(a) Carbon Monoxide: In collaboration with Prof. Jim Lyons at the School of Earth and Space Exploration, Arizona State University and with Dr. Alan Heays at the University of Leiden, I am studying the extreme ultraviolet photoabsorption spectrum of CO and its isotopologues.The origin and significance of unusual oxygen isotope ratios discovered in early condensates from the solar nebula have been an intriguing puzzle for the past 30 years. It has been recently proposed that isotope-selective photodissociation of CO in the solar nebula or parent molecular cloud is responsible for the oxygen isotope signatures. This theory places constraints on the formation conditions of the solar nebula, and is amenable to testing with accurate photodissociation calculations for CO isotopologues. To evaluate the CO photodissociation theory we are measuring high-resolution photoabsorption cross sections of CO and its isotopologues in the 91.2 to 111.8 nm region. The results of these measurements will be incorporated into radiative transfer calculations of the solar nebula to quantify oxygen isotope fractionation via CO photodissociation. The measurements are being carried out on the DESIRS beamline at SOLEIL. This work is supported by the NASA Origins of Solar Systems program.
(b) Sulfur Dioxide Isotopologues: In collaboration with Prof. Jim Lyons, Dr. Doug Blackie and Dr. Juliet Pickering at Imperial College, London, and researchers at the DESIRS beamline at SOLEIL, I am measuring, at high resolution, photoabsorption cross sections of several isotopologues of sulfur dioxide: 32SO2, 33SO2, 34SO2, and 36SO2. The cross sections will be used in models of the early earth atmosphere to test the hypothesis that "mass independent fractionation" of sulfur deposits in early rock samples is a marker of the rise of oxygen in the earth's atmosphere. This work is supported by the NASA Exobiology program.
(c) Carbon Dioxide: The photodissociation of CO2 is a fundamental photochemical process in the atmospheres of Mars and Venus. My research, carried out on the DESIRS beamline at SOLEIL centers on the measurement of high resolution cross sections from 87 to 200 nm. In collaboration with Prof. Jim Lyons, I am currently focusing on isotopic variations in these cross sections. This work is supported by the NASA Planetary Atmospheres program.
(d)Diatomic Sulfur: Interpretations of atmospheric (Io, Jupiter) S2 absorption features are hindered by a complete lack of laboratory cross sections in the ultraviolet. We have begun to quantify the photoabsorption spectrum of S2 from 200 to 300 nm based on laboratory measurements and theoretical calculations. Absorption measurements were recently completed at NIST, Gaithersburg, MD, in collaboration with Dr. Gillian Nave. Coupled-channel calculations, with Prof. Brenton Lewis and Dr. Stephen Gibson at the Australian National University, are underway to analyze the predissociation seen in S2. This work is supported by the NASA Planetary Atmospheres program.
(e) Molecular Nitrogen: I am currently collaborating with Dr. Charles Malone at the Jet Propulsion Laboratory, Cal Tech, to measure band f-values of the Lyman-Birge-Hopfield (LBH) bands in the vacuum ultraviolet. This band system is a prominent airglow feature in earth and other nitrogen-rich atmospheres. Measurements are currently being carried out on the DESIRS beamline of the SOLEIL synchrotron facility in St. Aubin, France.This work is supported by the NASA Planetary Atmospheres program.