High-resolution angle-resolved positron reemission spectra from metal surfaces.

Abstract

An electrostatically focused beam of monoenergetic positrons is used with a hemispherical energy analyzer to acquire the first high-resolution (\ensuremath{\approxeq}20 meV) total-energy spectra of positrons reemitted from metal single crystals. At 300 K the measured energy width of positrons elastically emitted (with kinetic energy corresponding to the magnitude of the negative positron work function) is \ensuremath{\approxeq}75\ifmmode\pm\else\textpm\fi{}5 meV for all the samples investigated: Ni(100), Ni(100)+CO, Ni(100)+S, Ni(100)+O, W(110)+C, W(110)+Cu, W(110)+O, Cu(111), Cu(111)+S, and Cu(100)+S. These results, along with angular distribution measurements on W(110)+C, W(110)+O, and Cu(111), are completely consistent with energy and angular widths as determined by a beam Maxwell-Boltzmann distribution---i.e., thermal broadening alone. A much narrower peak is observed at sample temperatures of 23 K, but uncertainty in the analyzer resolution limits us to conclude that the observed broadening is consistent with a thermal distribution with effective temperature less than 100 K. Discrete energy-loss peaks due to vibrational excitations of adsorbed molecules on the sample surface, first reported by Fischer et al. [Phys. Rev. Lett. 50, 1149 (1983)], are further investigated for Ni(100)c(2\ifmmode\times\else\texttimes\fi{}2)/CO. The first evidence for short-range positron ``impact'' scattering is found for OH adsorbed on NiO as a loss peak at 400 meV in the reemitted spectrum. Continuous inelastic scattering processes are also observed in the spectrum and a limit on wide-angle elastic scattering is determined from angular scans. A discussion comparing electron-energy-loss spectroscopy with positron-energy-loss spectroscopy is presented along with a brief discussion of possible improvements to positron-energy-loss spectroscopy, including the use of brightness-enhanced beams, spin-polarized positron beams, and liquid-helium-cooled samples for narrow-energy-width beams.