Theoretical studies of photodissociation of small molecules of astrophysical importance
The radicals and ions observed in comets result from photodissociation and photoionization of molecules. According to current models, a comet is composed chiefly of a large, solid nucelus of frozen gases (parent molecules) such as H2O, HCN, and NH3. It is believed comets were formed at the same time and in the same region of space as the major planets and that their chemical composition is the same as that of the early solar system. As the comet nears the Sun, the surface heats up, liberating the frozen gases as well as dust particles. Solar radiation photodissociates the parent molecules into fragments that are observed by resonance fluorescence. Both polyatomic molecules, present in the interstellar medium, and cometary radicals were observed. Using laboratory photo-dissociation data and computer models, astronomers are attempting to identify the parent molecules that account for all observed radicals and ions.