4) Innovative Methodology and Instrumentation Development

The spectroscopy of a molecule serves as a preliminary step to consider the study of spin conversion of that molecule, particularly to enable real-time measurements of the population of different isomers. We focused our efforts on the ammonia molecule (NH₃). This study proved to be very rich due to the presence of a fairly intense band within the range of our laser source (1470-1547 nm), the identification of which is partial (20%), and its complete analysis presents a challenge for spectroscopy. This area corresponds to an energy zone where about twenty combinations of interacting vibrational bands coexist (only 4 have been identified). To broaden our research theme, we took advantage of the possibility to record these spectra at variable temperatures (up to the disappearance of the gas phase) to provide valuable information for the identification of transitions.

The study of the evolution of the relative intensity of two transitions as a function of temperature allows us to access the energy difference of the initial states of these transitions. By using some already identified transitions, we can almost exhaustively obtain the initial state (J,K) of each transition, thus determining whether it is ortho or para. An article has been published for a limited energy range (40 cm⁻¹), and a publication covering a broader range (180 cm⁻¹) is in preparation. The data were recorded over the entire range of our sources (430 cm⁻¹).

An international community exists around this study, including experimental and theoretical teams. From an experimental standpoint, most teams use Fourier transform spectroscopy. Our team has expertise in laser spectroscopy. The former has the advantage of simultaneously recording a significant spectral range, but the latter offers superior resolution, especially at low temperatures where the Doppler effect is weaker. These low-temperature studies are made possible by a cooled Herriot-type cell, which allows access to spectra at all temperatures (122-296 K).

From a theoretical perspective, teams are attempting to reproduce ab initio the observed spectra by improving potential surfaces. The assignment of transitions relies on knowledge of their position, intensity, and, when available, the initial state of the transition. Given the precision of these calculations, which, although continually improving, remain far from experimental precision, this additional information can prove crucial when the spectrum is dense.

Laser spectroscopy is very labor-intensive but also highly effective, and we have focused our efforts on this information. In a similar spirit to the analysis techniques used by other international groups (the MARVEL program or band analysis), we coupled these techniques with 3D spectral recognition, where spectra are compared in a space (Energy, intensity, initial state energy). New assignments are still possible, supported by this three-dimensional representation. The collected data allow for the improvement of the potential surface and the proposal of new calculations that we compare again with the experimental results. This improvement also benefits the recognition of spectra of the isotopologue 15NH3​, for which we have also recorded all the spectra at various temperatures. Recently, we initiated the development of a CRDS experiment to access transitions of lower intensity. We have also begun studying the spectroscopy of deuterated substitutions of ammonia (NH₂D, ND₂H, and ND₃). For these isotopologues, due to the doubled mass of deuterium, the bands are generally shifted toward the red. A new piece of equipment we have just acquired (a 2-micron laser system, 5000 cm⁻¹) should enable fruitful studies.

Furthermore, work is being conducted on the methane molecule in an attempt to observe nuclear spin conversion in low-temperature spectra under saturated vapor pressure conditions (presence of the solid phase) up to the observation limit. The analysis of the absorption of transitions allows us to measure the saturated vapor pressure up to 40 K. The change in this pressure as a function of temperature yields the enthalpy of sublimation of methane over the range of our measurements (40-77 K). Two orders of magnitude are thus crossed in the verification of Clapeyron's law.

This research activity focuses on the modeling and determination of the electronic structure (potential energy curves, transition moments, static molecular polarizabilities, couplings) of neutral and ionic diatomic molecules for cold and astrophysical environments and comprises two themes.

1. The first theme, conducted in collaboration with national and international groups of experimentalists and theorists (M. Aubert-Frécon & A.J. Ross (LASIM, Lyon), A.M. Lyyra (Temple University, Philadelphia), Li Li (Tsinghua University, China), V.N. Sovkov (U. St. Petersburg, Russia)), concerns the spectroscopy of highly excited states of alkali dimers dissociating to and above the doubly excited np+np asymptote. Based on the programs we have developed, we determine the potential energy curves of numerous molecular states (100 states) over a wide range of internuclear distances (4a₀-100a₀), as well as transition moments and static molecular polarizabilities. Feasibility studies for experiments are then conducted. During the period from 2008 to 2013, these studies were carried out on the first excited states of several alkali dimers, and we obtained very satisfactory results for the Li₂ and Cs₂ molecules. In the case of the K₂ molecule, we recalculated the potential energy curves taking into account spin-orbit coupling for all states dissociating to 4p₃/₂+4p₃/₂, in order to interpret the energy transfer reaction between two potassium atoms K(4s)+K(8s) -> K(4s)+K(4f) (in collaboration with the experimental group of M. Glodz, Polish Academy of Sciences). Unfortunately, we were unable to describe the asymptotes 4s+8s and 4s+4f under spin-orbit coupling, but the effective cross-sections could be determined from our initial predictions within a semi-classical molecular dynamics framework. A satisfactory agreement was obtained between theory and experiment (σ_theoretical = 1.12 × 10⁻¹⁴ cm² and σ_experiment = 1.4 × 10⁻¹⁴ cm² at 450 K). In parallel to the dimers, studies are also conducted on alkali molecular ions using pseudopotential or model potential methods now including spin-orbit coupling. In collaboration with M. Lyyra, the study of the Rb₂ molecule has begun, with the goal of investigating not only the spectroscopy of highly excited states but also proposing new scenarios for forming cold molecules. In this context, we chose to use the model potential method and introduce effects related to electronic and nuclear spin in order to observe nuclear spin conversion in Rb₂.

2. The second theme concerns the description of alkali-rare gas mixtures and hydrogen compounds.

In parallel, as part of the development of an international collaboration with Lebanon (F. Taher), the molecules of Lutetium (LuBr, LuI, and LuO) are currently being studied using classical quantum chemistry methods (MOLPRO program) with the aim of creating a database useful for the astrophysics community.

1) Laboratory Astrophysics

2) Environmental Sciences

3) Metrology of molecules of defense interest