Experimental test of self-shielding in vacuum ultraviolet photodissociation of CO

Self- shielding of carbon monoxide ( CO) within the nebular disk has been proposed as the source of isotopically anomalous oxygen in the solar reservoir and the source of meteoritic oxygen isotopic compositions. A series of CO photodissociation experiments at the Advanced Light Source show that vacuum ultraviolet ( VUV) photodissociation of CO produces large wavelength- dependent isotopic fractionation. An anomalously enriched atomic oxygen reservoir can thus be generated through CO photodissociation without self- shielding. In the presence of optical self- shielding of VUV light, the fractionation associated with CO dissociation dominates over self- shielding. These results indicate the potential role of photochemistry in early solar system formation and may help in the understanding of oxygen isotopic variations in Genesis solar- wind samples.