Volatile organic compounds (VOCs) are trace atmospheric gases, widely present in the Earth's atmosphere. Despite their low concentration, VOCs play a role as primary pollutants and as precursors for secondary organic aerosols (SOA), which have an impact on the climate system and ecosystems. A number of VOCs compounds were observed using the Fourier transform Infrared spectrometers on board of satellites, and among them several internal rotors were observed and more detections are expected in the future. The high resolution spectra of those molecules showing large amplitude motion(s) cannot be treated by traditional Hamiltonian methods. I will first describe the various methods and codes existing to fit the spectra of internal rotors from the microwave through the infrared spectral range. I will show some applications to acetaldehyde and dimethyl sulfide and more recent results on isoprene infrared spectroscopy. Another topic I will cover is the new approaches to investigate the impact of complexes on the Earth atmospheric chemistry. By combining rotational spectroscopy and quantum chemical calculations the structures of water complexes formed between atmospherically relevant molecules can be estimated and validated by experimental data. Recent results and challenges in the spectral analysis of HONO-H₂O and NH₃-H₂O complexes will be shown.This work has been supported by French National program LEFE (Les Enveloppes Fluides et l'Environnement) of CNRS and by the European project “International Network for a Research and Innovation Staff Exchange” (RISE ATMOS), Marie Curie Action.html:<hr /><h3>Footnotes: This work has been supported by French National program LEFE (Les Enveloppes Fluides et l'Environnement) of CNRS and by the European project “International Network for a Research and Innovation Staff Exchange” (RISE ATMOS), Marie Curie Action.

The only “matrix” within which a wide variety of molecules can almost freely rotate is superfluid helium nanodroplets. For relatively small molecular complexes, this can allow for the observation of rotational substructure within their infrared bands, which offers structural insight. Here, we investigated the atmospherically relevant O₃-N₂ and O₃-O₂ van der Waals complexes by quantum cascade laser spectroscopy in the intense asymmetric stretching fundamental of O₃. For O₃-N₂, we observed a slightly blue-shifted perpendicular band with well-defined rotational substructure which is consistent with the approximately T-shaped structure that has been characterized by microwave spectroscopy [1]. For O₃-O₂, we observed a slightly red-shifted band with similar rotational substructure indicating that its structure is also approximately T-shaped. A variety of larger clusters have also been observed and the latest results in terms of their analysis will be presented at the symposium. [1] J. P. Connelly et al., J. Mol. Spec., 199, 205 (2000).

l0pt Figure The ozone molecule, O₃, is a non-linear triatomic molecule famous for its Hartely-Huggins absorption band in the UV region centered at about 5 eV.¹ Increasing the photon energies to above 12 eV leads to the region where the ozone's ionization spectrum begins.² The first band of the ozone's photoionization spectrum is the focus of this work. Within the Born-Oppenheimer separation of electronic and nuclear degrees of freedom, the ozone cation has two low-lying states which are vibronically coupled by the asymmetric stretch. Consequently, assignment of the ozone's photoelectron spectrum requires simulation methods that account for the correlated electron-nuclear motion. In my talk, I will present our simulations which use the Köppel-Domcke-Cederbaum model of the vibronic Hamiltonian.³ We use the model parameters derived from the coupled-cluster methods, where in particular, the diabatic couplings are calculated following the method of Ichino, Gauss, and Stanton.⁴ References: ¹ H. Keller-Rudek, G.K. Moortgat, R. Sander, and R. Sörensen, The MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest. Earth Syst. Sci. Data, 5, 365–373, (2013). ² S. Willitsch, F. Innocenti, J.M. Dyke, and F. Merkt, High-Resolution Pulsed-Field-Ionization Zero-Kinetic-Energy Photoelectron Spectroscopic Study of the Two Lowest Electronic States of the Ozone Cation O ₃ ⁺. J. Chem. Phys. 122, (2005). ³ H. Köppel, W. Domcke, and L.S. Cederbaum, Multimode Molecular Dynamics Beyond the Born-Oppenheimer Approximation. In Advances in Chemical Physics (eds I. Prigogine and S.A. Rice). (1984). ⁴ T. Ichino, J. Gauss, and J.F. Stanton, Quasidiabatic States Described by Coupled-Cluster Theory. J. Chem. Phys. 130, 174105 (2009).

l0pt Figure The formation of aerosols in the atmosphere is responsible for one of the largest uncertainty factors in current climate models. The rate limiting step of aerosol formation is often the initial dimerization and sequential addition of monomer units. For non-ionic species, the cluster formation is primarily stabilized by hydrogen bonding. It is paramount to understand the thermodynamics of these initial small clusters. We use a theoretical/experimental-hybrid approach to accurately determine the partial pressures of the MeOH dimer, trimer and tetramer in the gas phase at standard conditions. With conventional FTIR spectroscopy and precise pressure broadening of the sharp MeOH monomer ro-vib transitions, we can obtain high quality spectra of the MeOH cluster species. The intensity enhancement and redshift that occurs for the OH-stretching transition upon hydrogen bonding aids the separation and quantification of the different clusters. We determine the dimer-, trimer- and tetramer equilibrium partial pressures in the gas mixture by scaling the OH_b-stretching band areas by a calculated oscillator strength. We estimate a MeOH dimer equilibrium constant K_eq=0.03, similar to that of H₂O dimer. The higher order MeOH clusters have similar structures and hydrogen bonding interactions to those of the H₂O clusters, and we therefore propose that the determined trimer- and tetramer equilibrium constants for MeOH may serve as proxies for H₂O clusters.

