P.M. Savundararaj

Ultraviolet spectroscopy of CN− in alkali halides: Dynamics of the metastable triplet state

Abstract An ultraviolet emission spectrum attributed to cyanide ions in alkali halide lattices is excited by VUV irradiation and electron impact, using samples containing isotopically substituted 13 CN − as well as normal 12 CN − . Analysis of the spectrum yields ground state vibrational constants ω e = 2125 cm −1 and ω e χ e = 14.2 cm −1 for 12 CN − in KCl. The excited electronic state lies 5.6 eV above the ground state and has room-temperature decay lifetime of 80 ms in very dilutely doped KCl samples. This value is consistent with expectations for the forbidden triplet-singlet transition, but inconsistent with other studies of this system.

Erosion and glow in the near-earth space environment

The phenomena of spaceglow and material erosion, which are observed on spacecraft that orbit the earth at low altitudes, are described. Also described are the physical environment of the low earth orbit and the methods by which laboratory research on these phenomena is conducted.

Dependence of alignment and orientation induced by grazing-incidence and beam-foil electron-exchange interactions on surface electronic structure

Abstract We report first detailed measurements of the effect of adsorbates on final configuration of electrons after electron pickup by positive ions during grazing-incidence and beam-foil interactions. Positive hydrogen ions incident at grazing angles on metal surfaces are neutralized and coherently excited due to anisotropic electron pickup from the surface. As a result the emitted radiation is strongly circularly polarized and when the scattered ions pass through an external electric field, the Stokes parameters characterizing the polarization oscillate as a function of the electric field strength. These oscillations, quantum beats, arise from quantum mechanical phase interference between participating atomic states. The effect of surface modification on these quantum beats is studied by adsorbing oxygen onto the surface. In addition, we discuss the alignment induced by interactions of a 17 keV H + beam passing through thin carbon foils with and without a deposited layer of lithium. The results presented here demonstrate the sensitivity of these techniques to the influence of overlayers on surface conditions and support the idea that future experiments with detailed surface characterization could provide a basis for theories of the sensitivity of electron pickup to surface electronic structure.

Hot Hole Diffusion in the Electron-Stimulated Desorption of Metal Atoms from Alkali Halides

Within the framework of a realistic theoretical model, diffusion of hot holes in alkali-halide crystals prior to self trapping is shown to have a substantial influence on the time dependence of the flux of desorbed ground state neutral metal atoms resulting from pulsed electron beam bombardment of these crystals. Comparison of results from the model with new time dependent measurements of Li ESD from LiF suggest that substantial hot hole diffusion occurs in LiF, and that, depending on its preparation, the LiF surface can be an absorber or reflector of hot holes.

Surface adsorbates and ion-surface interactions at grazing incidence

Abstract The effect of adsorbate overlayers on surfaces is investigated by the scattering of a 10 keV proton beam by a nickel(100) surface at grazing incidence. The anisotropy at the surface during the electron pickup results in the orientation of the excited states and in the polarization of light emitted by the excited hydrogen atoms. The polarization of the 6563 A Balmer alpha (n = 3 → n = 2) transition is detected. The Stokes parameter S/I characterizing circular polarization and associated with an orientation in the distribution of orbital angular momentum, exhibits an oscillatory dependence in an external electric field. We are investigating the effect of overlayers on these quantum beats as a means to understand the ion-surface scattering mechanisms.

Desorption induced by ion- and neutral-beam excitation

Abstract Beam-induced desorption of particles from solid surfaces can be initiated by momentum-changing atomic collisions or by changes in the electronic environment of the desorbing particle. This report gives preliminary results on experiments designed to provide information about the mechanisms of electronic desorption through comparisons of the optical spectra of particles desorbed by beams of different species, charge state, and energy.

Optical Radiation from Electron, Photon and Heavy Particle Bombardment of Lithium Fluoride and Lithium-Dosed Surfaces

Comparative studies of electron, photon and ion bombardment of the same sample surface probe the various channels through which incident particle energy is dissipated, often leading to desorption. Electron or photon irradiation of alkali halides results in the swift ejection of halide atoms, leaving behind an enriched alkali metal surface from which the alkali atoms thermally desorb. Ion bombardment involves momentum transfer as well as electronic mechanisms, which results in a different surface stoichiometry at the time of desorption. This study explores how the degree of surface metallization influences the choice of the final excitation state of the desorbing particle. An important way to study the desorption products is to monitor the characteristic spectral radiation from the de-excitation of ejected species [1]. In this paper we report recent results of this type. Measurements were made to compare the desorption of excited state neutral lithium from lithium fluoride by electron, photon and ion bombardment and from lithium-dosed tungsten and lithium-dosed glass by electron and photon bombardment.

Atomic oxygen simulation and analysis

Abstract A technique has been developed for creating in the laboratory suprathermal, neutral atomic beams by means of grazing-incidence collisions between ion beams and metal surfaces. Residual ions are deflected by applied electric fields. The resulting neutral beams are pure, well-focused and mono-energetic. Energies from a few eV to tens of keV are obtainable. The technique permits the beams to be used in conjunction with electron and photon irradiation for studies of synergistic effects. Of particular interest is the creation of a low-energy, neutral, atomic-oxygen beam, which is of importance to space-related research.