Infrared spectroscopy of nano-sized carbon grains produced by laser pyrolysis of acetylene: Analog materials for interstellar grains

The infrared spectroscopic behavior of nano-sized carbon grains produced by laser-driven pyrolysis of acetylene (C2H2) is presented with respect to th e internal structure of the particles investigated by electron energy loss spectroscopy and high-resolution transmission electron microscopy. Carbon g rain samples were synthesized at different condensation conditions, and the effect of the pyrolysis temperature and pyrolysis mode (pulsed versus cont inuous wave) on the carbon structure was investigated. The size distributio n of the carbon clusters synthesized in the flow reactor was determined by means of time-of-flight mass spectroscopy. Despite the CH absorption featur es attributed to saturated aliphatic hydrocarbons adsorbed by the grain sur faces, the infrared spectra of the neat carbon grains show only weak CH fea tures. The investigations show that the formation and growth of polycyclic aromatic structural units are involved in the carbon grain condensation pro cess. We conclude that the low feature-to-continuum ratio in the IR spectra of the grains is a typical property of carbonaceous dust formed by the pyr olysis of acetylene. The lack of observational evidence for hydrocarbon dus t in the outflows of carbon-rich asymptotic giant branch (AGB) stars can be rationalized by our spectroscopic results. An evolutionary sequence of the circumstellar carbonaceous material during the AGE to planetary nebula (PN ) phase transition can be deduced from our results by comparison with the I R spectral behavior of carbonaceous grain materials synthesized in other co ndensation systems.