Size effects on dissociation rates of polycyclic aromatic hydrocarbon cations: Laboratory studies and astophysical implications

Abstract

The photostability of a series of polycyclic aromatic hydrocarbons (PAHs) has been studied experimentally by determining the internal energy E<SUB>int</SUB> of their monocations at which the dissociation rate is 10<SUP>4</SUP>/s. The results on the hydrogen atom loss reaction, fitted to an RRK model calculation, were then scaled to determine the internal energy E<SUB>crit</SUB> at the astrophysically critical dissociation rate 10<SUP>2</SUP>/s. Data were also obtained on H2 and C2H2 loss channels. The quasi-linear dependence of E<SUB>int</SUB> and E<SUB>crit</SUB> on three PAH size parameters is demonstrated and modeled. The results indicate that in H I regions, photoexcited regular PAHs containing less than 30-40 carbon atoms will dissociate rather than relax by infrared emission, whereas for N<SUB>c</SUB> greater than or = 30-40, and for analogous PAH photoions of any size, the principal relaxation channel will be infrared emission. Some other implications are discussed concerning the photophysics and photochemistry of PAHs in the interstellar medium.