Preferential emission of lighter isotopes in the initial stage of sputtering

Abstract

Initially, the flux of sputtered ions is strongly enriched in the lighter isotopes. During sample erosion, this enrichment gradually decreases and finally a steady-state value is reached. Isotopic variations were monitored for primary beam (14.5 keV O−) fluence increments of ∼1 × 10¹⁵ ions/cm² by means of secondary ion mass spectrometry. Extrapolation of the data to the zero-fluence limit shows that the ratio ⁴⁶Ti⁺/⁵⁰Ti⁺ (sputtered from TiO₂) is initially higher by a factor of ∼1.063 relative to the steady-state flux. The same value for ⁶Li⁺/⁷Li⁺ (sputtered from LiAlSi₂O₆) is ∼1.054. Enrichments of comparable magnitude were observed also for Ga and Mo isotopes sputtered from GaAs and Mo, respectively. Applying a theoretically predicted enrichment factor at low fluence of (M_j/M_i)^(2m), with M_j and M_j being the mass of the light and heavy isotope, respectively, values of m ranging from 0.16 to 0.45 are derived from the experiments. A momentum asymmetry in the collision cascade, as found in recent computer simulations, might cause these strong effects.

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