Arnaud Cuisset

This study presents a high-resolution rovibrational investigation of 1,4-dioxane in the mid-infrared region, motivated by the need for accurate spectroscopic data in the context of atmospheric monitoring of this emerging pollutant. Jet-cooled spectra were recorded using two complementary setups: a continuous supersonic jet coupled to the Fourier transform spectrometer of the AILES beamline at the SOLEIL synchrotron facility (Jet-AILES) and a quantum cascade laser spectrometer coupled to a pulsed jet (SPIRALES). These measurements were combined with long-path cell spectra at room temperature and supported by anharmonic quantum chemical calculations. Four fundamental vibrational bands (ν34, ν33, ν16, and ν15) were analyzed at high resolution, leading to the assignment of more than 6100 rovibrational transitions. A global fit yielded accurate ground- and excited-state rotational and centrifugal distortion parameters. The results significantly improve the spectroscopic characterization of 1,4-dioxane and provide reliable line lists for future atmospheric detection in the mid-infrared range.

Atmospheric emissions from waste recovery centers are strictly regulated by industrial emission directives which stipulate that they should be continuously monitored. In this work, we demonstrate and evaluate the capacity of terahertz rotational spectroscopy for real-time diagnosis and monitoring of gases of environmental interest on industrial sites. A spectrometer based on the use of a frequency multiplication chain operating at 530–620 GHz has been constructed, enabling the detection of most of the regulated compounds in waste incineration. The detection limits obtained are compatible with regulatory emission limit values. The installation of the instrument at the waste recovery center in Dunkirk demonstrated its ability to perform real-time quantitative multi-component in situ monitoring in an industrial environment. Continuous measurements of SO ₂ and CO over more than 24 h are compared with the measurements performed by two certified infrared instruments. The agreement between the three sets of measurements is excellent, demonstrating that high-resolution terahertz spectroscopy is relevant for real-time in situ monitoring of regulated industrial emissions gases, constituting an interesting alternative to other techniques thanks to a higher degree of selectivity.

A standard measuring gas cell used in absorption spectrometers is a cylinder enclosed by two transparent windows. The Fabry–Perot effects caused by multiple reflections of terahertz waves between these windows produce significant variations in the transmitted radiation intensity. Therefore, the Fabry–Perot effects should be taken into account to correctly measure absorption spectra in Bouguer law-based absorption spectroscopy. One approach to reducing the Fabry–Perot effects is based on inserting an additional external movable window with the standard measuring gas cell. This was proposed and numerically analyzed in our previous work. This paper is aimed at the experimental validation of this method when using amplitude modulation (AM) spectroscopy. Also, a comparison of the efficiency of reducing the Fabry–Perot effects using this method is experimentally compared to frequency modulation spectroscopy. The latter was shown to effectively reduce the Fabry–Perot effects compared to AM spectroscopy with the standard measuring gas cell, and the use of the external movable window was shown to further improve the elimination of Fabry–Perot effects.

Alternative energy sources require the search for innovative materials with promising functionalities. Systems with unusual chemical properties represent an insufficiently explored domain, concealing unexpected features. Using diffraction and Raman spectroscopy over a wide temperature range, supported by first‐principles simulations, a rare phenomenon is unveiled: phase‐dependent chemical interactions between binary components in the NaCl–Ga₂S₃ system. In this unique occurrence, previously intact binary crystalline species transform upon melting into mixed liquid structural isomers, forming bonds with new partners. The chemical combinatorics appears to be fully reversible for stable crystals and liquids. Despite this, rapidly frozen glasses out of thermodynamic equilibrium remain in a metastable isomeric state, offering remarkable properties, particularly a high room‐temperature Na⁺ conductivity, comparable to the best sodium halide superionic conductors and therefore encouraging for sodium solid‐state batteries and energy applications. A rigidity paradigm is responsible for the observed phenomenon, as the extremely constrained Ga₂S₃ crystal lattice does not survive viscous flow, breaking up at a short‐range level. The removal of rigidity constraints and dense packing leads to a significant increase in empty space, which is the origin of high sodium diffusivity. Broadly, the rigidity‐driven structural isomerism opens up an inspiring path to the discovery of atypical materials.

In the continuity of a previous jet-cooled rovibrational study of trans and cis conformers of 2-furfural in the mid-infrared region (700–1750 cm−1) [Chawananon et al., Molecules 28 (10), 4165 (2023)], the present work investigates the far-infrared spectroscopy of 2-furfural using a long path absorption cell coupled to a high-resolution Fourier transform spectrometer and synchrotron radiation at the AILES beamline of the SOLEIL synchrotron. Guided by anharmonic calculations, vibrational energy levels and excited-state rotational constants are sufficiently predictive for a complete assignment of all fundamental and combination bands up to 700 cm−1, as well as the rovibrational analysis of 4 (1) low-frequency modes of trans-(cis-)2-furfural. A global rovibrational simulation, including far-infrared rovibrational lines and microwave and millimeter-wave rotational lines assigned in a previous study [Motiyenko et al., J. Mol. Spectrosc., 244, 9 (2007)] provides a reliable set of ground- and excited-state rotational parameters involving ring torsion, bending, and ring puckering modes of 2-furfural. In a second step, a rovibrational analysis of several hot band sequences, mainly involving the lowest frequency ring CHO torsion mode, is carried out. Reliable values of some anharmonic coefficients are obtained experimentally and could serve as a benchmark for validating advanced anharmonic calculations related to these large amplitude motions of flexible molecules.

A novel high finesse cavity allowing THz CRDS has enabled accurate quantification of CF₄. Yielding spectral parameters with unprecedented precision, these finding are critical to monitor and understand CF₄'s contribution to global radiative forcing.

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200–350 K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200–350 K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42 m and by a THz radiation beam (82.5 GHz to 1.1 THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.