Development of a cryogenic source of polyatomic molecules for very high precision mid-infrared spectroscopy

This work aims to extend mid-infrared (MIR) frequency metrology to increasingly complex molecular systems, such as heavy chiral complexes that are promising candidates for measuring the parity-violating energy difference between molecular enantiomers, as well as polyatomic species of atmospheric or astrophysical interest. To this end, we have developed, characterized, and operated a cryogenic molecular source dedicated to high-resolution MIR spectroscopy of complex polyatomic molecules. This source relies on buffer-gas cooling in a cryogenic cell, an efficient method for producing cold gas-phase molecules. Collisions with a cryogenic noble gas at a few kelvin ensure molecular thermalization, reduce velocity spread, simplify congested spectra, and increase population in the lowest rovibrational states—conditions that are essential for precision measurements on large molecules.

The apparatus consists of a vacuum chamber, a two-stage pulse-tube cryocooler, two thermal radiation shields, and a cryogenic cell optimized to guarantee uniform temperature, precise control of gas flows, and suitable optical access. The cell was specifically designed to allow multiple molecular injection configurations, providing significant flexibility for different experimental needs. The thermal behavior of the entire setup has been experimentally characterized. In addition, probabilistic numerical simulations based on Direct Simulation Monte Carlo (DSMC) were developed to describe molecular cooling and diffusion, yielding a quantitative understanding of molecular dynamics and cooling processes. Finally, first absorption signals of methyltrioxorhenium (MTO) obtained in the cryogenic cell with a quantum cascade laser (QCL) frequency-locked to an ultra-stable CO$_2$ laser reference are presented. These results constitute a first step toward high-resolution measurements on complex polyatomic systems relevant to fundamental physics, atmospheric science, and astrophysics.

Jury member:
Claudine CREPIN-GILBERT, Université Paris-Saclay, ISMO, Examinatrice
Hans LIGNIER, Université Paris-Saclay, LAC, Rapporteur
Manuel GOUBET, Université de Lille, PhLAM, Rapporteur
Jean-Hugues FILLION, Sorbonne Université, MONARIS, Examinateur
Gabriel DUTIER, Université Sorbonne Paris Nord, LPL, Examinateur
Anne AMY-KLEIN, Université Sorbonne Paris Nord, LPL, Directrice de thèse
Benoît DARQUIE, CNRS, LPL, Co-encadrant
Mathieu MANCEAU, Université Sorbonne Paris Nord , LPL, Co-encadrant

Keywords: buffer-gas cooling, cryogenic source, DSMC, Mid-infrared spectroscopy, Quantum Cascade Lasers, frequency metrology, chiral molecules, parity violation

Soutenance: December 16, 2025 at 2:00 p.m. Amphi Dorothy Vaughan (amphi B)