Potentialities of fast ion neutralization at grazing incidence angles From crystal surfaces for development of new generation of uv laser sources

The paper considers processes of electron capture by fast ions scattered from metal surfaces at grazing incidence angles. It discuss-es the possibilities of excitation of high atomic levels in neutralization process and laser generation. The Doppler compression effectfor ion beams is shown to be rather essential for laser generation in the ion energy range of several tens keV. It is shown that the four-level laser generation scheme is the preferable one. The values of critical currents for laser generation are evaluated. The experimen-tal setup for ion beam neutralization is observed. The experimental results on nitrogen ion beam neutralization on silicon (001)wafers at grazing incident angles are presented. The dependence of the neutralization coefficient on the grazing incidence angle ismeasured, a good coincidence between peak maximum and Lindhard angles is demonstrated. It is shown that in case of neutraliza-tion without special vacuum surface cleaning the neutralization coefficient makes approximately 30 % for ion energy of 40 keV. Theone-electron resonant neutralization to nitrogen 2P1/2, 2D3/2, 4S3/2 atomic terms is discussed. The mechanism of resonant coher-ent excitation of fast nitrogen atoms in front of a Si (001) surface is observed, the resonant energy being estimated about 70 ke

The structure of Green functions in quantum field theory with a general state

In quantum field theory, the Green function is usually calculated as the expectation value of the time-ordered product of fields over the vacuum. In some cases, especially in degenerate systems, expectation values over general states are required. The corresponding Green functions are essentially more complex than in the vacuum, because they cannot be written in terms of standard Feynman diagrams. Here, a method is proposed to determine the structure of these Green functions and to derive nonperturbative equations for them. The main idea is to transform the cumulants describing correlations into interaction terms.Comment: 13 pages, 6 figure

The spatial extent of surface effects on electron inelastic scattering

We calculate the thickness of the surface scattering layer, defined as the region where electron inelastic scattering is affected by the surface, using the semi-classical treatment of electron energy loss provided by the Chen–Kwei theory. To this end, we consider the depth-dependent, surface-related contributions to the inverse inelastic mean free path, namely, the excitation of surface plasmons and the reduction in bulk plasmon excitation (Begrenzung effect). We find that surface effects extend further after electrons cross the surface than before they cross it. The ‘pre-surface thickness’ is given by the ratio of the electron velocity to the plasma frequency, the characteristic decay length for surface effects. All thickness estimates increase linearly with the electron velocity and decrease as (cosα)x with the angle α between the electron trajectory and the surface normal

Momentum transfer effects in near surface Electron Energy Loss

We examine two formulations for the differential surface excitation parameter (DSEP): one provided by Tung et al. and the other given by the Chen–Kwei position-dependent differential inverse inelastic mean free path integrated over the electron trajectory. We demonstrate that the latter converges to the former provided that the dielectric function of the solid does not depend on the momentum transfer or it depends on just the momentum transfer component parallel to the surface. Tung's DSEP represents therefore an approximation to the Chen–Kwei DSEP calculated for a dielectric function with no restrictions on the momentum dependence. The approximation is shown to work in the limit of small momentum transfer and to imply an error of 4%–5% for electrons traveling through the solid with energy E = 1 keV

Reflection electron energy loss spectroscopy: role of the Bethe–Born factor

The article deals with two issues concerning reflection electron energy loss spectroscopy (REELS), namely, which angular cutoff should be used to properly define the phase space of energy loss into plasmon excitations and how approximate evaluations of the surface component of the momentum transfer can affect the surface excitation parameter. With regard to the first point, we demonstrate the crucial role of plasmon energy dispersion in determining the angular range for inelastic scattering. As for the second point, we show that an exact evaluation of the surface component of the momentum transfer is needed if the surface excitation parameter has to be determined in a reliable way over the entire range of angles of surface crossing