Methanol is an attractive molecule in astrochemistry and the search for the variation of the proton-electron mass ratio. We have computed 100 vibrational energy levels of methanol using the GENIUSH-Smolyak approachG. Avila and E. Mátyus, J. Chem. Phys. 150, 174107 (2019).; E. Mátyus, A. Martín Santa Daría, and G. Avila, Chem. Commun. 59, 366 (2023). Our computations are based on Qu and Bowman's potential energy surface (PES)C. Qu and J. M. Bowman, Mol. Phys. 111, 1964 (2013). We have defined symmetrized eleven curve-linear normal coordinates for small amplitude motions with respect to an instantaneous reference configuration r0pt Figure changing as a function of the torsional degree of freedom, which is the internal rotation of the CH₃D. Lauvergnat and A. Nauts, Spectrochim. Acta A Mol. Biomol. Spectrosc. 119, 18 (2014). Our vibrational computations have converged more than 80 vibrational energy levels, up to 2000 cm⁻¹ beyond the zero-point energy, within 1.0 cm⁻¹ with respect to the basis set size. The deviation from the experiment is within a few cm⁻¹ for most states where experimental data is availableG. Moruzzi et al., "Microwave, Infrared, and Laser Transitions of Methanol Atlas of Assigned Lines from 0 to 1258 cm-1" We also report an idea to search for the parity violation shit (E_\textPV) utilizing chiral methanol-like molecules, including heavy atoms. E_\textPV has a strong scaling with nuclear charge. Transitions between torsional energy levels could lead to enhancement of PV shifts and long lifetime. The PES and E_\textPV with respect to the internal OH rotation of CHFIOH are shown in the figure. The PV shifts between vibrational levels will also be delineated.

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Footnotes:

G. Avila and E. Mátyus, J. Chem. Phys. 150, 174107 (2019).; E. Mátyus, A. Martín Santa Daría, and G. Avila, Chem. Commun. 59, 366 (2023).. C. Qu and J. M. Bowman, Mol. Phys. 111, 1964 (2013).. D. Lauvergnat and A. Nauts, Spectrochim. Acta A Mol. Biomol. Spectrosc. 119, 18 (2014).. G. Moruzzi et al., "Microwave, Infrared, and Laser Transitions of Methanol Atlas of Assigned Lines from 0 to 1258 cm-1".

Acetone, one of the most important atmospheric ketones, is produced in situ from a number of anthropogenic and natural VOCs. We present preliminary experimental and theoretical results obtained within the frame of the ATMOS H2020 project. Experimental Raman spectroscopy in supersonic gas jets, which is able to probe molecular gas samples with high spatial and spectral resolution, and highly correlated ab initio calculations are employed to characterize the vibrational spectrum of the main isotopologue of acetone. Isotopologues are also studied. The low temperature reaches in the jet causes an important simplification of the molecular spectra, facilitating the assignment of their internal energy levels. Assignments are achieved with the help of theoretical vibrational studies combining second order perturbation theory and a variational procedure of reduced dimensionality for non-rigid species. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 872081.

r0 pt Figure Perfluoroalkanes are potent greenhouse gases due to their infrared properties and long atmospheric lifetimes. Their climate impact can be quantified using climate metrics calculated from infrared absorption cross-sections. While measuring the infrared properties of perfluoro-n-pentane, we found that our experimental cross-sections differed significantly from those available on the HITRAN database (from Bravo et al. 2010 and Sharpe et al. 2004). We investigated this discrepancy using density functional theory. A likely explanation is the low sample purity (95%) of the literature perfluoro-n-pentane samples, compared to the 99% purity for our sample. We also calculated the radiative efficiency (RE) and global warming potential (GWP) using our cross-sections. The RE is 0.44±0.06 W m⁻² ppbv⁻¹ and the 100-year GWP is 9380±1230. These results show good agreement with literature values.

r0pt Figure CF₄ is the most abundant perfluorocarbon in the atmosphere. This greenhouse gas is very stable, has an atmospheric lifetime of 50,000 years, and a warming potential 6,500 times that of CO₂. Over the last 15 years, its atmospheric concentration has increased at a rate of 0.8 ppt/year. The accurate quantification of CF₄ is key to understanding the contribution of its emissions on the radiative forcing budget. In this study, a novel high finesse THz cavity (interaction length  > 1 km) has enabled highly resolved spectra of the weak transitions of CF₄ by Cavity Ring-Down Spectroscopy (CRDS). More than 50 pure rotational ν₃−ν₃ lines have been measured, yielding both position and intensity with unequalled precision. Several tetrahedral splitting are fully resolved and measured with sub-MHz accuracy. CRDS-THz has allowed the absolute intensities, used in the global fit, to be determined. A CF₄ dipolar parameter (&̃#956;_3,3) has been fitted to 106.38(53) mD. This value is in a very good agreement with that of the ab initio-based parameter deduced from a dipole moment surface. For the first time, a set of ab initio effective dipole moment parameters is derived for the computation of the hot bands of the type P_n−P_n (n=0,…,8) and the resulting line list composed of 25,863 transitions can be used to model the whole rotational spectrum. Finally, the TFMeCaSDa database is updated and is available for future spectroscopic and monitoring activities.

