Cavity-enhanced optical frequency comb spectroscopy for astrophysics – CECoSA

Uniform supersonic flows allow these kinetic studies to be carried out because they allow the effective cooling of gas samples to temperatures ranging from 13 to 150 K. However, probing the molecular processes taking place in these flows requires a rapid, quantitative detection technique. , sensitive and multispecies. The project aims to combine for the first time optical combs in the mid-infrared, optical cavities, supersonic flows and ultra-fast Fourier transform spectroscopy, which will make it possible to follow the evolution of chemical reactions with the sensitivity and the speed required.

This young researcher project started in March 2020 with an empty laboratory room, which we began to install at the end of the first containment due to the COVID-19 epidemic. The laboratory has been completely renovated, the original optical table has been replaced by a state-of-the-art table. The equipment was then ordered and the centerpiece, the femtosecond laser, arrived at the laboratory in March 2021. Scientific development could then begin and comprises five stages: 1 - development of a time-resolved Fourier transform spectrometer, 2 - conversion of the frequency comb to 3µm thanks to an optical parametric oscillator, 3 - installation of an optical cavity slaved in length in a supersonic flow chamber, 4 - diagnosis of a radical-molecule reaction at room temperature, 5 - diagnosis of a low temperature reaction (in a supersonic flow).
The first semester following the arrival of the laser was dedicated to setting up the first stage. Romain Dubroeucq, doctoral student paid by a doctoral contract from Rennes 1 University, joined the group in October 2020, to start the project. This work led to the development of a new technique, allowing decay time spectroscopy to be performed using a frequency comb source. A leading group had proposed a solution that pursued this goal, but failed to achieve sufficient sensitivity to make the approach viable [M.J. Thorpe, et al., Science, 311 (5767), 1595 (2006)]. The first results obtained with this approach are being formatted for submission to Optics Letters (impact factor = 3.589), for the first publication of the project.

The ANR CECoSA project made it possible to establish several new collaborations with the group of Fabien Goulay at the University of West Virginia (obtaining a Jefferson scholarship from the French Embassy in the United States) as well as the group of Guillaume Genoud at the VTT institute in Helsinki (Maupertuis program of the French Embassy in Finland). Last but not least, the CECoSA project enabled Rennes to host the 2021 Molecular Spectroscopy Days. This French-speaking congress mainly brings together French groups but also Swiss and Belgian groups in Rennes to discuss new developments underway at the IPR, in particular those based on frequency combs.

The next stages of the project will focus on the development of the uniform supersonic flow spectrometer and the development of the frequency comb source emitting in the mid infrared. To achieve these many objectives, Solène Perot will begin her thesis at IPR on October 1, 2021, paid for by the CECoSA project.

International conferences:
1. Poster presentation at the CLEO-EQEC 2021 conference (online, 5000 attendees)
2. Oral presentation at the ‘Sensing and optical sensors 2021’ conference (OSA, online, 500 attendees)
3. Poster presentation at the European Optical Society Annual Meeting 2021 (500 attendees, Rome, Italy)

National conference:
1. Poster presentation at the Journées de Spectroscopie Moléculaire 2021, Rennes
2. Poster presentation at the conference Optique 2021 (Société française d’optique), Dijon

The CECoSA project aims at employing for the first time optical frequency comb spectroscopy in the mid-infrared spectral range in combination with the CRESU technique (Reaction Kinetics in Uniform Supersonic Flows) to achieve universal time-resolved monitoring of low temperature reactions involving free radicals species.
Free radicals play an important role in astrochemistry due to their highly reactive nature. They are the cornerstone of the formation of complex species such as polycyclic aromatic hydrocarbons (PAH) and complex organic molecules (COM), whose presence in the interstellar medium has been observed and the process of formation still under investigation. Understanding the kinetics and pathways of the fundamental processes involving free radicals is critical for modelling the physics and chemistry of planetary atmospheres and interstellar clouds. It requires extensive laboratory studies at the temperature prevailing in extraterrestrial atmospheres such as that of Titan (70-150 K) or interstellar clouds (10-100 K). In particular, reactions between radical species are likely to play an important role at low temperature as the radical interactions can give rise to multiple potential energy surfaces (PESs) correlating adiabatically with the reactants. In order to quantify the various accessible exit channels of such reactions, simultaneous monitoring of the different product formation yields is necessary.
The CRESU technique, combined with laser photochemical methods, is the technique of choice to carry out low temperature kinetic investigations as it allows studying gas samples at temperature ranging from 13 to 150 K. Yet probing the molecular processes occurring in the flow requires a fast, quantitative, sensitive and multispecies detection technique. Using optical frequency comb spectroscopy as the diagnostic tool in CRESU offers all these advantages in a single measurement, opening up for monitoring the reagent consumption rate, the formation rate of the different product sets, and the potential formation of intermediate complexes.
The CECoSA project will tackle an ambitious experimental development, combining for the first time mid-infrared optical frequency combs with cavity enhancement and fast Fourier transform spectroscopy, which will make possible monitoring reactions with the necessary sensitivity and velocity. This instrument will be the first comb based apparatus operating in a uniform supersonic flow for low temperature kinetics. It will be employed to answer key scientific questions in the field of astrochemistry via the monitoring of reactions involving radicals. It will specifically target radical-radical reactions, which are a long standing experimental challenge to monitor and analyze. The PI’s expertise lies in the design of state-of-the-art spectrometer based on optical frequency combs. She pioneered two approaches with complementary characteristics: the Vernier spectrometer in continuous regime during her PhD, which is robust and compact, and the Fourier transform spectrometer with sub-nominal resolution during her postdoctoral research, which breaks the resolution limit of mechanical interferometers thanks to the comb coherence properties. She has the necessary expertise to build the optical system and perform successfully the experiment. She will benefit from a strong support from the group of Laboratory Astrophysics of the Institute of Physics of Rennes, internationally renowned for CRESU studies, concerning both the experimental implementation and results analysis, which further ensures a successful outcome of the project. The CECoSA project will also benefit from and complement the diagnostic apparatuses already under development within the group aiming at detecting the reaction products, namely the CRESUSOL and CRESUCHIRP projects, creating a high synergy within the Laboratory Astrophysics group and the QUADMARTS international research network led by the Rennes group.