  Rapid, efficient deactivation of the photoexcited PAH cations accounts for
their remarkable photostability and have important implications for
astrochemistry, as these cations are the leading candidates for the species
responsible for the diffuse interstellar bands (DIB) observed throughout the
Galaxy.Ultrafast relaxation dynamics for photoexcited PAH cations isolated in
boric acid glass have been studied using femtosecond and picosecond transient
grating spectroscopy. With the exception of perylene+, the recovery kinetics
for the ground doublet (D0) states of these radical cations are biexponential,
containing a fast (< 200 fs) and a slow (3-20 ps) components. No temperature
dependence or isotope effect was observed for the fast component, whereas the
slow component exhibits both the H/D isotope effect (1.1-1.3) and strong
temperature dependence (15 to 300 K). We suggest that the fast component is due
to internal Dn to D0 conversion and the slow component is due to vibrational
energy transfer (VET) from a hot D0 state to the glass matrix.
