Velocity Dependence of the Ionization Probability of Sputtered Atoms

The velocity dependence of the ionization probability of sputtered O^- 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_⊥>1×10⁶ cm/sec, the ionization probability shows an exponential dependence on v_⊥. However, this velocity dependence fails at lower velocities.