Significance of isotope effects for secondary-ion emission models

Differences in the ionization probabilities P of the isotopes of an element have been investigated and compared to the predictions of secondary-ion emission models. The relative differences in P for two isotopes depend inversely on the ion emission velocity v but approach a constant value for small v. Both this variation and the observed linear dependence on mass difference follow from an exponential correlation between P and v, P∝exp(-$v_0$/v), as predicted by the bond-breaking model. This agreement confirms the validity of this concept for secondary-ion emission from samples with some ionic character (halides and oxides). The parameter $v_0$ was determined for all ion-substrate systems investigated.