Isotope ratio measurements in strontium using two-photon two-colour resonance ionization mass spectrometry

Abstract

A simple two-photon two-colour excitation scheme has been investigated, using a UV Ar⁺-ion laser pumped-dye laser system, for isotopic analysis of Sr by continuous wave laser resonance ionization mass spectrometry (RIMS). Strontium ion yield measurements were made in an ion microprobe with multi-collection facility, employing both thermal evaporation and solid target sputtering by primary ion beams of O⁺, Ar⁺, and Xe⁺, for producing neutral Sr atoms. These measurements have displayed a consistent depletion of the odd ⁸⁷Sr : ⁸⁶Sr isotope ratio from the standard value by as much as 10%(or 100‰) and the dependence of this observed even–odd mass isotopic selectivity of the ionization process on the resonant dye laser power has been examined. The elemental selectivity of the ionization scheme for Sr has also been demonstrated to be in excess of 10⁶ over isobarically interfering Rb. The ionization yields obtained with both thermal evaporation and primary ion beam sputtering are compared with the results of simple mathematical analyses of the resonance ionization process and possible reasons for the quantitative differences observed between the measured and the predicted results are suggested and discussed.