Photoelectron imaging of interstellar medium anions

2007

An extensive program has been developed to characterize the physical and chemical properties of carbon molecules and ions in space and to describe how they influence the radiation and energy balance. We review recent progress in the experimental and observational studies of an important class of carbon materials (PAHs) in the diffuse interstellar medium and discuss global astrophysical implications and future directions.

Monthly Notices of the Royal Astronomical Society, 2017

Interstellar polycyclic aromatic hydrocarbon (PAH) molecules exist in diverse forms depending on the local physical environment. Formation of ionized PAHs (anions and cations) is favourable in the extreme conditions of the interstellar medium (ISM). Besides in their pure form, PAHs are also likely to exist in substituted forms; for example, PAHs with functional groups, dehydrogenated PAHs etc. A dehydrogenated PAH molecule might subsequently form fullerenes in the ISM as a result of ongoing chemical processes. This work presents a density functional theory (DFT) calculation on dehydrogenated PAH anions to explore the infrared emission spectra of these molecules and discuss any possible contribution towards observed IR features in the ISM. The results suggest that dehydrogenated PAH anions might be significantly contributing to the 3.3 µm region. Spectroscopic features unique to dehydrogenated PAH anions are highlighted that may be used for their possible identification in the ISM. A comparison has also been made to see the size effect on spectra of these PAHs.

Journal of Physics: Conference Series

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, C 48 H 20 + , and compare its behavior under VUV processing with that of smaller species. We suggest that C2 H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Al...

Astronomy & Astrophysics, 2002

Polycyclic Aromatic Hydrocarbons (PAHs) are thought to be the carriers of the ubiquitous infrared emission bands (UIBs). Data from the Infrared Space Observatory (ISO) have provided new insights into the size distribution and the structure of interstellar PAH molecules pointing to a trend towards larger-size PAHs. The mid-infrared spectra of galactic and extragalactic sources have also indicated the presence of 5-ring structures and PAH structures with attached side groups. This paper reports for the first time the laboratory measurement of the UV-Vis-NIR absorption spectra of a representative set of large PAHs that have also been selected for a long duration exposure experiment on the International Space Station ISS. PAHs with sizes up to 600 amu, including 5-ring species and PAHs containing heteroatoms, have been synthesized and their spectra measured using matrix isolation spectroscopy. The spectra of the neutral species and the associated cations and anions measured in this work are also compared to astronomical spectra of Diffuse Interstellar Bands (DIBs).

Astronomy & Astrophysics, 2003

The so-called ``strong'' Polycyclic Aromatic Hydrocarbons (PAHs) hypothesis postulates that isolated PAHs, which are thought to be the carriers of the Unidentified Infrared Bands, ought to be also responsible for a large number of Diffuse Interstellar Bands (DIBs). In this framework, the spectral profile of such DIBs should be due to unresolved rotational structure of vibronic absorption bands, the rotation

Astronomy & Astrophysics, 2020

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 fo...

Faraday Discussions, 2006

Laboratory measurements of the recombination of polycyclic aromatic hydrocarbon (PAH) ions with electrons are presented. Experimental data have been obtained at room temperature for azulene (C 10 H 8) and acenaphthene (C 12 H 10) cations by the Flowing Afterglow with PhotoIons method. The results confirm that the recombination of PAH ions is fast although well below the geometrical limit. The set of our recent and present measurements reveal a definite trend of increasing rate with the number of carbon atoms of the PAH. This behaviour that needs further characterization is potentially of great interest for charge state models as recombination is a dominant mechanism of PAH ion destruction in the interstellar medium. The design of experiments to measure the recombination of larger PAHs and their temperature dependence is discussed.

The Astrophysical Journal, 2015

Interstellar Polycyclic Aromatic Hydrocarbons (PAH) are expected to be strongly processed by Vacuum UltraViolet (VUV) photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C 24 H 12), ovalene (C 32 H 14) and hexa-peri-hexabenzocoronene (HBC; C 42 H 18) cations by exposure to synchrotron radiation in the range of 8 − 40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. However, as the size increases, ionization becomes more and more important and for the HBC cation, ionization dominates. These results are discussed and it is concluded that, for large PAHs, fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible. This implies that PAHs are even more photo-stable than previously thought. The implications of this experimental study for the photo-chemical evolution of PAHs in the interstellar medium (ISM) are briefly discussed.

Journal of Chemical Physics, 2018

The discovery of interstellar anions has been a milestone in astrochemistry. In the search for new interstellar anions, CP and C 2 P are viable candidates since their corresponding neutrals have already been detected astronomically. However, scarce data exist for these negatively charged species. Here we report the electron affinities of CP and C 2 P along with the vibrational frequencies of their anions using high-resolution photoelectron imaging. These results along with previous spectroscopic data of the neutral species are used further to benchmark very accurate quartic force field quantum chemical methods that are applied to CP, CP , C 2 P, and two electronic states of C 2 P. The predicted electron affinities, vibrational frequencies, and rotational constants are in excellent agreement with the experimental data. The electron affinities of CP (2.8508 ± 0.0007 eV) and C 2 P (2.6328 ± 0.0006 eV) are measured accurately and found to be quite high, suggesting that the CP and C 2 P anions are thermodynamically stable and possibly observable. The current study suggests that the combination of high-resolution photoelectron imaging and quantum chemistry can be used to determine accurate molecular constants for exotic radical species of astronomical interest.