Gas-phase PAH research for the interstellar medium – GASPARIM

For astronomers, polycyclic aromatic hydrocarbon (PAH) molecules refer to the carriers of a set of emission bands in the mid-infrared (mid-IR) that are observed in many places of our Galaxy where ultraviolet (UV) photons drive the chemistry. Their observation extends now to external galaxies up to redshifts of z>~3. The success of the PAH model in astrophysics lies on the emission mechanism; following the absorption a single UV photon, PAHs could reach high enough temperatures to emit in the mid-IR bands including the 3.3 micron band. A drawback of the PAH model remains up to now the lack of identification of individual species. Since spectroscopy is the main diagnostic tool to probe interstellar matter, this motivates dedicated laboratory studies in the whole spectral range where these species have characteristic signatures. Measurements have to be performed in conditions that mimic those found in interstellar space and involve good laboratory analogues, which implies the simulation of the molecular mechanisms that play a role in the physics and chemistry of the interstellar species. The GASPARIM project has the ambition to address these questions by gathering an interdisciplinary team with strong expertise in this field, both on the experimental and theoretical aspects. We propose to bring new and original data that could be compared directly to astronomical data: IR emission spectra of UV-excited PAHs, far-IR emission signatures of selected PAHs, electronic signatures of new types of PAH species. The mid-IR spectrum brings interesting insights into the nature of the emitting species providing one can solve the effect of anharmonicity. One important challenge for GASPARIM is to design experimental set-ups that will provide IR spectra of hot (UV-excited) PAHs. Another challenge will be to provide a theoretical analysis of vibrational anharmonicity for systems of increasing sizes. Some studies will also be performed in the far-IR in connection with the observations of the Herschel space observatory that explores this spectral range with an unprecedented sensitivity and spectroscopic resolution. Low-frequency vibrational modes are more suitable to identify individual species than mid-IR modes that are specific of local chemical bonds. Electronic transitions are also very good fingerprints of molecular identity and we will search for a possible link between these bands and diffuse interstellar bands (DIBs) that are observed mainly in the visible. In this context, we will take advantage of experimental tools that are developed in our laboratories to record the electronic spectrum of cold species that are difficult to isolate, such as large PAHs and dehydrogenated species. Combined with theoretical calculations, these measurements will allow evaluating the capability of any new family of compounds to present properties that are compatible with the DIBs, in particular in terms of oscillator strength and band profile. Finally, an important aspect to consider is that there is no obvious chemical scenario to rebuild PAH molecules in molecular clouds and this puts drastic constraints on the lifetime of PAH species in the cosmic dust cycle. We will explore the possibility to form PAH clusters and larger PAH systems from non-reactive or reactive collisions. We will study the spectroscopic signatures of PAH clusters using a PEPICO spectrometer at the SOLEIL synchrotron facility. From a theoretical point of view, we will address the question of charge delocalisation in charged PAH clusters.