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Project Progress

The evolving isotope composition of sulfur compounds pertains to the questions of the composition of the Earth’s early atmosphere. Investigations are underway to elucidate the influence of photolysis, and the subsequent oxygen dissociation of SO₂ on the isotope ratios. During the report period, we developed and constructed a new detection system for our femtosecond-laser-based time-of-flight mass spectrometer that enabled us to overcome conventional difficulties encountered in making such measurements. This new scheme facilitated the study of individual isotopes without the need for “spiking” ones of small abundance, which is fraught with difficulties, due to acquiring accurate and uniform mixtures. The new detection system has undergone extensive testing and evaluation, through the use of a Xenon gas calibration, which proved quite successful. This assessment was performed to ensure our detector had the ability to better analyze minor isotopes, embedded along with others isotopes of greater abundance. We then progressed to our primary objective of learning about effects in the ensuing chemistry of sulfur compounds that give rise to isotope effects in the dissociation process.

Experiments conducted in our laboratory show that heavier isotopes of sulfur undergo different temporal dynamics in a dissociative process than the light isotopes. Photo-dissociation of SO₂, excited to electronic states with conical intersections, has been found to display dependence on the sulfur isotope. Pump-probe experiments on the SO₂ monomer have been performed to elucidate the sulfur isotope effect on the temporal dynamics. The studies show that there is a pronounced pump wavelength effect on the type of isotope effect observed. At 197.5 nm, an inverse kinetic isotope effect is observed, while no other wavelengths studied (between 197 and 200 nm) show reproducible isotope effects. These findings, along with an observed change in absorption cross section, can be explained by a shift in absorption origin for the various isotopes. Current work continues to asses the net isotope effect of photo-dissociation of SO₂ at these wavelengths, and to identify the states involved.