Oxygen isotope fractionation in the vacuum ultraviolet photodissociation of carbon monoxide: Wavelength, pressure, and temperature dependency

摘要

Several absorption bands exist in the vacuum ultraviolet (VUV) region of carbon monoxide (CO). Emission spectra indicate that these bands are all predissociative. Experimental results of CO photodissociation by vacuum ultraviolet photons (90 to 108 nm; ～13 to 11 eV) from the Advanced Light Source synchrotron by measurement of the oxygen isotopic composition of the products are presented here. A large (few hundred per mil) range of oxygen isotopic compositions are observed in the CO photodissociation product and are wavelength dependent. Slope values (δ ′~(17)Oδ ′~(18)O) ranging from 0.72 to 1.36 were observed in the oxygen three-isotope space (δ ′~(18)O vs. δ ′~(17)O), which anti-correlated with increasing synchrotron photon energy, and indicates a dependency on the upper electronic state specific dissociation dynamics (e.g., perturbation and coupling associated with a particular state). An unprecedented magnitude in isotope fractionation was observed for photodissociation at 105 and 107 nm and is found to be associated with accidental predissociation of the vibrational states (ν 0 and 1) for the upper electronic state E ~1Π. A significant temperature dependency in oxygen isotopic fractionation was observed, indicating a rotational level dependency in the predissociation process.