  The physical and chemical conditions in photodissociation regions (PDRs) are
largely determined by the influence of far ultraviolet radiation. Far-UV
photons can efficiently dissociate molecular hydrogen, a process that must be
balanced at the HI/H2 interface of the PDR. Given that reactions involving
hydrogen atoms in the gas phase are highly inefficient under interstellar
conditions, H2 formation models mostly rely on catalytic reactions on the
surface of dust grains. Additionally, molecular hydrogen formation in
polycyclic aromatic hydrocarbons (PAHs) through the Eley-Rideal mechanism has
been considered as well, although it has been found to have low efficiency in
PDR fronts. In a previous work, we have described the possibility of efficient
H2 release from medium to large sized PAHs upon photodissociation, with the
exact branching between H-/H2-loss reactions being molecule dependent. Here we
investigate the astrophysical relevance of this process, by using a model for
the photofragmentation of PAHs under interstellar conditions. We focus on three
PAHs cations (coronene, ovalene and circumcoronene), which represent three
possibilities in the branching of atomic and molecular hydrogen losses. We find
that, for ovalene (H2-loss dominated) the rate coefficient for H2 formation
reaches values of the same order as H2 formation in dust grains. This result
suggests that this hitherto disregarded mechanism can account, at least partly,
for the high level of molecular hydrogen formation in dense PDRs.
