Interpretation of interstellar medium observations, in extinction (absorption and scattering) and in emission, relies on a complex modelling of the photophysical properties (i.e. competition between all radiative and non-radiative relaxation channels of the isolated systems) of the interstellar molecules and dust components using laboratory data on analogues. Up to now, analogues of the interstellar species from a few tens to few hundreds carbon atoms have been scarcely explored in the laboratory although they may contain up to several tens of % of the available interstellar carbon and they are suspected to play a crucial role in the physics and chemistry of the interstellar medium. Their properties have been extrapolated from smaller species or nanoparticle analogues. The main reason, above all, is the lack of sources of large molecular systems in the gas phase. In the recent years, albeit the soot nucleation process in sooting flames remains largely unexplained and has challenged researchers for at least the last 40 years [1], it has been shown that the soot nucleation process efficiently generates species of sizes in between the soot molecular precursors (up to about the C₂₄H₁₂ molecular size) and the smallest particles of the soot nuclei distribution (containing about a hundred carbon atoms).
The goal of the project is to tune original flames and efficiently produce such systems of similar size and structures as those suspected in space, from the nucleation zone, and study their photophysics in conditions mimicking those of the interstellar medium. Structural characterisation, electronic absorption, electronic fluorescence and the peculiar recurrent fluorescence (although called Poincaré fluorescence [3]) will then be probed performing online and ex situ experiments completed by theoretical modelling.
In more details, the project will combine several online laser diagnostics of laboratory flames including Raman scattering spectroscopy [2], laser induced fluorescence, laser induced incandescence and resonant multiphoton ionization. These online experiments will be performed on cooled sampled species to drastically reduce the spectral congestion and clean up the flame background (target T=50K, a unique setup), completed by highly sensitive time-of-flight mass spectrometry (TOFMS) using various photoionization (PI) schemes from resonant 2 color to direct VUV processes. Precise identification of the large molecular structures involved in the soot nucleation will thus be accessible, as well as their radiative properties. The recurrent fluorescence will then be at the focus to precisely explore these radiative processes and investigate its probable manifestation in space. It should be noted that recurrent fluorescence has only been observed in the laboratory few times in the last years after being predicted almost 40 years ago. This project should thus allow to progress on the identification of carriers of interstellar spectral features observed in absorption and in emission. In addition, the species and their spectral responses will guide interpretation of the on-going JWST (James Webb Space Telescope) satellite observations from the NIR to FIR range of wavelength.

References :
[1] Martin J.W., Salamanca M. Kraft M. ; Prog. Energ. Comb. Sci. (2022) 88, 100956 ; Soot inception : Carbonaceous nanoparticle formation in flames
[2] K.C. Le, C. Lefumeux, T. Pino, Comb & Flame 236, 111817 (2022), DOI : 10.1016/j.combustflame.2021.111817 , Watching soot inception via online Raman spectroscopy
[3] O. Lacinbala, F. Calvo, C. Dubosq, C. Falvo, P. Parneix, M. Rapacioli, A. Simon, and T. Pino, J. Chem. Phys. (2022), DOI : 10.1063/5.0080494 ; Radiative relaxation in isolated large carbon clusters : vibrational emission versus recurrent fluorescence