Isotopic fractionation effects under laser ablation of silicon from pure silicon and silicon oxide targets

The isotopic and elemental composition of bodies in the solar system is of great interest due to the information it will yield as to the formation and evolution of the bodies and the solar system in general. One possible method to quickly measure isotopic ratios on airless bodies is to use laser ablation coupled with ion mass spectrometry at a stand off distance. Specifically a Nd:YAG laser operating at 1.064μm was used to ablate the target and the resulting plasma was measured with a time of flight mass spectrometer (TOFMS) placed >3m away from the target.; The detector used was a Ceramic Channel Electron Multiplier (CCEM) with an Electro-Static Analyzer (ESA) for ion energy selection. This set up allowed the measurement of mass spectrums from a variety of targets with ion energies running from 25 eV to 1 KeV created by laser energy densities from 21 to 77 J/cm2. Because of a non-linear response of the CCEM, the relative amounts of the different isotopes could not be accurately compared, as the more abundant isotopes flux rate causes the CCEM to respond with a lower gain per particle then less abundant isotopes, yielding an apparent enhancement in the minor isotopes. This saturation effect was proven by comparing the ion flux measured by the CCEM to that measured by a faraday cup from an aluminum target.; However, by making comparisons between the flux measured by the faraday cup and the changes in isotopic ratios observed in the CCEM data for silicon targets, a trend appears which is independent of ion energy or laser fluence, indicating that any fractionation is caused by a process which is independent of laser fluence and ion energy. Further, by making comparisons between the measured major isotope flux and the changes in isotopic ratios for both silicon and SiO₂, another trend emerges, namely independence from materials. Finally, several changes must be made to the proposed instrument to make it viable for exploration purposes, most importantly a detector with a wider dynamic range in incident flux must be chosen to make accurate measurements of isotopic ratios.