SIZE EFFECTS ON DISSOCIATION RATES OF POLYCYCLIC AROMATIC HYDROCARBONCATIONS - LABORATORY STUDIES AND ASTROPHYSICAL IMPLICATIONS

The photostability of a series of polycyclic aromatic hydrocarbons (PA Hs) has been studied experimentally by determining the internal energy E(int) of their monocations at which the dissociation rate is 10(4) s -1. The results on the hydrogen atom loss reaction, fitted to an RRK m odel calculation, were then scaled to determine the internal energy E( crit) at the astrophysically critical dissociation rate 10(2) s-1. Dat a were also obtained on H-2 and C2H2 loss channels. The quasi-linear d ependence of E(int) and E(crit) on three PAH size parameters is demons trated and modeled. The results indicate that in H I regions, photoexc ited regular PAHs containing less than 30-40 carbon atoms will dissoci ate rather than relax by infrared emission, whereas for N(C) greater-t han-or-equal-to 30-40, and for analogous PAH photoions of any size, th e principal relaxation channel will be infrared emission. Some other i mplications are discussed concerning the photophysics and photochemist ry of PAHs in the interstellar medium.