Image potential and ion trajectories in secondary-ion mass spectrometry

The angle and energy distributions of Ni+ ions ejected from ion-bombarded Ni(001) c(2×2)-CO are shown to be in excellent agreement with classical trajectory calculations for Ni atoms if the calculations are corrected for the presence of an image force. Two important consequences of this observation are that the ionization probability R+ is nearly isotropic and that it is only weakly dependent on particle velocity. These constraints impose severe restrictions on proposed ionization theories.