Soft-x-ray spectroscopy of coronene plus and (coronene plus H) plus cations: The influence of hydrogenation on electronic structure and photofragmentation

We have studied soft-x-ray absorption in coronene and in singly hydrogenated coronene cations. Hydrogenation on the one hand closes the singly occupied molecular orbital of the coronene radical cation and thus decreases the reactivity of the molecule. On the other hand, the additional H atom breaks the coronene D6h symmetry and exhibits a comparably small binding energy. The effect of single hydrogenation on soft-x-ray photoionization is therefore hard to predict. In our experiments, the mass selected polycyclic aromatic hydrocarbons' cations were stored in a cryogenic radio-frequency ion trap and exposed to monochromatic x rays with energies around the carbon K edge. The partial photoion yields were recorded as a function of photon energy by means of time-of-flight mass spectrometry. The experimental data was compared to time-dependent density-functional theory calculations based on a short-range corrected functional. The combination of experiment and theory allowed for precise assignment of 1s excitations to the resonances in the spectra. We found that these resonant transitions as well as direct 1s ionization into the continuum could induce nondissociative ionization, H/H2 loss, as well as fragmentation of the carbon backbone. For multiple ionization, neutral hydrogen loss was found to compete with fission reactions where H+ or H2+ is released. H attachment was found to substantially decrease backbone fragmentation.