EFFECT OF METAL BAND CHARACTERISTICS ON RESONANT ELECTRON-CAPTURE - H- FORMATION IN THE SCATTERING OF HYDROGEN-IONS ON MG, AL, AND AG SURFACES

We present the results of an experimental and theoretical study of H- formation in collisions of 1 to 4 keV positive and negative hydrogen i ons with clean Mg, Al, and Ag surfaces. Ion fractions and energy loss spectra of scattered particles were measured in a large angular range allowing us to investigate the characteristics of the resonant charge transfer process for a wide range of collision velocities normal to th e surface, thus probing the charge-transfer process in different atom- surface distance ranges. We compare H- formation on metal surfaces, wi th different valence-band characteristics involving the work function and Fermi energies (epsilon(F)) The experimental results are found to be in good agreement with the predictions of the nonperturbative coupl ed angular mode method, used in conjunction with a semiclassical rate- equation approach. The importance of the parallel velocity effect is d emonstrated and the differences between the various metal targets are interpreted in terms of differences between both work functions and al so the valence-band width or Fermi energies.