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We have formed an energetic (10-100 eV) beam of positronium (Ps) by scattering a well-collimated, small-diameter beam of 30-400-eV positrons off surfaces of Al(llO) and clean and oxygen-coated Cu(l 00) at glancing angles of 6°-42° with respect to the surface. We determined the angular distribution (FWHM -20 0 ) of the Ps beam and measured the relative Ps formation efficiency (3%-5%) as a function of incident positron beam energy and angle. Future uses of the Ps beam in atomic and surface physics will be discussed.PACS numbers: 78.70.Bj, 36.10.Dr, 61.80.Fe We report the first investigation of glancing-angle scattering and neutralization of a positron beam on surfaces. The glancing-angle geometry permits the study of a wide range of physical processes with positrons and positronium for which analogous work is currently under investigation in surface and atomic physics with more conventional electron, ion, and neutral-atom beams, e.g., diffraction, reflection, inelastic scattering, surface-state resonances, and electron capture. Multiple elastic positron scattering at a surface 1 is closely analogous to lowenergy electron diffraction (LEED). However, a major difference emerges at glancing incidence in that positron reflection 2 is much more probable than electron reflection because the inner potential is repulsive rather than attractive. Also at glancing incidence, the positively charged positron's interaction with the surface will have features similar to low-energy ion scattering 3 and ion neutralization. 4 In this latter case the capture of a surface electron to form the bound state, positronium (Ps), should complement studies of ion neutralization processes as probes of surfaces 4 and surface magnetism 5 by, for example, lowering the mass range of the projectile by 3 orders of magnitude. Finally, we note that electron capture by surface-reflected positrons is a potential source of a variable "fast" (10-100 eV) energy Ps beam. With its low mass, charge neutrality, and relatively high energy, a variable-energy Ps beam could (in analogy with He-atom diffraction) be a useful probe for surface diffraction and reflection studies. 1 The elastically scattered Ps will sample the cores of the outer surface layer only, and will not undergo multiple scattering which tends to complicate the interpretation of the data. Such a beam might be useful in performing the first measurements of Ps-atom collision cross sections. 6 In this Letter we will focus on the surface neutralization process as a means of forming a beam of fast Ps. Several methods of forming fast Ps have been or currently are under investigation. Mills and Crane 6 have shown that kiloelectronvolt positrons propelled through a 50-A carbon foil form Ps over a wide range of energies (10-500 eV) with an integral formation probability of -0.5%. Brown is investigating charge capture in a low-pressure gas cell, and Mills 8 has suggested a method in which Ps ~ is accelerated and the electron subsequently stripped off or photoionized. Howell et al. 9 have produced Ps wi...

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The results of an experimental study and quantitative analysis of the intensity versus energy (I-V) curves are reported for low-energy electron diffraction and low-energy positron diffraction (LEPD) with a brightness-enhanced electrostatically focused positron beam. In a close comparative study, the incident electrons and positrons scattered at a large incident polar angle (0& 50') with respect to the surface normal off clean Cu(100) and the I-V spectra from six and seven diffraction beams were taken with electrons and positrons, respectively. The analysis of the experimental data from the electron studies indicates firstand second-layer relaxation that is consistent with earlier results.Use of the structure derived from the electron studies, analysis of the I-V curves from the LEPD studies suggests that the attenuation for positrons is greater than the value for electrons over the energy range 50 -400 eV, possibly as a result of the enhanced electron-image cloud surrounding the positron. The real part of the inner potential is 0 eV for positrons compared with 11 eV for electrons, in rough agreement with predictions. Further, the best agreement between experiment and calculation for LEPD I-V curve analysis tends to favor the potential formed by changing of the sign of the Coulomb term (relative to electrons), eliminating the exchange, and retaining the correlation term.