Photolysis-induced scrambling of PAHs as a mechanism for deuterium
  storage
  Aim. We investigate the role of PAHs as a sink for deuterium in the
interstellar medium and study UV photolysis as a potential process in the
variations of the deuterium fractionation in the ISM. Methods. The UV
photo-induced fragmentation of various isotopologues of D-enriched, protonated
anthracene and phenanthrene ions was recorded in a FTICR mass spectrometer.
IRMPD spectroscopy using FELIX provided the IR spectra that were compared to
DFT vibrational spectra; reaction barriers and rates were also calculated and
related to the product abundances. Results. The mass spectra for both UV and
IRMPD photolysis show the loss of H from [D-C$_{14}$H$_{10}$]$^+$, whereas
[H-C$_{14}$D$_{10}$]$^+$ shows a strong preference for D loss. Calculations
reveal facile 1,2-H and -D shift reactions, with barriers lower than the energy
supplied by the photo-excitation process. Together with confirmation of the
ground-state structures via the IR spectra, we determined that the photolytic
processes in the 2 PAHs are largely governed by scrambling where the H and the
D atoms relocate between different peripheral C atoms. The $\sim$0.1 eV
difference in zero-point energy between C-H and C-D bonds ultimately leads to
faster H scrambling than D scrambling, and increased H atom loss compared to D.
Conclusion. Scrambling is common in PAH cations under UV radiation. Upon
photoexcitation of deuterium-enriched PAHs, the scrambling results in a higher
probability for the aliphatic D atom to migrate to an aromatic site, protecting
it from elimination. This could lead to increased deuteration as a PAH moves
towards more exposed interstellar environments. Also, large, compact PAHs with
an aliphatic C-HD group on solo sites might be responsible for the majority of
aliphatic C-D stretching bands seen in astronomical spectra.
