Measurements of the secondary ion mass spectrometry isotope effect

Abstract

The heavier isotopes of a given element are generally underrepresented in the secondary ion mass spectrometry mass spectrum since they emerge from the sample with lower average velocity and are therefore less likely to be emitted as ions. The relative deviation of an isotope ratio measurement from its expected value is represented in the literature by the parameter α. In this work we report measurements of α as a function of secondary ion energy in high‐purity Ge, Mo, Pd, Cd, Sn, and W samples; each of which offers several abundant isotopes for examination. A Cameca IMS‐3f magnetic sector instrument is operated in the high‐energy resolution mode for these measurements. The α values are determined from weighted least‐squares fits to the isotope ratio deviations. Appropriate precautions against beam instability, mass interference, and detector saturation effects are employed. Positive and negative ion emission, induced by O+2 and Cs+ bombardment, respectively, are examined. For monatomic positive ion (M+) emission, the magnitude of α correlates roughly with the secondary ion yields and generally increases with increasing emission energy. In contrast, α values for oxide species (MO+) display an inverse dependence on energy indicating surface emission. For negative ion (M−) emission, α changes sign at low energies indicating that, at low energy, ion emission is dominated by the survival probability of the negative ion while, at higher energies, emission depends on the probability of ion formation by electron attachment. Measured α values for diatomic (M+2, M−2) emission are examined with respect to current models of molecular ion formation.