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Accueil > Équipes scientifiques > Systèmes Moléculaires, Astrophysique et Environnement (SYSTEMAE) > Offres de stages, thèses et post-docs > Quantum-state labelling and chemical discrimination using broadband MW-VUV double resonance absorption spectroscopy

M2 internship

Quantum-state labelling and chemical discrimination using broadband MW-VUV double resonance absorption spectroscopy

M2 or PhD

Absorption spectroscopy is a powerful tool to identify and quantify molecules thanks to the unique fingerprint that possess each species and is used in the entire electromagnetic spectrum domain (from microwave to Vacuum Ultra- Violet (VUV) and even at higher energy). In the VUV ( 60 nm <lambda < 200 nm), increasing molecular complexity or gas mixtures lead to spectral congestions which often prevent the obtention of rotationally resolved VUV spectra and thus unambiguous spectroscopic assignments and chemical discriminations. These difficulties are less problematic in the microwave (MW) domain were higher spectral resolution can be reached.

In the present proposal, we propose a novel approach based on doubly resonant absorption of MW and VUV photons i) to greatly facilitate spectral assignment of rotational levels and ii) to unambiguously discriminate and identify the VUV absorption of a given species in a complex mixture (mixture of various gases, chemical composition of plasma discharges…). Two resonant photons experiment is a well-established technique in the high-resolution community [Park2015], often using laser techniques. The principle is as follow : (i) the MW frequency remains fixed to an intense rotational transition of the vibronic ground state of the molecule and (ii) the VUV frequency is scanned over a range which encompasses the transition of interest. Either the MW or VUV signals can be monitored to detect the effects of doubly resonant absorptions and thus obtained a VUV absorption spectrum involving only the rotational levels involved in the rotational transition.

This technique will allow a drastic simplification of the spectra by limiting the number of lines and, above all, a powerful molecule-specific selectivity. Indeed, as pure rotational lines are very specific to a given molecule, this approach will provide powerful chemical discrimination in the VUV spectrum of a gas mixture, as frequently occurring in plasmas or in various discharges aiming at producing radicals of astrophysical relevance. The MW+VUV doubly resonant absorptions approach will allow to distinguish unambiguously the lines belonging to the molecule of interest. This powerful and original instrumental method will open new perspectives for ultra-high resolution VUV spectroscopy that we would like to explore in the future.

During the internship, a proof of principle experiment will be built. As preliminary tests, a tunable visible or UV laser coupled with a microwave source will be used to get familiar with the doubly resonant technique. The NO molecule will be used as reference for this preliminary experiment as its MW and UV/visible spectroscopy are very well known. Then, the same setup will be coupled with the unique high-resolution VUV laser system recently built at ISMO. In parallel to this work, experiments on the VUV DESIRS beamline of the SOLEIL synchrotron will be performed on the same project (a proposal application has been deposited for beam time during the first semester of 2022). The use of the DESIRS beam line will allow a large tunability of the VUV source.

The first proof of concept will be performed during the internship using already available equipment (collaboration with O. Pirali), and the numerous perspectives we expect can be followed by a PhD thesis.
The candidate will be in trained to state of the art instrumentation in both VUV and MW spectral ranges. The internship is experimental and requires good understanding of molecular quantum physics, optics, and electronics.

Voir en ligne : Astrophysique et édifices moléculaires