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 extende d sequence of aromatic carbon clusters containing in excess of 30 carb on 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 o ccur in a series of stages in which small (<20 atom) molecules includi ng 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 r eleased, we observe a strong mass peak near 500 amu corresponding to c lusters with similar to 40 C atoms. These clusters, which are protogra phitic aromatic structures, have been detected in IR spectra of HAC an d 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 tha t the decomposition of HAC in interstellar shocks may be a source of l arge PAH and fullerene molecules.