The decomposition of hydrogenated amorphous carbon: A connection with polycyclic aromatic hydrocarbon molecules

We have produced thin solid films of hydrogenated amorphous carbon (HAC) by vapor deposition in vacuum and have investigated the structure and infrared spectra of these materials under conditions in which they are at the point of decomposition. We find that the IR absorption spectrum of HAC under these conditions contains all the major spectral components seen in emission in nebulae. In addition, high-resolution scanning tunneling microscopy of HAC prior to decomposition reveals protographitic islands with dimensions similar to 1-5 nm showing that polycyclic aromatic hydrocarbon (PAH)-like molecular groups evolve from HAC. These experimental results suggest for the first time how PAHs and HAC may be related and how PAH molecules could evolve from HAC in regions of high excitation. The material produced as HAC decomposes is a very low density carbonaceous ''aerogel'' consisting primarily of these aromatic protographitic clusters in a weakly connected friable network. These protographitic clusters may be the source of interstellar graphite grains.