NASA Astrobiology Institute (NAI)

The evolving isotope composition of sulfur compounds pertains to the questions of the composition of the Earth’s early atmosphere. We are currently in the process of finalizing our interpretation of our investigations on the influence of photolysis, and the subsequent oxygen dissociation of sulfur dioxide on the isotope ratios. During the report period, we began the interpretation of the experimental results obtained from our spectrometer, which, coupled with a new detection scheme, has enabled us to overcome conventional difficulties encountered in making isotopic measurements. This scheme facilitates the study of individual isotopes without the need for a “spiked sample”, a process that is fraught with difficulties due to the inability to reliably acquire accurate and uniform mixtures.

Experiments that have been conducted in our laboratory show that the heavier isotopes of sulfur undergo different temporal dynamics in a dissociative process than the lighter isotopes. The photo-dissociation of SO₂, excited to electronic states with conical intersections, has been found to display dependence on the sulfur isotope. In an effort to assess the viability of the photochemical processes in the Achaean era that may have resulted in an unusual sulfur isotope signature in the rock record, we have performed pump-probe experiments on SO₂ at ~200 to 197 nm. As part of the continuing effort to interpret our experimental findings on SO₂ has led to a publication this year [Chemical Physics 350 (2008) 212-219], and we shall publish the remainder of our results in the near future. In brief, we have found that when SO₂ is excited by ~200 nm light, it can undergo one of three different processes: (1) promptly dissociate an oxygen (between 100 and 265 fs); (2) internally redistribute of the energy through an avoided crossing (a 15 to 28 ps decay process); or (3) enter a bound state (lifetime > 150 ps). In addition, we have no evidence for dissociation beyond one oxygen atom. Finally, regarding an ongoing debate associated with this system – a singlet or triplet dissociation pathway – our most recent results support the singlet mechanism, as we observe a prompt dissociation.