Velocity Dependence of the Ionization Probability of Sputtered Atoms

The velocity dependence of the ionization probability of sputtered ${\mathrm{O}}^{\ensuremath{-}}$ from chemisorbed oxygen layers on V and Nb is studied. The ionization probability is found to depend on the normal component of the emission velocity, which suggests that the ionization process is an ion-surface interaction and not an ion-atom binary interaction. For ${v}_{\ensuremath{\perp}}g1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ cm/sec, the ionization probability shows an exponential dependence on ${v}_{\ensuremath{\perp}}$. However, this velocity dependence fails at lower velocities.