A study of the electronic excitation of he from grazing incidence ion-surface interaction

The neutralization of He⁺ at grazing incidence from a Ni(111) surface leads to excited states. The light emission from the 3d ³ D and 3d ¹ D states is studied as a function of the beam energy and angular parameters. The observed circular polarization dependence on the crystallographic surface directions is interpreted in terms of the Brillouin zone of the solid.     

Resonant coherent excitation in atom-surface scattering at grazing incidence

Within a semiclassical approximation the excited-state density matrix is calculated for a resonant 1s→2p transition in a hydrogen-like projectile. The resonant coherent excitation process is induced by the periodic surface potential seen by the particle during its surface passage at grazing incidence. We find an anisotropic magnetic sublevel population which yields a high degree of polarization in a subsequent photon emission. The dependence of the Stokes parameters on both the relative orientation between the beam velocityv and the crystal axes and the surface potential decaying along the surface-normal are studied.     

Electron emission induced by keV protons from tungsten surface at different temperatures

The electron emission yield is measured from the tungsten surface bombarded by the protons in an energy range of 50 keV-250 keV at different temperatures. In our experimental results, the total electron emission yield, which contains mainly the kinetic electron emission yield, has a very similar change trend to the electronic stopping power. At the same time, it is found that the ratio of total electron emission yield to electronic stopping power becomes smaller as the incident ion energy increases. The experimental result is explained by the ionization competition mechanism between electrons in different shells of the target atom. The explanation is verified by the opposite trends to the incident energy between the ionization cross section of M and outer shells.     

Influence of the surface electric field in ion-beam-surface interaction at grazing incidence

We have measured the polarization of light emitted after ion surface scattering at small angle of incidence. The measurements are carried out with H⁺-, H ₂ ⁺ - and He⁺-ions under UHV-conditions with mono- and polycrystalline targets. We explain the typical variation of the polarization as “post collision Stark interaction” (PCSI) in the surface electric field, which can force transitions between nearly degenerate terms. The electric field is composed of two different contributions, a strong but short range surface field which is “seen” by atomsand ions and a long range but weak field due to the image charge which is “seen” to first orderonly by ions. The influence of the electric field on H-Balmer radiation is negligible at typical survival distances r_s≧0.35nm. But in contrast to H-atoms He⁺-ions feel the additional influence of the image field leading to a strong alteration of the polarization of the emitted light. The polarization of the Balmer-radiation stemming from Coulomb exploding H ₂ ⁺ -beams is observed to be modified by the electric field of the “spectator proton”.     

Band structure effects in high energy ion beam surface interaction at grazing incidence

The analysis of charge state distributions after the interaction of fast Li- and N-ions with a surface at grazing incidence at energies between 50 and 350 keV yields for Li a strongly suppressed and for N an enhanced fraction of neutrals in comparison to the beam-foil interaction. These findings are supported by corresponding alkali-spectra which are dominated by lines from transitions in singly ionized atoms. The experiments are consistently interpreted in terms of a two step model: (1) collisional excitation in the close vicinity of the surface and (2) modification of this population by resonant electron transfer from (to) non localized states in the conduction (valence) band to (from) the ion. The model is also applied to interpret recent beam-foil experiments where preferential populations of Rydberg levels in highly ionized atoms were found.     

Polarized beam of unstable nuclei via ion beam surface interaction at grazing incidence

We have measured the polarization of unstable nuclei produced by the technique of ion beam surface interaction at grazing incidence (IBSIGI). A 60 keV ¹²⁴Cs beam from the on-line mass separator was used. The NMR technique was employed for the observation of the nuclear polarization. A small polarization of 0.22(13)% was observed. The small value was interpreted in terms of the velocity dependence of nuclear polarization, comparing with the results observed with stable nuclei. This revised version was published online in August 2006 with corrections to the Cover Date.     

