Hydrogenated pyrene: Statistical single-carbon loss below the knockout threshold

TL;DR

This study investigates how hydrogenation affects carbon loss in pyrene cations during collisions, revealing that hydrogenated molecules exhibit significant non-statistical fragmentation at energies below traditional knockout thresholds.

Contribution

It provides the first detailed measurements of single-carbon loss cross sections in native and hydrogenated pyrene, combined with MD simulations to understand energy transfer and threshold energies.

Findings

Hydrogenation alters fragmentation pathways in pyrene.

Non-statistical carbon knockout occurs below classical thresholds.

Hydrogenated pyrene shows large CHx-loss cross sections at low energies.

Abstract

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$_{26}^{+}$.