Pierre Asselin

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.

N-bearing derivatives of polycyclic aromatic hydrocarbons (PANHs) are considered to be relatively abundant in the interstellar medium. Thanks to their significant permanent dipole moment, several pure rotation spectroscopy studies of PANHs were performed in the microwave and millimeter-wave ranges enabling radio astronomical searches, but were unsuccessful. In contrast, very few studies reported rotationally resolved spectra of the vibrational bands of PANHs in the far-infrared region. In the continuity of a recent high resolution rovibrational study of two-ring centrosymmetric PAHs [S. Chawananon, O. Pirali, M. Goubet and P. Asselin, J. Chem. Phys., 2022, 157, 064301], the present work targets four PAHs with one or two N atoms substituted on one ring, namely quinoline and isoquinoline (C₉H₇N) and quinazoline and quinoxaline (C₈H₆N₂), mainly using jet-cooled infrared laser experiments in both in-plane ring C–H bending and C–C stretching regions. Very distinct spectral behaviors are observed: on one hand, quinoline and quinoxaline exhibit unperturbed a-type band contours leading to derivation of accurate spectroscopic parameters for five vibrationally excited states of quinoline and one excited state of quinoxaline. In the case of isoquinoline and, to a lesser extent, quinazoline, the presence of vibrational perturbations revealed by distorted Q-branches could be systematically correlated with the presence of a closely lying vibrational mode and related to vibrational interactions and coupling effects. The rovibrational analyses of these perturbed states underlined substantial variations with respect to calculated rotational constants leading to experimental inertial defect values far from theoretical predictions. The larger number of vibrational modes perturbed in isoquinoline compared to quinoline is finally discussed on the grounds of density of states in both spectral regions investigated.

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.

Heather Marie Stur’s Saigon at war: South Vietnam and the global sixties tells an important yet largely neglected story: that of South Vietnam generally and Saigon specifically during the Vietnam W...

The Tricontinental Revolution - January 2022

High resolution jet-cooled spectrum and simulation of the Π _e/f ( j = 1, k = 0) ← Σ _f ( j = 1, k = 0) transition of NH ₃ -Kr.

The high resolution far-infrared spectrum of trans-butadiene has been re-investigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm^-1, ν₁₀ and ν₂₄, have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν₂₄ analysis was rather straightforward, ν₁₀ exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν₉+ν₁₂ state. Effective rotational constants have been derived for both the v₁₀ = 1 and v₂₄ = 1 states. Since only one weak, infrared active fundamental band (ν₂₃) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

Precise spectroscopic analysis of polyatomic molecules enables many striking advances in physical chemistry and fundamental physics. We use several new high-resolution spectroscopic devices to improve our understanding of the rotational and rovibrational structure of methyltrioxorhenium (MTO), the achiral parent of a family of large oxorhenium compounds that are ideal candidate species for a planned measurement of parity violation in chiral molecules. Using millimetre-wave and infrared spectroscopy in a pulsed supersonic jet, a cryogenic buffer gas cell, and room temperature absorption cells, we probe the ground state and the Re=O antisymmetric and symmetric stretching excited states of both CH3 187 ReO3 and CH3 185 ReO3 isotopologues in the gas phase with unprecedented precision. By extending the rotational spectra to the 150-300 GHz range, we characterize the ground state rotational and hyperfine structure up to J = 43 and K = 41, resulting in refinements to the rotational, quartic and hyperfine parameters, and the determination of sextic parameters and a centrifugal distortion correction to the quadrupolar hyperfine constant. We obtain rovibrational data for temperatures between 6 and 300 K in the 970-1015 cm-1 range, at resolutions down to 8 MHz and accuracies of 30 MHz. We use these data to determine more precise excited-state rotational, Coriolis and quartic parameters, as well as the ground-state centrifugal distortion parameter D K of the 187 Re isotopologue. We also account for hyperfine structure in the rovibrational transitions and hence determine the upper state rhenium atom quadrupole coupling constant eQq' .