Photo-stability of super-hydrogenated PAHs determined by action spectroscopy experiments

TL;DR

This study investigates how super-hydrogenation affects the photo-stability of pyrene cations, revealing that hydrogenation weakens the carbon backbone and influences fragmentation behavior under photon irradiation.

Contribution

The paper provides experimental data on the photon energy required for backbone fragmentation of super-hydrogenated pyrene ions, highlighting the impact of hydrogenation on their stability.

Findings

Super-hydrogenated pyrene requires fewer photons for backbone fragmentation.

Hydrogenation does not offset the weakening of the carbon backbone.

Carbon backbone fragmentation competes with H2 formation in photodissociation regions.

Abstract

We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations C$_{16}$H$_{10+m}^+, m = 0,6, \mathrm{\ or\ } 16$) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions (C$_{16}$H$_{10}^+$) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C$_{16}$H$_{16}^+$) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C$_{16}$H$_{26}^+$) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H$_2$-formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H$_2$-formation at least for small hydrogenated PAHs like pyrene.