The velocity dependence of the ionization probability of sputtered ${\mathrm{O}}^{-}$ 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 $v_{\perp }>1\mathrm{}\times {10}^6$ cm/sec, the ionization probability shows an exponential dependence on $v_{\perp }$. However, this velocity dependence fails at lower velocities.

• Received 17 July 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.1325