Photodissociation Rate, Excess Energy, and Kinetic Total Energy Release for the Photolysis of H2O Producing O(1S) by Solar UV Radiation Field

Abstract

Forbidden atomic oxygen lines in emission are ubiquitous for cometary spectra in the visible region, and the oxygen atoms in metastable states causing the forbidden emission lines are considered as a proxy of H2O in coma. However, the photodissociation rate and related quantities for the dissociation reaction producing O(1S) from H2O have never been estimated based on experimental studies. Based on the recent laboratory study of the photodissociation reaction of H2O producing O(1S) by Chang et al., we derived the photodissociation rates of the reactions for both the O(1S) and O(1D) channels, consistent with the green-to-red line ratios observed in comets so far. Furthermore, the total kinetic energies released for the photodissociation products are also consistent with the intrinsic line widths of forbidden atomic oxygen emission lines observed in comets. The photodissociation rates of H2O leading to O(1S) and O(1D) calculated here do not significantly change the previous estimates of CO2/H2O in comets based on the green-to-red line ratios of the comets if we use the photodissociation rates of CO2 (calculated elsewhere) with a correction for the difference of solar UV spectra used for calculating photodissociation rates of H2O and CO2.