Laser-induced fragmentation of coronene cations.

Polycyclic aromatic hydrocarbons are an important component of the interstellar medium of galaxies and photochemistry plays a key role in the evolution of these species in space. Here, we explore the photofragmentation behaviour of the coronene cation (C24H12˙+) using time-of-flight mass spectrometry. The experiments show photodissociation fragmentation channels including the formation of bare carbon clusters (Cn˙+) and hydrocarbon chains (CnHx+). The mass spectrum of coronene is dominated by peaks from C11˙+ and C7H+. Density functional theory was used to calculate relative energies, potential dissociation pathways, and possible structures for relevant species. We identify 6-6 → 5-7 ring isomerisation as a key step in the formation of both the bare carbon clusters and the hydrocarbon chains observed in this study. We present the dissociation mechanism outlined here as a potential formation route for C60 and other astrochemically relevant species.

Evolution of the ionisation energy with the stepwise growth of chiral clusters of [4]helicene.

Polycyclic aromatic hydrocarbons (PAHs) are widely established as ubiquitous in the interstellar medium (ISM), but considering their prevalence in harsh vacuum environments, the role of ionisation in the formation of PAH clusters is poorly understood, particularly if a chirality-dependent aggregation route is considered. Here we report on photoelectron spectroscopy experiments on [4]helicene clusters performed with a vacuum ultraviolet synchrotron beamline. Aggregates (up to the heptamer) of [4]helicene, the smallest PAH with helical chirality, were produced and investigated with a combined experimental and theoretical approach using several state-of-the-art quantum-chemical methodologies. The ionisation onsets are extracted for each cluster size from the mass-selected photoelectron spectra and compared with calculations of vertical ionisation energies. We explore the complex aggregation topologies emerging from the multitude of isomers formed through clustering of P and M, the two enantiomers of [4]helicene. The very satisfactory benchmarking between experimental ionisation onsets vs. predicted ionisation energies allows the identification of theoretically predicted potential aggregation motifs and corresponding energetic ordering of chiral clusters. Our structural models suggest that a homochiral aggregation route is energetically favoured over heterochiral arrangements with increasing cluster size, hinting at potential symmetry breaking in PAH cluster formation at the scale of small grains.

Photoionization Cross Section of the NH2 Free Radical in the 11.1-15.7 eV Energy Range.

The NH2 radical is a key component in many astrophysical environments, both in its neutral and cationic forms, being involved in the formation of complex N-bearing species. To gain insight into the photochemical processes into which it operates and to model accurately the ensuing chemical networks, the knowledge of its photoionization efficiency is required, but no quantitative determination has been carried out so far. Combining a flow-tube H-abstraction radical source, a double imaging photoelectron-photoion spectrometer, and a vacuum-ultraviolet synchrotron excitation, the absolute photoionization cross section of the amino radical has been measured in the present work for the first time at two photon energies: σionNH2(12.7 eV) = 7.8 ± 2.2 Mb and σionNH2(13.2 eV) = 7.8 ± 2.0 Mb. These values have been employed to scale the total ion yield previously recorded by Gibson et al. ( J. Chem. Phys. 1985, 83, 4319-4328). The resulting cross section curve spanning the 11.1-15.7 eV energy range will help in refining the current astrophysical models.

Five Birds with One Stone: Photoelectron Photoion Coincidence Unveils Rich Phthalide Pyrolysis Chemistry.

