We use the Newns-Anderson Hamiltonian to describe many-body electronic processes that occur when hyperthermal alkali atoms scatter off metallic surfaces.Following Brako and Newns, we expand the electronic many-body wavefunction in the number of particle-hole pairs (we keep terms up to and including a single particle-hole pair). We extend their earlier work by including level crossings, excited neutrals and negative ions. The full set of equations of motion are integrated numerically, without further approximations, to obtain the many-body amplitudes as a function of time. The velocity and work-function dependence of final state quantities such as the distribution of ion charges and excited atomic occupancies are compared with experiment. In particular, experiments that scatter alkali ions off clean Cu(001) surfaces in the energy range 5 to 1600 eV constrain the theory quantitatively. The neutralization probability of Na + ions shows a minimum at intermediate velocity in agreement with the theory. This behavior contrasts with that of K + , which shows virtually no neutralization, and with Li + , which exhibits a monotonically increasing neutral fraction with decreasing velocity. Particle-hole excitations are left behind in the metal during a fraction of the collision events; this dissipated energy is predicted to be quite small (on the order of tenths of an electron volt). Indeed, classical trajectory simulations of the surface dynamics account well for the observed energy loss, and thus provide some justification for our truncation of the equations of motion at the single particle-hole pair level. Li + scattering experiments off low work-function surfaces provide qualitative information on the importance of many-body effects. At sufficiently low work function, the negative ions predicted to occur are in fact observed. Excited neutral Li atoms (observed via the optical 2p → 2s transition) also emerge from the collision. A peak in the calculated Li(2p) → Li(2s) photon intensity occurs at intermediate work function in accordance with measurements.

We have measured the initial (clean surface) reflectance of F2 and 0 2 normally incident on Si(100)-2x 1 and Si(l11)-7x7 as a function of incident kinetic energy Ei at different surface temperatures T,. For 02, the technique of King and Wells yields the initial sticking probability, SO, which increases monotonically with E, on both surfaces for nearly all Ei studied. For F2, the presence of abstractive chemisorption complicates the interpretation of the measurements. We find that the apparent sticking probability of F2 increases monotonically with E, on both Si(100)-2x 1 and Si(ll1)-7x7, consistent with the idea that F2 does not undergo precursormediated chemisorption on these clean surfaces. Using a crude model, we show that the data obtained with F2 on Si( 1 1 1)-7 x 7 are consistent with F2 abstractive chemisorption dominating for Ei < 0.1 eV and dissociative chemisorption becoming more probable as Ei is increased. We find that the apparent initial sticking probability of F2 depends linearly on the fluorine coverage, which is consistent with a stepwise chemisorption mechanism.For both F2 and 02, the sticking increases with T, for intermediate incident energies (0.1 eV < Ei 0.3 eV).The increase with T, in the case of F2 is consistent with a surface dynamical effect whereas for 0 2 the increase may be explained by the existence of a negative ion-like intermediate state.

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Microscopes are natural objects of study in introductory and upper level courses that cover optics because they are used in most science and engineering disciplines. The solid immersion microscope has been developed to study a variety of physical systems with high resolution and we suggest its inclusion in upper level optics courses. We briefly describe the solid immersion microscope in the context of geometrical optics and a desktop demonstration. We use the angular spectrum representation to calculate the focal fields produced by a conventional microscope and a solid immersion microscope. We also suggest a simple model for lens aberration and perform numerically the focal field calculations with and without aberrations to enable users to compare the performance of conventional and solid immersion microscopes. These calculations can help users develop intuition about the sensitivity of microscope performance to real-world manufacturing tolerances and to the limitations and capabilities of microscopy.

We have studied the dependence of the saturation coverage of fluorine on the Si( 100)-2 x 1 and Si( 11 1)-7 x 7 surfaces upon F2 incident energy by measuring the desorption yields of SiF2 and SiF4 as a function of Fz exposure generated by normally incident, nearly monoenergetic molecular beams. When the F2 molecules have incident translational energy Ei = 0.060 eV, the desorption yields rapidly approach limiting values with increasing exposure. However, when Ei = 0.275 eV, the SiF4 desorption yields on both surfaces slowly approach limiting values which are increased by nearly a factor of 4 over that for Ei = 0.060 eV. These observations indicate that although there is no barrier to the dissociative adsorption of F2 onto the Si dangling bonds of the clean surfaces, there exists a barrier, between 0.060 and 0.275 eV, to the insertion of fluorine into the most vulnerable Si-Si backbonds on the saturated silicon surfaces.