Silicon tetrafluoride (SiF₄) is an important gas in semiconductor technology in the electronic and optical industries, as a precursor for the preparation of various silicon-based materials. It is a trace component of fumarole gases on volcanoes and it has also been suggested that it is present on Io, a volcanic moon of Jupiter. Studying the structural and spectroscopic properties of SiF₄ is also interesting in itself, since spectroscopic studies of this molecule in different spectral ranges provides a detailed understanding of its structure (bond lengths and angles) and properties, such as quantum mechanical features, as well as intra- and intermolecular dynamical processes. Therefore, both line positions and ro-vibrational energy values can be of primary importance. New spectra were recorded on the AILES Beamline of the SOLEIL Synchrotron facility. Due to the presence of many hot bands, a low temperature (160 K) was required. Silicon tetrafluoride in a natural isotopic composition produces complex infrared spectra. Not less than three stable isotopologues exist in natural abundance: ²⁸ SiF₄ (92.23 %), ²⁹ SiF₄ (4.67 %) and ³⁰ SiF₄ (3.10 %). Thus, the ν₃+ν₄ fundamental band of SiF₄ could be analyzed for these three isotopologues. In this band we found four sub-bands whose centres are located very close to each other (less than 5 cm⁻¹). This fact, as well as the possible presence of hot bands of weak intensity, complicates the process of assignment of the lines for this band. As a result, for the ν₃+ν₄ band it was possible, based on the assignment of about 2200 lines, to fit more than 30 spectroscopic parameters that allow us to reproduce the experimental spectrum with a standard deviation d_rms = 0.733 ×10⁻³ cm⁻¹. Analyzing ν₁+ν₂ band was difficult due to its weak intensity and overlap of the P-branch with the strong ν₃ fundamental band. However, despite this, it was possible for the first time to analyse this band up to the value J_max = 50 with an accuracy of a few 10⁻⁴ cm⁻¹ and obtain values of the main parameters with an acceptable standard deviation.

We present recent updates in the measurements and analysis of the rotational spectra of ammonia-water complex in the frequency range 50 to 300 GHz using FLASH (Fast Lille Absorption emiSsion High-resolution) spectrometer coupled to a pulsed supersonic jet. The NH₃−H₂O complex is characterized by two large amplitude motions: almost free internal rotation of ammonia owing to very low torsional barrier ( ≈  10 cm⁻¹), and the inversion of water hindered by relatively high barrier ( ≈  700 cm⁻¹). The rotational spectrum of NH₃−H₂O was first analyzed using the rho-axis method HamiltonianE. Herbst, J. Messer, F. C. De Lucia, P. Helminger, J. Mol. Spectrosc. 108, 42 (1984); V. V. Ilyushin, Z. Kisiel, L. Pszczólkowski, H. Mäder, J. T. Hougen, J. Mol. Spectrosc. 259, 1 (2010)o account for the NH₃ torsion. It allowed us to fit the data from previous studiesP. A. Stockman, R. E. Bumgarner, S. Suzuki, & G. A. Blake, J. Chem. Phys. 96, 2496 (1992); G. T. Fraser & R.D. Suenram, J. Chem. Phys. 96, 7287 (1992) and more than 160 transitions measured in this study. Then, the "hybrid" Hamiltonian approachI. Kleiner & J. T. Hougen, J. Mol. Spectrosc. 368, 111255 (2020)as applied for the analysis of both torsional and inversion motions. The inversion splittings were resolved owing to the implementation of special nozzle allowing for sub-Doppler measurements. The analysis is in progress, the latest results will be presented.

This work has been supported by the French PN LEFE and ANR Labex CaPPA through the PIA under Contract No. ANR-11-LABX-0005-01

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Footnotes:

E. Herbst, J. Messer, F. C. De Lucia, P. Helminger, J. Mol. Spectrosc. 108, 42 (1984); V. V. Ilyushin, Z. Kisiel, L. Pszczólkowski, H. Mäder, J. T. Hougen, J. Mol. Spectrosc. 259, 1 (2010)t P. A. Stockman, R. E. Bumgarner, S. Suzuki, & G. A. Blake, J. Chem. Phys. 96, 2496 (1992); G. T. Fraser & R.D. Suenram, J. Chem. Phys. 96, 7287 (1992), I. Kleiner & J. T. Hougen, J. Mol. Spectrosc. 368, 111255 (2020)w