly independent of surface temperature over a large range.