Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft x-ray absorption in native and hydrogenated coronene cations, C_24H_12+m^+ m=0–7, led to the conclusion that additional hydrogen atoms protect (interstellar) polycyclic aromatic hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30–200 eV) He atoms and pyrene (C_16H_10+m^+, m=0, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.