Oxygen Isotopic Fractionation in Vacuum Ultraviolet Photodissociation of Co: Lack of Self-shielding and Relevance to the Early Solar System

Introduction: Isotopically selective photodissociation of CO at vacuum ultraviolet (VUV) wavelengths has been invoked as a photochemical process in interstellar molecular clouds to explain the observed abundance variation of minor isotopologues of CO [1-2], a process known as isotopic self-shielding (SS). Isotopic SS of CO within the nebular disk has also been advocated as the source of isotopically anomalous oxygen in the solar reservoir and has been suggested as a mechanism for production of meteoritic oxygen isotopic compositions [3-6]. Assumptions in the SS Models: The SS models [1, 6 and references therein] have assumed the following: (i) the quantum yield of dissociation is unity (e.g., each photon absorption after light shielding leads to immediate dissociation), (ii) the oscillator strengths and predissociation probability for different isotopomers are same as those for CO [1] (e.g., no isotope selectivity). Consequently, the isotope selectivity derives from only differential photon absorption and line saturation of the major isotopologue. This is incorporated into the various models through a parameter termed ‘shielding function’, which primarily depends on column densities of CO and H2 [1, 6]. This readily yields a slope of unity in three-isotope oxygen space for the product atomic oxygen. Recently, spectroscopic studies show that the oscillator strengths and predissociation rates for different isotopologues of CO is different for a particular absorption band [7] and the photodissociation yield arises from the combination of: (i) band oscillator strength (f-value), and (ii) predissociation rate. Therefore, the basic assumptions of the SS models are invalid. Experimental Data Analysis: Recently we published new oxygen isotope data from the VUV photodissociation of CO through a series of CO photodissociation experiments at the Advanced Light Source synchrotron [8], the only experimental tests of SS in CO. Four different synchrotron bands (s-bands) centered at 107.61, 105.17, 97.02, and 94.12 nm were used to photolyze CO of various column densities in different sets of experiment, and chosen such that CO is optically thick (σcl = τ > 1) and the minor isotopologues are optically thin (τ << 1). Results show that the product atomic oxygen is highly enriched and lies over regression line of various slope values in three-isotope oxygen space as shown in Figure 1. Figure 1. Oxygen isotopic composition of atomic oxygen produced by VUV photodissociation CO [8].