FORMATION OF POLYCYCLIC AROMATIC HYDROCARBONS AND CARBONACEOUS SOLIDS IN GAS-PHASE CONDENSATION EXPERIMENTS

Abstract

Carbonaceous grains represent a major component of cosmic dust. In order to\nunderstand their formation pathways, they have been prepared in the laboratory\nby gas-phase condensation reactions such as laser pyrolysis and laser ablation.\nOur studies demonstrate that the temperature in the condensation zone\ndetermines the formation pathway of carbonaceous particles. At temperatures\nlower than 1700 K, the condensation by-products are mainly polycyclic aromatic\nhydrocarbons (PAHs), that are also the precursors or building blocks for the\ncondensing soot grains. The low-temperature condensates contain PAH mixtures\nthat are mainly composed of volatile 3-5 ring systems. At condensation\ntemperatures higher than 3500 K, fullerene-like carbon grains and fullerene\ncompounds are formed. Fullerene fragments or complete fullerenes equip the\nnucleating particles. Fullerenes can be identified as soluble components.\nConsequently, condensation products in cool and hot astrophysical environments\nsuch as cool and hot AGB stars or Wolf Rayet stars should be different and\nshould have distinct spectral properties.\n