Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

TL;DR

This study challenges previous claims by showing that hydrogenation does not protect polycyclic aromatic hydrocarbons from fragmentation; instead, it increases the likelihood of backbone fragmentation during collisions.

Contribution

The paper provides experimental evidence that hydrogenation does not shield PAHs from fragmentation, contradicting earlier findings based on soft X-ray absorption studies.

Findings

Hydrogenation increases backbone fragmentation cross section.

Experimental results oppose previous protection hypothesis.

Fragmentation likelihood rises with hydrogenation degree.

Abstract

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, 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$_{16}$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.