Astrochemical relevance of VUV ionization of large PAH cations

Abstract

Context. As part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by an interaction with vacuum ultraviolet (VUV) photons emitted by hot stars. This interaction leads to the emission of not only the well-known aromatic infrared bands, but also electrons, which can significantly contribute to the heating of the interstellar gas. Aims. Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs. Methods. Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9–20 eV from the DESIRS beamline at the synchrotron SOLEIL. All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy. Results. Photoionization is found to be predominant over dissociation at all energies, which differs from the conclusions of an earlier study on smaller PAHs. The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV, in agreement with the evolution of the ionization potential with size. Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections, which were calculated using time-dependent density functional theory. An analytical function was derived to calculate this yield for a given molecular size. Conclusions. Large PAH cations could be efficiently ionized in H I regions and contribute to the heating of the gas by the photoelectric effect. Also, at the border of or in H II regions, PAHs could be exposed to photons of energy higher than 13.6 eV. Our work provides recipes to be used in astronomical models to quantify these points.

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

“…Atom positions were kept fixed during the time evolution, so that the resulting spectra represent vertical excitations, neglecting vibronic structure. This combination of simulation box size, grid spacing, and exchange-correlation functional was previously shown to provide good numerical convergence (Wenzel et al 2020) and an overall acceptable agreement with available spectra of PAHs (Malloci et al 2004).…”

“…Subsequently, we used the OCTOPUS implementation (Tancogne-Dejean et al 2020) of TD-DFT in real time and real space (Yabana & Bertsch 1996) to evaluate the complete electronic photoabsorption spectrum of each species. These calculations, following calibrations performed in previous works (Malloci et al 2004(Malloci et al , 2007Wenzel et al 2020), were carried out with a simulation box defined by the union of spheres centered on each atom of the given molecule, each with a 8 Å radius. OCTOPUS represents all physical quantities such as Kohn-Sham wavefunctions, electron density, etc., in a discrete grid in the simulation box, whose spacing was chosen as 0.18 Å.…”

The excited-state dynamics of the radical anions of cyanoanthracenes

“…[1][2][3][4][5][6][7] In astrochemistry, fluorescence from radical cations of aromatic hydrocarbons has been suggested to be at the origin of emissive diffuse interstellar bands. [8][9][10][11] In photoredox chemistry, the observation that bimolecular electron transfer (ET) between two neutral reactants is diffusion controlled even at driving forces at which Marcus inverted region is predicted, was explained by the formation of the ion-pair product in an electronic excited state. [12][13][14][15] Despite the relevance of photoexcited open-shell ions, not much is known about their dynamics.…”

Charge-Transfer-Induced Predissociation in Rydberg States of Molecular Cations: MgAr+

“… 5 − 9 Now that photoionization studies of molecular cations are becoming feasible at synchrotron and free-electron-laser facilities, the prospects of systematic studies of Rydberg states of molecular cations are excellent. 10 − 12 Rydberg electrons are primarily located outside the ionic core, so that, in first approximation, Rydberg states have potential-energy functions that closely resemble those of the ion-core electronic states to which the Rydberg series converge. 13 − 17 In the case of molecular cations M + = F 1 F 2 + , the ionic core (M 2+ ) is doubly charged and prone to dissociation into two charged atomic or molecular fragments F 1 + + F 2 + on a repulsive Coulomb potential (see Figure 1 a).…”