Polycyclic aromatic hydrocarbons and fullerenes as decomposition products of hydrogenated amorphous carbon

The gaseous products evolved from solid hydrogenated amorphous carbon (HAC) under UV irradiation have been sampled using time of flight (TOF) mass spectrometry. A notable feature is the appearance of an extended sequence of aromatic carbon clusters containing in excess of 30 carbon atoms and including fullerenes such as C-50, C-60, and C-70. Under certain conditions, clusters containing up to several hundred C atoms are detected. The radiation-induced decomposition of HAC is found to occur in a series of stages in which small (<20 atom) molecules including alkanes and unsaturated carbon chain (C=C)(n)H-m radicals with m = 1-4 are the first to be released. There is also evidence in these mass spectra for the ejection of small dehydrogenated polycyclic aromatic hydrocarbon (PAH) molecules. After these lighter molecules have been released, we observe a strong mass peak near 500 amu corresponding to clusters with similar to 40 C atoms. These clusters, which are protographitic aromatic structures, have been detected in IR spectra of HAC and show absorption and emission features at 3.3, 3.4, and 6.2 mu m and other wavelengths characteristic of PAH molecules. These data show that the decomposition of HAC in interstellar shocks may be a source of large PAH and fullerene molecules.