The photodissociation of interstellar and cometary CH2

The photodissociation rates of CH2 are calculated in interstellar clouds and in cometary atmospheres using fully ab initio cross sections for the lowest three excited triplet states and estimates of the contributions from higher excited triplet states. Lower limits to the photodissociation rates are obtained taking only the lowest three excited states into account. The best estimates assume a dissociation probability eta = 0.2 for higher excited states. The upper limits to the rates use a dissociation probability of unity for all excited states. In the interstellar medium, the best estimate of the photodissociation rate in the unattenuated interstellar radiation field is 5.8 x 10(-10) s(-1). In comets, a lifetime of 7700-14800 s at 1 AU from the quiet Sun is computed, which differs significantly from previous estimates. The cometary lifetime of CHD is similar to that of CH2, but production of CD is favored by a factor of similar to 2 over C. The photodissociation rate of cometary CH2 is compared with those of the related molecules CH4, CH3 and CH.