Infrared Spectroscopy of Nano‐sized Carbon Grains Produced by Laser Pyrolysis of Acetylene: Analog Materials for Interstellar Grains

Abstract

The infrared spectroscopic behavior of nano-sized carbon grains produced by laser-driven pyrolysis of acetylene (C2H2) is presented with respect to the internal structure of the particles investigated by electron energy loss spectroscopy and high-resolution transmission electron microscopy. Carbon grain samples were synthesized at different condensation conditions, and the effect of the pyrolysis temperature and pyrolysis mode (pulsed versus continuous wave) on the carbon structure was investigated. The size distribution of the carbon clusters synthesized in the flow reactor was determined by means of time-of-flight mass spectroscopy. Despite the CH absorption features attributed to saturated aliphatic hydrocarbons adsorbed by the grain surfaces, the infrared spectra of the neat carbon grains show only weak CH features. The investigations show that the formation and growth of polycyclic aromatic structural units are involved in the carbon grain condensation process. 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 pyrolysis of acetylene. The lack of observational evidence for hydrocarbon dust 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 AGB to planetary nebula (PN) phase transition can be deduced from our results by comparison with the IR spectral behavior of carbonaceous grain materials synthesized in other condensation systems.