Suppression of coherent inelastic Mössbauer scattering of synchrotron radiation at grazing angles of incidence

Coherent inelastic Mössbauer scattering (CIMS) of synchrotron radiation (SR) by an isotopic interface (a flat interface between two regions of matter differing in the concentration of the Mössbauer isotope) at grazing angles of incidence of the SR beam is investigated theoretically. The qualitative features of CIMS as compared with conventional optics at small grazing angles of the SR beam are determined. Specifically, it is shown that for CIMS by an isotopic interface, instead of the phenomenon of total external (internal) reflection well known in optics, total suppression of reflection of the CIMS beam occurs at grazing angles of the SR beam less than a critical angle, and a maximum of the transmission and reflection at distinguished CIMS frequencies appears near the critical angle.     

Generation of coherent thermal phonons in amorphous dielectrics at room temperature induced by four-photon interaction

Resonant responses of Mandelshtam-Brillouin scattering from longitudinal and transverse acoustic waves at frequencies of ν _LS = ±1.15 cm⁻¹ and ν _TS = ±0.7 cm⁻¹, respectively, and at frequency of ν _SS = ν _TS $ sqrt 3 $ sqrt 3 = ±0.43 cm⁻¹ were detected in K8 optical glass at room temperature using four-photon spectroscopy. We attribute this effect to the induced generation of a second sound wave (coherent thermal phonons). The mechanisms of generation and the possibility of practical application of the observed effect for express diagnostics of the quality of transparent materials are discussed.     

Orientation of Na I 3p after ion beam surface interaction at grazing incidence

The orientation of orbital angular momenta of Na I 3p after the interaction of 3–30 keV Na⁺ ions with a monocrystalline Ni (111) surface at grazing incidence is measured. The data show a monotomic increase of the orientation with energy and extrapolate to vanishing orientation at zero ion velocity.     

Transitions from coherent to incoherent state induced by anharmonic interaction

A non-linear stochastic oscillator driven by a coherent radiation field is studied. Analog simulation is shown to support the arguments of theory on the specific role of the fluctuation-dissipation process and/or anharmonic interaction in making valid a treatment in terms of a Boltzmann-like distribution of energy. The results of analog simulation show that at certain values of the excitation frequency a superimposition of a coherent and an incoherent state may appear thereby opening the way to a more exhaustive theoretical treatment of this problem.     

On high-frequency scattering by a strip at nearly grazing incidence

The problem of diffraction by an absolutely soft segment is considered in the high-frequency approximation. The asymptotic field decomposition is obtained, which makes it possible to trace the transition from classical asymptotics valid for grazing incidence to geometrical optics asymptotics, which describes scattering at a finite (not small) angle.     

Carbon sputtering yield measurements at grazing incidence

In this investigation, carbon sputtering yields were measured experimentally at varying angles of incidence under Xe⁺ bombardment. The measurements were obtained by etching a coated quartz crystal microbalance (QCM) with a low energy ion beam. The material properties of the carbon targets were characterized with a scanning electron microscope (SEM) and Raman spectroscopy. C sputtering yields measured under Ar⁺ and Xe⁺ bombardment at normal incidence displayed satisfactory agreement with previously published data over an energy range of 200 eV-1 keV. For Xe⁺ ions, the dependence of the yields on angle of incidence θ was determined for 0° ≤ θ ≤ 80°. Over this range, an increase in C sputtering yield by a factor of 4.8 was observed, with the peak in yield occurring at 70°. This is a much higher variation compared to Xe⁺ → Mo yields under similar conditions, a difference that may be attributed to higher scattering of the incident particles transverse to the beam direction than in the case of Xe⁺ → C. In addition, the variation of the yields with θ was not strongly energy dependent. Trapping of Xe in the surface was observed, in contrast to observations using the QCM technique with metallic target materials. Finally, target surface roughness was characterized using atomic force microscope measurements to distinguish between the effects of local and overall angle of incidence of the target.