Visible photodissociation spectroscopy of PAH cations and derivatives in the PIRENEA experiment

The electronic spectra of gas-phase cationic polycyclic aromatic hydrocarbons (PAHs), trapped in the Fourier Transform Ion Cyclotron Resonance cell of the PIRENEA experiment, have been measured by multiphoton dissociation spectroscopy in the 430-480 nm spectral range using the radiation of a mid-band optical parametric oscillator laser. We present here the spectra recorded for different species of increasing size, namely the pyrene cation (C16H10+), the 1-methylpyrene cation (CH3-C16H9+), the coronene cation (C24H12+), and its dehydrogenated derivative C24H10+. The experimental results are interpreted with the help of time-dependent density functional theory calculations and analysed using spectral information on the same species obtained from matrix isolation spectroscopy data. A kinetic Monte Carlo code has also been used, in the case of pyrene and coronene cations, to estimate the absorption cross-sections of the measured electronic transitions. Gas-phase spectra of highly reactive species such as dehydrogenated PAH cations are reported for the first time

PAH chemistry and IR emission from circumstellar disks

Aims. The chemistry of, and infrared (IR) emission from, polycyclic aromatic hydrocarbons (PAHs) in disks around Herbig Ae/Be and T Tauri stars are investigated. The equilibrium distribution of the PAHs over all accessible charge/hydrogenation states depends on the size and shape of the PAHs and on the physical properties of the star and surrounding disk. Methods. A chemistry model is created to calculate this equilibrium distribution. Destruction of PAHs by ultraviolet (UV) photons, possibly in multi-photon absorption events, is taken into account. The chemistry model is coupled to a radiative transfer code to provide the physical parameters and to combine the PAH emission with the spectral energy distribution (SED) from the star+disk system. Results. Normally hydrogenated PAHs in Herbig Ae/Be disks account for most of the observed PAH emission, with neutral and positively ionized species contributing in roughly equal amounts. Close to the midplane, the PAHs are more strongly hydrogenated and negatively ionized, but these species do not contribute to the overall emission because of the low UV/optical flux deep inside the disk. PAHs of 50 carbon atoms are destroyed out to 100 AU in the disk's surface layer, and the resulting spatial extent of the emission does not agree well with observations. Rather, PAHs of about 100 carbon atoms or more are predicted to cause most of the observed emission. The emission is extended on a scale similar to that of the size of the disk. Furthermore, the emission from T Tauri disks is much weaker and concentrated more towards the central star than that from Herbig Ae/Be disks. Positively ionized PAHs are predicted to be largely absent in T Tauri disks because of the weaker radiation field.Comment: 13 pages, 8 figures, accepted for publication in A&

Plasma structure in a pulsed discharge environment

A pulsed slit discharge nozzle (PDN) has been developed in our laboratory to generate molecular ions in an astrophysically relevant environment. The free cold molecular ions are formed through soft (Penning) ionization of the neutral precursor molecules seeded in a supersonic free jet and are probed with cavity ringdown spectroscopy. An attempt is made to characterize the nature and the structure of the plasma that is generated in these experiments to optimize the yield of formation of ions in the jet. The experimental conditions are characterized by a strong pressure gradient in a short discharge zone. We find that the plasma generated in the PDN source is best characterized as an intense abnormal glow discharge and that its structure is reduced to a negative glow and to dark zones near the electrodes. We have calculated the parameters (length, thickness and cathode voltage fall) that are associated with the Crookes dark space and the negative glow in the plasma. We have also estimated the electron temperature (Te) and density (ne) in the plasma. All these parameters are required to optimize the yield of formation of ions and radicals in the jet expansions, a key requirement in our experiments

Ion Chemistry in Uniform Supersonic Flows

International audienceIon processes are a key driver of chemistry in a wide variety of natural dilute environments such as the ionosphere of Earth, the atmosphere of solar system planets, and satellites and interstellar clouds. They also play a role in low-temperature plasmas that are commonly encountered in industrial settings. Uniform supersonic flows have proved to be pivotal in gaining insights into the kinetics of ionic processes. In more than three decades, a variety of ion-molecule reactions has been investigated with the CRESU (French acronym standing for Kinetics of Reactions with Uniform Supersonic Flows) method. They include bimolecular and termolecular reactions and often depart from predictions made by simple empirical models. Neglected in the 90s for the benefit of the study of less predictable radical-neutral reactions, the exploration of ion-molecule reactive collisions has seen a recent revival. It has been in particular stimulated by the discovery of molecular anions in astrophysical environments for which the formation and destruction processes remain elusive, mostly due to the lack of kinetic and branching ratio data. Despite the growing sophistication of competing methods which include crossed beams or cooled ion traps, uniform supersonic flows continue to be a method of choice, one of the rare techniques to provide rate coefficients obtained under well-controlled thermalized conditions over a wide range of low temperatures. Uniform supersonic flows turn out to be well suited for heavy neutral co-reactants. Some challenges remain and call for new directions of action. In a last section, the most promising routes and the limits of the method are outlined. The extension of the method to the study of dissociative recombination sounds perilous, and state-selective chemistry with ions could only marginally benefit from the CRESU. From a technical point of view, the implementation of isomer-specific detection schemes could, however, greatly expand the scope of ion-molecule reaction studies. In tight connection with planetary sciences, ion-induced nucleation appears to be a topic within reach today using the CRESU technique. Further development of the approach will contribute to validate approximate treatments and push further the knowledge of ion-molecule reactions at low temperatures. © 2022 by World Scientific Publishing Europe Ltd

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