Phthalide pyrolysis has been assumed to be a clean fulvenallene source. We show that this is only true at low temperatures, and the C7H6 isomers 1-, 2-, and 5-ethynylcyclopentadiene are also formed at high pyrolysis temperatures. Photoion mass-selected threshold photoelectron spectra are analyzed with the help of (time-dependent) density functional theory, (TD-)DFT, and equation-of-motion ionization potential coupled cluster, EOM-IP-CCSD, calculations, as well as Franck-Condon simulations of partly overlapping bands, to determine ionization energies. The fulvenallene ionization energy is confirmed at 8.23 ± 0.01 eV, and the ionization energies of 1-, 2 and 5-ethynylcyclopentadiene are newly determined at 8.27 ± 0.01, 8.49 ± 0.01 and 8.76 ± 0.02 eV, respectively. Excited state features in the photoelectron spectrum, in particular the Ã+ 2A' band of 1-ethynylcyclopentadiene, are shown to be practical to isomer-selectively detect species when the ground-state band is congested. At high pyrolysis temperatures, the C7H6 isomers may lose a hydrogen atom and yield the fulvenallenyl radical. Its ionization energy is confirmed at 8.20 ± 0.01 eV. The vibrational fingerprint of the first triplet fulvenallenyl cation state is also revealed and yields an ionization energy of 8.33 ± 0.02 eV. Further triplet cation states are identified and modeled in the 10-11 eV range. A reaction mechanism is proposed based on potential energy surface calculations. Based on a simplified reactor model, we show that the C7H6 isomer distribution is far from thermal equilibrium in the reactor, presumably because irreversible H loss competes efficiently with isomerization.

To see C2: Single-photon ionization of the dicarbon molecule.

The C2 carbon cluster is found in a large variety of environments including flames, electric discharges, and astrophysical media. Due to spin-selection rules, assessing a complete overview of the dense vibronic landscape of the C2 + cation starting from the ground electronic state X Σg+1 of the neutral is not possible, especially since the C2 + ground state is of X+ Σg-4 symmetry. In this work, a flow-tube reactor source is employed to generate the neutral C2 in a mixture of both the lowest singlet X Σg+1 and triplet a 3Πu electronic states. We have investigated the vibronic transitions in the vicinity of the first adiabatic ionization potential via one-photon ionization with vacuum ultraviolet synchrotron radiation coupled with electron/ion double imaging techniques. Using ab initio calculations and Franck-Condon simulations, three electronic transitions are identified and their adiabatic ionization energy is determined Ei(a+ 2Πu←X 1Σg +)=12.440(10) eV, Ei(X+ 4Σg -←a 3Πu)=11.795(10) eV, and Ei(a+2Πu ← a3Πu) = 12.361(10) eV. From the three origin bands, the following energy differences are extracted: ΔE(a - X) = 0.079(10) eV and ΔE(a+ - X+) = 0.567(10) eV. The adiabatic ionization potential corresponding to the forbidden one-photon transition X+ ← X is derived and amounts to 11.873(10) eV, in very good agreement with the most recent measurement by Krechkivska et al. [J. Chem. Phys. 144, 144305 (2016)]. The enthalpy of formation of the doublet ground state C2 + cation in the gas phase is determined at 0 K, ΔfH0(0K)(C2 +(Πu2))=2019.9(10) kJ mol-1. In addition, we report the first experimental ion yield of C2 for which only a simple estimate was used up to now in the photochemistry models of astrophysical media due to the lack of experimental data.

Isotope Effects in the Predissociation of Excited States of N2 + Produced by Photoionization of 14N2 and 15N2 at Energies Between 24.2 and 25.6 eV.

Photoelectron/photoion imaging spectrometry employing dispersed VUV radiation from the SOLEIL synchrotron has been used to study the predissociation of N2 + states located up to 1.3 eV above the ion's first dissociation limit. Branching ratios for unimolecular decay into either N2 + or N+ were obtained by measuring coincidences between threshold electrons and mass-selected product ions, using a supersonic beam of either 14N2 or 15N2 as photoionization target. The results confirm that predissociation of the C 2 Σ u + state of 14N2 + is faster than emission to the electronic ground-state by a factor 10 or more for all vibrational levels v' ≥ 3, while for 15N2 + the two decay modes have comparable probabilities for the levels v' = 3, 4, and 5. In contrast, no significant isotope effect could be observed for the other states of N2 + identified in the photoelectron spectrum. For both 14N2 + and 15N2 + isotopologues all vibrational levels of these other states decay to an extent of at least 95% by predissociation.