The Decomposition of Hydrogenated Amorphous Carbon: A Connection with Polycyclic Aromatic Hydrocarbon Molecules

Abstract

We have produced thin solid films of hydrogenated amorphous carbon(HAC) by vapor deposition in vacuum and have investigated the structureand infrared spectra of these materials under conditions in whichthey are at the point of decomposition. We find that the IR absorptionspectrum of HAC under these conditions contains all the major spectralcomponents seen in emission in nebulae. In addition, high-resolutionscanning tunneling microscopy of HAC prior to decomposition revealsprotographitic islands with dimensions ~1-5 nm showing that polycyclicaromatic hydrocarbon (PAH)-like molecular groups evolve from HAC.These experimental results suggest for the first time how PAHs andHAC may be related and how PAH molecules could evolve from HAC inregions of high excitation. The material produced as HAC decomposesis a very low density carbonaceous "aerogel" consisting primarilyof these aromatic protographitic clusters in a weakly connected friablenetwork. These protographitic clusters may be the source of interstellargraphite grains.