D.P. Russell

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.

Temperature dependence of surface charging of NaCl under electron irradiation: role of secondary electron emission

Abstract In initial observations during Auger studies of NaCl, we have observed stable, clearly resolved Auger peaks. With time, however, some of the Auger spectra reveal the occurrence of strong negative charging. This charging appears to be a result of irradiation-induced accumulation of sodium metal on the sample surface. We report the dependence of the strong negatively charged state on temperature. The results are consistent with the above model; the strongly negatively charged surface cannot be maintained at temperatures sufficiently high that sodium metal evaporation occurs.

Generation of excited CN molecules electronically desorbed from alkali-rich surfaces

Abstract Exposure of alkali-halide crystal surfaces and alkali-metal surfaces to nitrogen- and carbon-containing gases is found to greatly enhance electron- and photon-stimulated desorption (ESD, PSD) of excited CN. The observations that the desorption yield of CN ∗ grows as a function of surface exposure to CO 2 and N 2 gases provide strong insight into the origin of the desorbed CN and the desorption process itself. In addition, the data unambiguously show that pre-irradiation of the alkali-halide surface by electrons or photons is required for this dose-dependent enhancement to occur. The rate of the growth is found to be correlated to the alkali component of the alkali-halide substrate. On the basis of this work we present a new model to explain electronic desorption of excited molecules from alkali-halide surfaces. The model involves three processes: (1) pre-irradiation produces alkali metal-rich surfaces via defect-mediated processes, (2) when the surface is exposed to CO 2 and N 2 , surface reactions generate CN molecules bonded to the alkali-rich surface, and (3) electron or photon bombardment induces the desorption of excited CN molecules from the surface by direct bondbreaking.

F center random walk diffusion after electron irradiation of alkali halides

Abstract The random walk of individual F centers, which causes F center diffusion in alkali halides after electron irradiation, is simulated in three dimensions with a Monte Carlo program. The F center random walk is considered to be thermally driven along the halogen sublattice. The resulting diffusion process supplies F centers to the surface. For cold (about room temperature) surfaces this causes alkali metal surface enrichment. The surface enrichment time dependence is predicted by the three-dimensional Monte Carlo simulation and compared with experimental data. Three different initial distributions for the F centers are considered. These distributions are chosen to coincide with the depth profile for: (i) the energy deposited by the irradiating electrons; (ii) the density of hot holes after their migration; and (iii) a Gaussian distribution located well below the surface. Although all three simulations with these initial distributions compare reasonably well with the experimental data, the third provides the best agreement with the data.

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.

Nonthermal energy level populations in excited oh electronically desorbed from alkali halide surfaces

Abstract Analysis of the A 2 Σ + → X 2 Π emission from excited OH radicals desorbed from alkali halide surfaces by low energy electron bombardment indicates strongly nonthermal energy partitioning among the rotational levels of the excited state. In particular, a difference in the population of the F 1 and F 2 states is observed. This alternation becomes stronger as the sample temperature is lowered below room temperature, indicating that the broken symmetry between the F 1 and F 2 states is sufficiently weak that thermal effects can partly mask it near room temperature.

Auger Studies of Damage and Charging in NaCl

In this study we observe that the positions of Auger signals shift from slightly below (∼10 eV) the tabulated values to well above (>150 eV). The observed shifts can be understood by a simple model of surface charging based on the change in the secondary electron emission of the damaged region.

Interactions of hydrogen with magnetic and nonmagnetic surfaces

Abstract We report on recent measurements from two experiments in which we attempt to probe the electron-transfer process that occurs between a proton and a metallic solid. In a beam-tilted-foil experiment, we have observed an anomalous tilt-angle dependence of the circular polarization of Balmer-alpha radiation emitted following the transmission of low-energy protons through a thin carbon foil. This anomalous behavior is correlated with structure changes in the foil induced by the proton beam. In a grazing-incidence scattering experiment, we have made the first observation of quantum beats from the spin-polarized electron capture from Fe(11O). We have also measured the energy dependence of the circular polarization of Balmer-alpha radiation emitted following the grazing-incidence scattering of protons from a magnetized Fe(110) surface. Our results show that the contribution of the spin of the captured electrons to the observed circular polarization is constant over the energy range studied.

A Monte Carlo model of alkali halides under low energy electron irradiation

Abstract A Monte Carlo random walk model was used to study the creation and dynamics of F- and H-centers induced by electron irradiation in alkali-halide single crystals. The purpose of the study was to observe the effects of depth dependent electron energy deposition near the surface region using beam energies of 500 eV or less and relatively low currents (< 100 μA/cm2). The calculations were carried out using the parameters appropriate for an FCC LiF single crystal at room temperature. Depth dependent dynamics of F- and H-centers are also presented as well as F—H center recombination rates, and thermal and non-thermal halogen production rates at the surface. The model demonstrates that F—H center recombination dramatically affects the F-center distribution. F-center distributions are found to be peaked at the surface and do not reflect the initial energy deposition profile.

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.