Hydrogenation of PAH cations: a first step towards H2 formation
  Molecular hydrogen is the most abundant molecule in the universe. A large
fraction of H2 forms by association of hydrogen atoms adsorbed on polycyclic
aromatic hydrocarbons (PAHs), where formation rates depend crucially on the H
sticking probability. We have experimentally studied PAH hydrogenation by
exposing coronene cations, confined in a radiofrequency ion trap, to gas phase
atomic hydrogen. A systematic increase of the number of H atoms adsorbed on the
coronene with the time of exposure is observed. Odd coronene hydrogenation
states dominate the mass spectrum up to 11 H atoms attached. This indicates the
presence of a barrier preventing H attachment to these molecular systems. For
the second and fourth hydrogenation, barrier heights of 72 +- 6 meV and 40 +-
10 meV, respectively are found which is in good agreement with theoretical
predictions for the hydrogenation of neutral PAHs. Our experiments however
prove that the barrier does not vanish for higher hydrogenation states. These
results imply that PAH cations, as their neutral counterparts, exist in highly
hydrogenated forms in the interstellar medium. Due to this catalytic activity,
PAH cations and neutrals seem to contribute similarly to the formation of H2.
