Keith T. Gillen

Computer simulations and rainbow patterns of alkali ion scattering from metal surfaces

Abstract In a previous article [Surface Sci. 172 (1986) 90] the necessity of 3-dimensional computer simulations has been shown for the modelling of the scattering of 10–100 eV K + from W(110). The measured triple differential cross section shows a complicated peak structure which can only be understood with the help of trajectory calculations. A simple model potential is given that can describe the scattering at 35 eV. The calculated trajectories show from which impact regions in the surface unit cell the peaks in the spectra originate. This mapping also allows a rainbow analysis for a triple differential cross section.

Energy and angular distributions for scattering of K+ from W(110) at normal incidence

Abstract Using a K + ion beam and a K neutral beam at normal incidence on a W(110) surface, we have measured the angle and energy distributions of scattered K + ions at different azimuthal angles. The primary energy is in the range 12–100 eV. The measured distributions show a complex peak structure. The peaks can be assigned to different scattering mechanisms, giving rise to various rainbow phenomena.

Elastic scattering measurements of the Ar2* (3Σ+u) well depth

Angular distribution measurements of the scattering of Ar(3P2) from Ar(1S0) at center of mass collision energies from 5 to 10 eV yield a prominent rainbow maximum at τ=Eϑ=81±3 eV‐deg. Combined with recent high energy ground state scattering data and information derived from the Ar2* emission continua, the result implies that De=0.78±0.04 eV and Re=2.33±0.02 A in an assumed Morse form for the lowest 1u, 0−u(3Σ+u)Ar2* excimer potential.

Magnetic bottle electron spectrometer using permanent magnets

Design principles and initial results are presented for a magnetic bottle electron spectrometer of high collection efficiency (2π sr) and good energy resolution. The instrument duplicates the carefully shaped axially decreasing magnetic field configuration of Kruit and Read [J. Phys. E 16, 313 (1983)], but replaces the high‐field electromagnet central to the Kruit–Read design with two simple pole pieces energized by permanent magnets. The use of permanent magnets requires modifications of the transition region (between the high‐field electron production zone and the low‐field drift tube analyzer) to avoid axial field reversals associated with fringing fields.

Ion-pair formation in fast collisions of metastable argon with iodine

Abstract Differential ion-pair production cross sections for Ar * —I 2 collisions at center of mass energies of 25–133 eV are quite similar to analogous alkali—I 2 results despite the addition of competing channels (Penning ionization and excitation transfer) and possible multiple electronic surface crossings.

Scattering of low energy K+ and Na from W(110)

Abstract Low energy scattering experiments of K+ and Na from W(110) have been performed. The triple differential cross section σ(θ, φ, E′) has been measured for 35 eV K+ as a function of the angle of incidence θi. As θi, increases, the complex σ(θ,φ,E′) structure reported previously at normal incidence simplifies considerably. At the largest experimental θi (30°), the triple differential cross section and the energy spectra are simple and reminiscent of energy spectra as measured in low energy ion-scattering (LEIS) at higher energies. Most of the structures observed can be attributed to electronically elastic scattering, and can be reproduced in classical trajectory calculations using a simple model potential. The calculations indicate that many of the features observed can be identified as surface rainbows. The surprising additional existence of “quasi-specular” scattering for this system is due to symmetrical zig-zag collisions. In addition electronic excitation of the projectile can be observed. Scattering of 20–70 eV Na atoms from the W(110) surface shows that in this energy range Na atoms behave similarly to K+ ions, although the higher electronic excitation energy of Na+ precludes its observation.

Depth profiling using intense untuned UV laser ionization of sputtered neutrals

Abstract Surface analysis by laser ionization (SALI) in a versatile technique that combines photoionization with various desorption tools, especially ion beam sputtering. In this method, neutral atoms and molecules removed from a surface under ultrahigh vacuum are multiphoton-ionized by an intense focused untuned UV laser beam, and the photoions are then analyzed by reflecting time-of-flight mass spectrometry. This paper describes several features of the instrument including mass resolution, which was measured to be 1000, and depth profiling capabilities. Depth profiles are presented of a B implant in Si, and unannealed and annealed three-layer, fluorine-implanted, electronic test devices showing F migration.

Surface and interface analysis by nonresonant multiphoton ionization of sputtered neutrals

Abstract The surface analysis by laser ionization (SALI) technique has unique advantages for mass spectrometric examination of materials, surfaces, and interfaces. Initial results from a second generation SALI instrument are presented to illustrate current capabilities of the technique when used to examine neutral atoms and molecules sputtered from inorganic materials by an at + probe beam. Results show good mass resolution, very high sensitivities, good quantitation capabilities, and excellent depth resolution at sharp interfaces.

State purification of a fast neon metastable beam by collinear optical pumping

An apparatus has been constructed to produce well‐characterized fast beams of rare gas metastable atoms for subsequent scattering experiments. Optical pumping by a collinear laser beam can be used to selectively remove either of the two metastable components from the beam. Laser‐induced‐fluorescence (LIF) techniques can be used to determine the original beam composition and to monitor composition changes resulting from optical absorption. These techniques have been successfully applied to a metastable neon beam produced by near‐resonant charge transfer of Ne+ in Na. A simple model has been developed which accurately reproduces the observed LIF lineshape generated by tuning the laser throught the optical transition frequency.

Comparison of ion pair formation in the systems Ar*+I2 and K+I2

A simple model that has been used extensively by Los and co‐workers to treat ion pair formation in collisions of alkali atoms with diatomic molecules is extended to include continuum coupling via a competing Penning ionization channel. This extended model is then used to calculate the differential cross sections for ion pair formation for the system Ar*+I2 over the energy range 28–154 eV and to compare with a previous treatment of K+I2. In the absence of significant competition from continuum processes, Ar* is expected to behave in a manner similar to K, since the active electron is an unpaired 4s electron in both cases. We perform model calculations for Ar*+I2 to investigate the effects of varying the potential curves and charge exchange matrix elements and of including a continuum coupling function Γ (R). Comparison with previous calculations for K+I2 suggests increased repulsion on the Ar*–I2 surfaces relative to those of K–I2. The competing mechanisms of excitation transfer and Penning ionization may ...