Small molecules from the decomposition of interstellar carbons

We have studied the molecular products of the photo-induced decomposition of hydrogenated amorphous carbon (HAC) and solid hexane, C6H14, using mass spectroscopy. Mass spectra of HAC are dominated by simple hydrocarbon molecules having fewer than four carbon atoms. Notable products include C3H2, phenyl, C6H5, benzene, C6H6, and a variety of partially dehydrogenated alkane molecules with the composition C n H2n-1. Hexane, chosen as a representative solid alkane, has a more complex mass spectrum which includes C n and a number of hydrocarbon molecules with up to 10 carbon atoms. As alkyl radicals, C n H2n-1, are commonly found in the decomposition of alkanes, we have used high precision density functional theory to simulate the infrared spectrum of 1-, 2-, and 3-hexyl radicals as well as that of the 3-hexyl ion, C6H13 +. The latter could be detectable in interstellar/circumstellar sources via a strong feature at 3.66 μm. The appearance of C3H2 as a decomposition product of photodissociated HAC may be related to the ubiquitous presence of c-C3H2 in the interstellar medium. The production of such molecules in the interstellar medium through a ‘top-down’ chemistry deriving from the decomposition of HAC is discussed.