Electron capture from a metal surface by slow, multicharged aluminum and carbon ions

A time-of-flight technique has been used to measure residual charge in the scattering of laser-produced pulses of ${\mathrm{C}}^{\mathrm{k}+}$ (k=1 to 4) and ${\mathrm{Al}}^{\mathrm{m}+}$ (m=2 to 5) ions from a well-outgassed amorphous gold-iridium surface under UHV conditions (2×${10}^{\mathrm{-}9}$ Torr). Ions incident at 7° to the surface were specularly reflected. The analysis showed the survival of singly charged ions in the case of scattering 300-, 400-, and 500-eV/charge Al ions with neutrals representing the majority species. This is equivalent to a kinetic energy in a direction transverse to the surface of 4.5, 5.9, and 7.4 eV/charge, respectively, which ensures only minimal surface penetration. In the case of 280-eV/charge carbon ions, only neutrals were detected. No residual ions were detected in either Al or C ions scattered through an angle of deviation equal to or greater than 45° within experimental error. In a separate experiment no residual ions were detected in the case of 400-eV/charge Al ions incident at 22.5° to a gold surface and specularly reflected. The results are explained in terms of Auger neutralization of the multicharged ions on the incoming pass and resonance ionization and neutralization of low-charge-state ions that emerge from the surface and change their charge on the outgoing pass. Under near adiabatic conditions, no residual charge is expected for either the aluminum or carbon projectiles. The presence of ${\mathrm{Al}}^{+}$ under grazing-incidence and specular reflection is analyzed and discussed in terms of the nonadiabatic behavior of ${\mathrm{Al}}^{+}$ ions emerging from the surface.