Deexcitation Dynamics of Superhydrogenated Polycyclic Aromatic Hydrocarbon Cations after Soft-x-Ray Absorption

We have investigated the response of superhydrogenated gas-phase coronene cations upon soft x-ray absorption. Carbon $(1\mathrm{s})⟶{\pi }^{\star }$ transitions were resonantly excited at $h\nu =285\mathrm{eV}$. The resulting core hole is then filled in an Auger decay process, with the excess energy being released in the form of an Auger electron. Predominantly highly excited dications are thus formed, which cool down by hydrogen emission. In superhydrogenated systems, the additional H atoms act as a buffer, quenching loss of native H atoms and molecular fragmentation. Dissociation and transition state energies for several H loss channels were computed by means of density functional theory. Using these energies as input into an Arrhenius-type cascade model, very good agreement with the experimental data is found. The results have important implications for the survival of polyaromatic hydrocarbons in the interstellar medium and reflect key aspects of graphene hydrogenation.

Figure 1

(a) A schematic representation of the relevant processes: photoionization (left) and photoexcitation (right). In both cases the inner shell vacancy gets filled by an auger decay. (b) NEXAMS scans of the dication (blue) and trication (red) yield at $m=292–300\mathrm{amu}$ after soft x-ray absorption of coronene cations, normalized to initial trap content, photon flux, and irradiation time. As the target is cationic, photoionization (red) mainly leads to tricationic and photoexcitation (blue) to dicationic products.

Figure 2

Mass spectra of (superhydrogenated) coronene after irradiation with 285 eV photons (black). The mass spectra in red represent the initial hydrogenation state distribution of the target cations. The cyan spectra show the corresponding model fragmentation spectra. Details about the model and the labels, $m$ and $n$, are given in the text. The model spectra are normalized to the $M=300$ peak in the black fragmentation spectrum. The red reference spectra are not normalized.

Figure 3

Overview of dissociation energies of H atoms, relative to the total energy of coronene $+4\mathrm{H}$. The levels in purple are the dissociation energies of single H atoms and the blue levels are the barriers for emission of ${\mathrm{H}}_2$. The displayed molecular structures of $m=2$ (top) and $n=3$ (bottom) clearly show breaking of planar symmetry after superhydrogenation.