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2005 Annual Science Report

Pennsylvania State University Reporting  |  JUL 2004 – JUN 2005

Isotope Effects in Atmospheric Sulfur Chemistry (Castleman)

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

Studies are underway to elucidate the influence of oxidation reactions, and subsequent photolysis on the evolving isotope composition of sulfur compounds that pertain to questions of the origin of the earth’s early atmosphere. We continued our development of a detector system for our femtosecond-laser-based time-of-flight mass spectrometer system that will overcome conventional difficulties encountered in making such measurements. The objective is to enable 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. We have constructed a more sensitive detection system to better analyze minor isotopes embedded along with others of proximate mass having much greater abundance. This system is undergoing extensive testing and evaluation, with the objective of learning about effects in the ensuing chemistry of sulfur compounds that give rise to mass independent ratios.

Recent experiments conducted in our laboratory have shown that heavier isotopes of sulfur can undergo different temporal dynamics in a dissociative process than the light isotopes. Dissociative processes of SO2, photo-excited through intermediate states with conical intersections have been found to display unexpected behavior on isotope mass. Photo-ionization studies of SO2 and SO2 clusters also have been performed to assess what effects ionic state dissociation has on the relative ratios of the various sulfur isotopes. Covariance mapping is being employed to identify the dissociative paths and sequential order of the reaction pathways being observed. These experiments give us reactant/product information that allows for comparison of individual species in the mass spectra to all other species present. Dynamics experiments are also performed as a complement to the photo-ionization investigations. These experiments allow us to track reactants and products on a femtosecond time scale, thus addressing the temporal behavior of the reactions in comparison to the respective isotopes.