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Relaxation processes induced by VUV photoexcitation in astrophysical media – PRIMA


The relaxation of molecular species following photoexcitation in the vacuum ultraviolet is very complex and constitutes a key step in the photochemical networks of astrophysical environments. Experimental data recorded in the laboratory concerning this relaxation are essential to understand the evolution of these environments.

Studies of the different relaxation channels of molecules following absorption of vacuum ultra-violet radiations applied to astrophysics

The physicochemical evolution of many astrophysical media is influenced by the interaction of dilute phase matter with local radiation in the vacuum ultraviolet (VUV) region. Excitation of these species in this spectral domain is at the origin of complex relaxation processes such as ionization, fragmentation, isomerization or ion pair formation. The PRIMA research project aimed at providing experimental data to the astrophysics and planetology communities in order to feed their photochemical models. The target species were gas-phase molecules or radicals, neutral or ionic, containing carbon and nitrogen. To fulfill this objective, two complementary approaches have been undertaken in parallel: (i) experiments with synchrotron radiation to study absorption and photoionization (DESIRS beamline of the SOLEIL synchrotron,) and (ii) laser experiments with the completion of an experimental set-up to study photodissociation and photoionization at the ”Institut des Sciences Moléculaires d’Orsay”.

Photoelectron spectroscopy and mass spectrometry are very sensitive techniques that allow to study in detail the ionization processes and in some cases the photodissociation. During this project these techniques were used either coupled with synchrotron radiation or with a VUV laser. This double approach allows to benefit from the advantages of each source, namely the tunability for the synchrotron and the high spectral resolution for the laser.
The main results of this project concern the ionization processes of stable or radical species which could be studied with State-of-the-art threshold photoelectron methods (« TPES », for Threshold PhotoElectron Spectroscopy, on the DESIRS beamline of the SOLEIL synchrotron, or « PFI-ZEKE PES », for Pulsed-Field-Ionization ZEro Kinetic Energy PhotoElectron Spectroscopy, with the VUV laser of ISMO).
Absorption experiments with the Fourier transform spectrometer in the vacuum ultraviolet of the SOLEIL synchrotron were also carried out within the framework of this project in order to study the photoexcitation step of a few systems.

The major results of this project are divided into two parts: studies at the SOLEIL synchrotron and studies at ISMO.
At the synchrotron, the most striking facts are the multiple studies of photoionization of radical species that have led to numerous publications (18 articles in total).
For the laser experiment part, which has benefited from almost all the financial support allocated by the ANR, the major result is the development and finalization of a state-of-the-art experimental set-up called VULCAIM («VUv Laser for Considering Astrophysical and Isolated Molecules«) dedicated to laser spectroscopy in the VUV domain, which has just given rise to a first publication. This setup is equipped with a photon source unique in France: a VUV laser tunable between 6 and 17 eV, with high spectral resolution (< 10 µeV). Only two other lasers of this type currently exist in Europe.
Finally, this project has largely contributed to the research training of a PhD student, Oliver J. Harper (thesis co-supervised by B. Gans and S. Boyé-Péronne and defended in July 2020) who obtained the 2020 thesis prize of the Graduate School of Physics of the University Paris-Saclay (Pôle Physique des Ondes et de la Matière).

The 4-year ANR PRIMA project has allowed to finalize a unique laser experiment in France optimized for spectroscopy in the vacuum ultraviolet range and also to consolidate a collaboration between the ISMO (S. Boyé-Péronne and B. Gans), the DESIRS line of the SOLEIL synchrotron (G.A. Garcia) and the ISM of Bordeaux (J.-C. Loison). This collaboration will be extended thanks to an ANR project ZEPHIRS started in January 2022, carried by ISMO (by S. Boyé-Péronne), in partnership with SOLEIL and ISM. This project will allow to optimize even more the complementarity between the laser experiment of ISMO (VULCAIM) and the synchrotron experiment of the DESIRS line (SAPHIRS). This last project in the continuity of the ANR PRIMA could lead to new collaborations. Indeed, a large part of the users of the SAPHIRS experiment could benefit from the complementarity of the VULCAIM and SAPHIRS setups and could therefore perform experiments at the ISMO through collaborations for better resolved measurements.

The most important outcome of this project is the laser experiment whose first paper has just been accepted in PCCP, and selected for coverage and in the «2021 PCCP HOT Articles«:
- O. J. Harper, N. L. Chen. S. Boyé-Péronne, and B. Gans, Phys. Chem. Chem. Phys., (2021), doi : 10.1039/D1CP04569E

The synchrotron part of the project allowed the publication of 18 papers. 2 of them used funds allocated to this project
- B. Gans, N. Lamarre, J.-C. Guillemin, S. Douin, C. Alcaraz, C. Romanzin, G. A. Garcia, J. Liévin, and S. Boyé-Péronne, J. Phys. Chem., 150(24) pp. 244304, 2019, doi : 10.1063/1.5097688
- B. Gans, S. Boyé-Péronne, and J. Liévin, J. Phys. Chem., 150(24) pp. 244303, 2019, doi : 10.1063/1.5097691

The PRIMA project is devoted to the experimental characterization of relaxation processes resulting from the absorption of Vacuum Ultra-Violet (VUV) radiations by gaseous molecules present in astrophysical media such as the interstellar medium, planetary atmospheres and comets. The proposed experiments in this project will be performed at Institut des Sciences Moléculaires d’Orsay (ISMO) and on the DESIRS beamline of SOLEIL synchrotron. These experimental facilities will provide us with the required tools to carry out the complete overview studies of the targeted molecular and radical species. A special attention will be paid to the characterization of carbon- and nitrogen-bearing radicals for which only scarce information are available. Emphasis will be given to (azo)-carbonated molecules and to ammonia (NH3) in order to retrieve new and reliable data for the astrophysics/astrochemistry community. Quantitative data such as photodissociation branching ratios and absolute photoionization or photoabsorption cross sections will be measured for the first time. These results and the spectral analysis will be supported by theory in collaboration with theoreticians from ISMO and ULB (Belgium).
The financial support requested in this application will allow us to build the first pulsed high-resolution and broadly-tunable VUV laser source in France. This system will be the master piece of the experimental setup needed for the planned absolute measurements described above. The complementarity of this new VUV source with the ones already available in the “plateau de Saclay” environment (DESIRS beamline of SOLEIL, CLUPS laser center) will strengthen the position of Paris-Saclay University as a world leader in the field of molecular VUV spectroscopy and photodynamics.

Project coordination

Bérenger Gans (Institut des Sciences Moléculaires d'Orsay)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


ISMO Institut des Sciences Moléculaires d'Orsay

Help of the ANR 291,978 euros
Beginning and duration of the scientific project: - 48 Months

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