Structure-dependent photostability of polycyclic aromatic hydrocarbon cations:: Laboratory studies and astrophysical implications

Following a previous experimental study of the photostability of a series of polycyclic aromatic hydrocarbons (PAHs) this work extends to irregularly shaped PAHs, methyl-substituted PAHs, and related compounds. A photostability index R has been devised, which measures the propensity for H-loss of PAH monocations relative to that for regular catacondensed or pericondensed PAHs containing the same number of atoms. This index is also valid, to a good approximation, for neutral PAHs. Structural effects (presence of a methyl group; dihydro groups; H-H steric hindrance; degree of pi conjugation; isomers) and size-dependent effects on R are demonstrated and discussed. Astrophysical applications of the propensity of PAH photodissociation include an estimation of the maximum amount of energy that can be deposited in the nascent parent PAH ion in H I regions, and an evaluation of the lower limit to the size of photostable PAHs in these regions, for various types of PAHs and their derivatives. The results provide a possible explanation for the existence of a limited number of PAHs in the ISM as required by the interstellar carbon budget determined by Snow & Witt. The information obtained is also relevant to interpretation of the presence and formation of PAHs and their methyl derivatives in meteoritic, micrometeoritic, cometary, and interplanetary particle materials.