  The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum
ultraviolet (VUV) photons triggers the emission of the well-known aromatic
infrared bands but other mechanisms such as fragmentation can be involved in
this interaction. Fragmentation leads to selection effects favoring specific
sizes and structures. We investigate the impact of aliphatic bonds on the VUV
photo-stability of PAH cations under conditions applicable for
photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene
(C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups,
superhydrogenation) are submitted to VUV (10.5 eV) photons over long timescales
(~1000 s) in the cryogenic PIRENEA setup. The fragmentation cascades are
analyzed with a simple kinetics model; fragmentation pathways, rates and
branching ratios are derived. Aliphatic PAH derivatives are found to have a
higher fragmentation rate and carbon loss compared to regular PAHs. The
fragmentation of PAHs with alkylated sidegroups forms species with peripheral
pentagonal cycles, which can be more stable than the bare PAH cations. This
stability is quantified and the most stable species, for which there is an
effective competition of fragmentation with isomerization and radiative
cooling, are identified. This work supports a scenario in which the evaporation
of nanograins with mixed aliphatic and aromatic composition followed by VUV
photoprocessing results in both the production of the carriers of the 3.4um AIB
by methyl sidegroups and in an abundant source of small hydrocarbons at the
border of PDRs. An additional side effect is the efficient formation of stable
PAHs containing some peripheral pentagonal rings. Our experiments also support
the role of isomerization processes in PAH photofragmentation, including the
H-migration process, which could lead to an additional contribution to the
3.4um AIB.
