Preferential sputtering of isotopes: Fluence and emission-angle dependence

The isotopic composition of the flux of sputtered neutrals was monitored for very low primary beam fluences (≲1×${10}^{15}$ ${\mathrm{Ar}}^{+}$ ions/${\mathrm{cm}}^2$) and different emission angles (0° to 60°) by means of secondary neutral mass spectrometry. Initially, the emitted material is enriched in the lighter isotopes by a factor of about 1.05 for ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}$ ${/}^{76}$Ge and ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}$ ${/}^{100}$Mo. This initial enrichment is independent of the emission angle and gradually decreases during sample erosion. A steady-state value is reached upon sputter removal of a layer of roughly 15 nm.