Gas-phase photolysis as a source of mass-independent fractionation

Photolysis of gas-phase molecules can yield MIF signatures via several mechanisms. Here we discuss these mechanisms with a focus on oxygen isotopes in CO and sulfur isotopes in SO2, with application to the solar nebula and early Earth atmosphere, respectively. We consider three MIF mechanisms: 1) self-shielding in a line-type absorption spectrum by the most abundant isotopologue; 2) isotopic variations in the magnitude of the absorption features (e.g., due to the Franck-Condon envelope, Tokue & Nanbu 2010); 3) isotopic variations in the dissociation probability, which reflect the likelihood of curve-crossing from a bound excited state to a dissociating state. All three of these effects are accounted for in the equation for the photodissociation rate coefficient for each isotopologue (e.g., Lyons 2007 GRL), but the dominant effect depends on the nature of the absorption spectrum and the extent of dissociation, among other factors. In the astrochemically important range from 91 to 108 nm the absorption spectrum of CO consists of ~ 40 bands, many of which have rotational (line-type) features and a deep continuum (line peak/continuum ~ 102-104). Clayton (2002) proposed that self-shielding by C16O during photolysis of CO isotopologues in the solar nebula produced O with δ17O/δ18O ~ 1.0. Subsequent formation of H2O from that O yielded a water reservoir in the solar nebula that formed the 16O-poor end-member of the CAI mixing line. Experiments by Chakraborty et al. (2008) have shown that CO photolysis yields O with δ17O/δ18O ~ 0.6 to 1.8, depending on wavelength, which they argue rules out CO self-shielding as the origin of the CAI mixing line. However, in their experiments only ~ 1 % or less of the CO in the photocell was dissociated before being replaced by fresh CO from the gas bottle. Chemical kinetics simulations of their experiments by one of us (JRL) have yielded O with δ17O/δ18O ~ 0.6 to 2.0 for ~ 1% CO dissociated, and δ17O/δ18O ~ 0.9-1.1 for > 10% CO dissociated. The latter results hold even when (artificially) large isotopic variations in band intensity and dissociation probability are assumed. These results support the CO self-shielding scenario. In the atmospherically important range from 190-220 nm the spectrum of SO2 consists of a progression of vibrational bands densely packed with rotational features. The continuum is shallow compared to CO, with peak/continuum ~ 2-5. Self-shielding by 32SO2 occurs, but isotopic variations in the continuum absorption will also contribute to sulfur MIF effects. Low-resolution spectral measurements of xSO2 isotopologues by Danielache et al. (2008) do not resolve lines, but should capture isotopic variations in the continua. Higher resolution spectra obtained with the Imperial College FTS (Blackie et al., this meeting) capture both lines and continuum. However, we have not yet been able to reconcile the two sets of spectral data. Progress on spectral data comparison, simulation of the SO2 photolysis experiments of Pen and Clayton (2008), and implications for early Earth sulfur MIF will be reported.