Hydrogenated pyrene: Statistical single-carbon loss below the knockout
  threshold
  An ongoing discussion revolves around the question of what effect
hydrogenation has on carbon backbone fragmentation in Polycyclic Aromatic
Hydrocarbons (PAHs). In order to shed more light on this issue, we have
measured absolute single carbon loss cross sections in collisions between
native or hydrogenated pyrene cations (C$_{16}$H$_{10+m}^{+}, m$ = 0, 6, 16)
and He as functions of center-of-mass energies all the way down to 20 eV.
Classical Molecular Dynamics (MD) simulations give further insight into energy
transfer processes and also yield $m$-dependent threshold energies for prompt
(femtoseconds) carbon knockout. Such fast, non-statistical fragmentation
processes dominate CH$_x$-loss for native pyrene ($m=0$), while much slower
statistical fragmentation processes contribute significantly to single-carbon
loss for the hydrogenated molecules ($m=6$ and $m=16$). The latter is shown by
measurements of large CH$_x$-loss cross sections far below the MD knockout
thresholds for C$_{16}$H$_{16}^{+}$ and C$_{16}$H$_{26}^{+}$.
