The dissociation of OH and OD in comets by solar radiation

Abstract

The radiative lifetimes of cometary OH are calculated as a function of the heliocentric velocity of the comet and the velocity distributions of the product atoms are determined. At a distance of 1 AU from the Sun, the lifetimes vary between 1.2×10 5 and 1.9×10 5 sec at solar minimum and between 1.0×10 5 and 1.4×10 5 sec at solar maximum, depending upon velocity. Continuous absorption into the repulsive 1 2Σ - state is major destruction path. The calculated lifetimes are generally consistent with the lifetimes inferred from observations, but suggest some elaboration of the models is necessary. Photodissociation of OH produces a low-velocity component of hydrogen atoms at 8 km sec −1 relative to the parent OH molecule and a high-velocity component between 17 and 27 km sec −1. Photodissociation of OH leads to metastable O( 1D) and O( 1S) and is an additional source of the red and green line emission of atomic oxygen. The lifetime of OD is estimated to be about 4.3× 10 5 sec at solar minimum and 2.6×10 5 sec at solar maximum so that the OD/OH ratio in comets is enhanced relative to the HDO/H 2O production ratio by a factor between 2 and 3. Photodissociation of OD produces only high-velocity D atoms with a mean value of 17 km sec −1.