Superhydrogenated polycyclic aromatic hydrocarbon (PAH) molecules have been demonstrated to act as catalysts for molecular hydrogen formation under interstellar conditions. Here we present combined thermal desorption mass spectrometry measurements and density functional theory calculations that reveal the most stable configurations in the superhydrogenation sequence of the PAH molecule coronene (C 24 H 12 ). Specifically, the experiments demonstrate the presence of stable configurations of superhydrogenated coronene at specific hydrogenation levels of 2, 10, 14, 18, and 24 extra hydrogen atoms. Density functional theory calculations of binding energies and barrier heights explain why these configurations are particularly stable and provide new insights into the superhydrogenation process of PAH molecules under interstellar conditions. Furthermore, an experimental cross-section for the first hydrogen atom addition to the neutral coronene molecule of σ add = 2.7 +2.7 −0.9 × 10 −2 Å 2 is derived from the experimental hydrogenation data.

The reactions of low energy (<100 eV) electrons with organometallic precursors underpin the fabrication of metalcontaining nanostructures using focused electron beam-induced deposition. To understand these reactions at a molecular level, we have studied the electron-induced reactions of Ru(CO) 4 I 2 in three different environments: as isolated molecules in the gas phase, adsorbed as thin films on surfaces, and as used in electron beaminduced deposition (EBID) in an Auger spectrometer. Gas-phase studies show that dissociative electron attachment (DEA) to Ru(CO) 4 I 2 predominantly results in the loss of two CO ligands, while dissociative ionization (DI) of Ru(CO) 4 I 2 leads to significantly more extensive fragmentation. Surface science studies of thin films of Ru(CO) 4 I 2 adsorbed on gold at −100 °C and irradiated with 500 eV electrons show that decomposition proceeds in two distinct steps: (1) an initial loss of two CO ligands, followed by (2) a slower step, where the residual two CO ligands desorb, leaving RuI 2 on the surface. EBID using Ru(CO) 4 I 2 and its brominated analogue, Ru(CO) 4 Br 2 , produced deposits with a ruthenium-to-halide ratio of ≈1:2 and minimal carbon and oxygen contamination. These results suggest that DEA is dominant over DI in the initial deposition step on the surface. This step produces a partially decarbonylated Ru(CO) 2 I 2 species, which is then subject to CO desorption under further electron irradiation, findings likely generalizable to other Ru(CO) 4 X 2 species (X = halide). The desorption of CO from the partially decarbonylated intermediate differs markedly from the results obtained for other metal carbonyls (e.g., W(CO) 6 ), a difference tentatively ascribed to the presence of M− X bonds.

Context. The mid-infrared emission features commonly attributed to polycyclic aromatic hydrocarbons (PAHs) vary in profile and peak position. These profile variations form the basis of their classification: Classes A, B, C reflect profiles with increasing central wavelength while Class D has similar central wavelength as Class B but a similar broad shape as Class C. A well-known empirical relationship exists between the central wavelength of these emission features in circumstellar environments and the effective temperature of their central stars. One posited explanation is that the presence of aliphatic hydrocarbons contributes to the variations in the shapes and positions of the features. Aims. We aim to test this hypothesis by characterising the aliphatic emission bands at 6.9 and 7.25 µm and identifying relationships between these aliphatic bands and the aromatic features. Methods. We have examined 5–12 µm spectra of 63 astronomical sources exhibiting hydrocarbon emission which have been observed by ISO/SWS, Spitzer/IRS, and SOFIA/FORCAST. We measured the intensities and central wavelengths of the relevant features and classified the objects based on their 7–9 µm emission complex. We examined correlations between the intensities and central wavelengths of the features, both aliphatic and aromatic, and investigated the behaviour of the aliphatic features based on the object type and hydrocarbon emission class. Results. The presence of the 6.9 and 7.25 µm aliphatic bands depends on (aromatic) profile class, with aliphatic features detected in all Class D sources, 26% of the Class B sources, and no Class C sources. The peak position of the aliphatic features varies, with more variability seen in Class B sources than Class D sources, mimicking the degree of variability of the aromatic features in these classes. Variations are observed within Class D 6–9 µm profiles, but are significantly smaller than those in Class B. While a linear combination of Classes B and C emission can reproduce the Class D emission features at 6.2 and 7.7–8.6 µm, it cannot reproduce the aliphatic bands or the 11–14 µm hydrocarbon features. A correlation is found between the intensities of the two aliphatic bands at 6.9 and 7.25 µm, and between these aliphatic features and the 11.2 µm feature, indicating that conditions required for a population of neutral hydrocarbon particles are favourable for the presence of aliphatic material. A comparison with experimental data suggests a different assignment for the aliphatic 6.9 µm band in Class D and (some) Class B environments. Finally, we discuss evolutionary scenarios between the different classes.