Energy shifts and broadening of excited hydrogen-atom levels in the vicinity of a metal surface

Effects of the atomic level shift in the Auger neutralization rates of noble metal surfaces

In this work we compare characteristics of Auger neutralization of [Formula: see text] ions at noble metal and free-electron metal surfaces. For noble metals, we find that the position of the energy level of He with respect to the Fermi level has a non-negligible influence on the values of the calculated Auger rates through the evaluation of the surface dielectric susceptibility. We conclude that even though our calculated rates are accurate, further theoretical effort is needed to obtain realistic values of the energy level of He in front of these surfaces.

Localized description of surface energy gap effects in the resonant charge exchange between atoms and surfaces

The resonant charge exchange between atoms and surfaces is described by considering a localized atomistic view of the solid within the Anderson model. The presence of a surface energy gap is treated within a simplified tight-binding model of the solid, and a proper calculation of the Hamiltonian terms based on a LCAO expansion of the solid eigenstates is performed. It is found that interference terms jointly with a surface projected gap maximum at the Γ point and the Fermi level inside it, lead to hybridization widths negligible around the Fermi level. This result can explain experimental observations related to long-lived adsorbate states and anomalous neutral fractions of low energy ions in alkali/Cu(111) systems.

Charge transfer in single and multiple scattering events at metal surfaces: a wavepacket study of the Na(+)/Cu(100) system

Resonant neutralization of hyperthermal energy Na(+) ions impinging on Cu(100) surfaces is studied, focusing on two specific collision events: one in which the projectile is reflected off the surface, the other in which the incident atom penetrates the outer surface layers initiating a series of scattering processes, within the target, and coming out together with a single surface atom. A semi-empirical model potential is adopted that embeds: (i) the electronic structure of the sample, (ii) the central field of the projectile, and (iii) the contribution of the Cu atom ejected in multiple scattering events. The evolution of the ionization orbital of the scattered atom is simulated, backwards in time, using a wavepacket propagation algorithm. The output of the approach is the neutralization probability, obtained by projecting the time-reversed valence wavefunction of the projectile onto the initially filled conduction band states. The results are in agreement with available data from the literature (Keller et al 1995 Phys. Rev. Lett. 75 1654) indicating that the motion of surface atoms, exiting the targets with kinetic energies of the order of a few electronvolts, plays a significant role in the final charge state of projectiles.

Density functional theory of complex transition densities

We present an extension of Hohenberg-Kohn-Sham density functional theory to the domain of complex local potentials and complex electron densities. The approach is applicable to resonance (Siegert) [Phys. Rev. 56, 750 (1939)] states and other scattering and transport problems that can be described by a normalized state of a Hamiltonian containing a complex local potential. Such Hamiltonians are non-Hermitian and their eigenvalues are in general complex, the imaginary part being inversely proportional to the lifetime of the system. The one-to-one correspondence between complex local potentials nu and complex electron densities rho is established provided that the complex variables are sufficiently close to real local potentials and densities of nondegenerate ground states. We show that the exchange-correlation functionals, contributing to the complex energy, are determined through analytic continuation of their ground-state-theory counterparts. This implies that the exchange-correlation effects on the lifetime of a resonance are, under appropriate conditions, already determined by the functionals of the ground-state theory.

First principles resonance widths for Li near an Al(001) surface: Predictions of scattered ion neutralization probabilities

By combining a first-principles periodic density functional theory calculation of adsorbate resonance widths with a many-body dynamical theory of charge transfer, we assess charge-transfer rates for ions scattering off metal surfaces. This goes beyond previous approaches, which have been limited to modeling the surfaces with either static potentials or finite clusters. Here we consider Li(+) scattering from an Al(001) surface. We show how the Li 2s orbital hybridizes with metal valence bands, near the surface, increasing the width of the 2s energy level. This in turn affects the charge-transfer rates between the ion and the metal surface. Our predictions for Li(+)-Al(001) scattering yield the correct angular dependence of the fraction of neutral Li atoms formed when compared to experiment.

Survival of He+ ions during grazing scattering from a Ag(111) surface

He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.

Chemical dynamics at metal surfaces

Theoretical aspects of dynamical processes at metal surfaces are reviewed. Experimental challenges to theory are presented and progress toward meeting these challenges is appraised. Topics include adsorbate vibrational energy flow, inelastic molecule-surface scattering, adsorption, transient mobility, dissociation, desorption, photochemistry, and electron-induced chemistry at metal surfaces. Experimental examples cited illustrate the richness of dynamical phenomena to be understood and the necessity of developing multidimensional, beyond Born-Oppenheimer, formulations of adsorbate dynamics. Classical mechanical and quantum mechanical treatments of dynamics are contrasted. The importance of including phonon and electron-hole pair dissipation in theories of adsorbate dynamics is emphasized, and strategies for doing this in classical and quantum treatments are presented.

Role of the 2D surface state continuum and projected band gap in charge transfer in front of a Cu(111) surface

Electron capture by Li+ and H projectiles in grazing scattering from Cu(111) and Cu(110) surfaces is studied experimentally and theoretically. Whereas data for Cu(110) can be described by established theoretical methods treating resonant charge transfer with a free-electron metal, data for Cu(111) show pronounced deviations from this approach. We interpret our observations by the effect of the projected L-band gap of the Cu(111) surface. In particular, the quantum states of reduced dimension (2D surface state continuum) play a dominant role in electron transfer.

Ultrafast Dynamics at Semiconductor and Metal Surfaces

A variety of important dynamical phenomena at metal and semiconductor surfaces are now being investigated with the use of new ultrafast measurement techniques involving lasers and nonlinear optics. Understanding of the rates and mechanisms for relaxation of optical excitations of the surface itself as well as those of adsorbates on the surface is providing new insight into surface chemistry, surface phase transitions, and surface recombination of charge carriers in semiconductors.