Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

Abstract

A recent study of soft x-ray absorption in native and hydrogenated coronene cations, ${\mathrm{C}}_{24}{{\mathrm{H}}_{12+m}}^{+}\phantom{\rule{4pt}{0ex}}m=0--7$, 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 (${\mathrm{C}}_{16}{{\mathrm{H}}_{10+m}}^{+}, m=0$, 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.