Molecular lines observed in high resolution far infrared spectra are associated with pure rotation transitions or belong to low-energy vibrational bands. Together with other parameters characterizing them, the intensities and broadening coefficients of these lines are required for example to analyze spectra of planetary atmospheres or to gain insight into the physics of the studied molecules or intermolecular interactions. Pure rotation line intensities can also be used to determine the particle density of chemically unstable species, allowing to obtain line intensities in another spectral range as was for example done for hypochlorous acidJ. Vander Auwera, J. Kleffmann, J.-M. Flaud, G. Pawelke, H. Bürger, D. Hurtmans, R. Pétrisse, J. Mol. Spectrosc. 204 (2000) 36-47.nd ozone.B.J. Drouin, T.J. Crawford, S. Yu, J. Quant. Spectrosc. Radiat. Transf. 203 (2017) 282-292.his lecture will deal with the measurement of the intensities and broadening coefficients of molecular lines observed in high resolution far infrared absorption spectra. It will skim over measurements carried out using THz spectroscopy and focus on Fourier transform spectroscopy. This latter technique is now commonly associated with synchrotron radiation, the high brightness and highly collimated nature of which being big advantages at low energies over conventional sources such as mercury lamps.A.R.W. McKellar, J. Mol. Spectrosc. 262 (2010) 1-10.he lecture will present and discuss some recent and ongoing measurements carried out relying on Fourier transform far infrared spectra recorded using synchrotron radiation, highlighting some aspects specific to these retrievals.

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J. Vander Auwera, J. Kleffmann, J.-M. Flaud, G. Pawelke, H. Bürger, D. Hurtmans, R. Pétrisse, J. Mol. Spectrosc. 204 (2000) 36-47.a B.J. Drouin, T.J. Crawford, S. Yu, J. Quant. Spectrosc. Radiat. Transf. 203 (2017) 282-292.T A.R.W. McKellar, J. Mol. Spectrosc. 262 (2010) 1-10.T

RuO₄ is a heavy tetrahedral molecule of interest in several fields. Due to its chemical toxicity and radiological impact of its 103 and 106 isotopologues, the possible remote sensing of this compound in the atmosphere in case of possible severe nuclear accident has renewed interest in its spectroscopic properties. We investigate here, for the first time at high resolution, the bending modes region in the far infrared. High resolution FTIR spectra have been recorded near room temperature, using a specially constructed cell and an isotopically pure sample of ¹⁰²RuO₄. New assignments and effective Hamiltonian parameter fits for the main isotopologue (¹⁰²RuO₄) have been performed, treating the whole ν₂-ν₄ bending mode dyad. We provide precise effective Hamiltonian parameters, including band centers and Coriolis interaction parameters [1]. [1] S. Reymond-Laruinaz, M. Faye, V. Boudon, D. Doizi, L. Manceron, “High-resolution Infrared Spectroscopy and analysis of the ν₂-ν₄ bending dyad of Ruthenium Tetroxide”, J. Mol. Spectrosc. 336 (2017) 29.

The ν₂ and ν₅ fundamental bands of thionyl chloride (SOCl₂) were measured in the 420 cm⁻¹- 550 cm⁻¹region using the FT-Far-IR spectrometer exploiting synchrotron radiation on the AILES beamline at SOLEIL. A straightforward line-by-line analysis is complicated by the high congestion of the spectrum due to both the high density of SOCl₂ rovibrational bands and the presence of the strong ν₂ fundamental band of sulfur dioxide produced by hydrolysis of SOCl₂ with residual water. To overcome this difficulty, our assignment procedure for the two isotopologues ³²S¹⁶O³⁵Cl₂ and ³²S¹⁶O³⁵Cl³⁷Cl alternates between a direct fit of the spectrum, via a global optimization technique, and a traditional line-by-line analysis. The global optimization, based on an evolutionary algorithm W. Leo Meerts and Michael Schmitt, Int. Rev. Phys. Chem. 25 (2006) 353-406 produces rotational constants and band centers that serve as useful starting values for the subsequent spectroscopic analysis. This work also helped to identify the pure rotational submillimeter spectrum of ³²S¹⁶O³⁵Cl₂ in the v₂=1 and v₅=1 vibrational states. A global fit gathering all the data of SOCl₂ from the microwave, submillimeter, and far-infrared spectral regions M. A. Martin-Drumel et al. J. Mol. Spectrosc. 315, (2015), 30-36M. A. Martin-Drumel et al., J. Chem. Phys., 144(8), (2016), 084305 has been performed A. Roucou et al. J. Chem. Phys. 147, (2017), 054303 showing that no major perturbation of rovibrational energy levels occurs for the main isotopologue of the molecule.

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W. Leo Meerts and Michael Schmitt, Int. Rev. Phys. Chem. 25 (2006) 353-406, M. A. Martin-Drumel et al. J. Mol. Spectrosc. 315, (2015), 30-36

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A. Roucou et al. J. Chem. Phys. 147, (2017), 054303,

Methyl formate (HCOOCH₃), is an important interstellar molecule, first detected about 40 years ago in a giant molecular gas cloud SGR B2. R. D. Brown, J. G. Crofts, P. D. Godfrey, F. F. Gardner, B. J. Robinson, & J. B. Whiteoak, Astrophys. J. Lett. 197, L29 (1975). More than 1000 rotational transitions including those of its isotopologues and in the torsional excited states have been observed from several astrophysical sources. The laboratory spectra of methyl formate (HCOOCH_3) exhibit many unassigned transitions and many of them would be due to rotational transitions in the low−lying excited states. Previous astronomical identification of the torsional excited states indicates that rotational transitions in the more excited vibrational states can also be observed in astrophysical sources. In laboratory, two new series of transitions have been identified in the rotational data, and based on intensity, they lie about 300 cm^-1 and 450 cm^-1 above the ground state. Y. Sakai, K. Kobayashi, M. Tsukamoto, M. Fujitake, & N. Ohashi, International Symposium on Molecular Spectroscopy, 67th meeting. RF05 (2012). There are many candidate vibrational excited states in this region and therefore, we decide to observe high-resolution far-infrared spectra to identify the responsible vibrational excited state and also to provide a feedback to the assignment of the microwave spectra. The experiment was performed on the Far-Infrared Beamline of the Canadian Light Source synchrotron. Methyl formate at a pressure of about 2-8 mTorr was admitted into a 2-m-long White cell cooled to 198K. The cell was set to provide 36 transits of the far-infrared synchrotron radiation, for a total path length of 72 m. Spectra were obtained with both a Si:bolometer and Cu:Ge detector at full resolution (0.00096 cm⁻¹). Very dense spectra of the C-O-C deformation and C-O torsional modes were obtained with high signal-to-noise ratio between 300-360 cm⁻¹. More than 30000 transitions were observed and the detail of analysis will be reported.

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R. D. Brown, J. G. Crofts, P. D. Godfrey, F. F. Gardner, B. J. Robinson, & J. B. Whiteoak, Astrophys. J. Lett. 197, L29 (1975).\end Y. Sakai, K. Kobayashi, M. Tsukamoto, M. Fujitake, & N. Ohashi, International Symposium on Molecular Spectroscopy, 67th meeting. RF05 (2012).

Highly-symmetric molecules like spherical-top possess no permanent dipole moment. Thus, at least in first approximation, their pure rotation spectrum is forbidden (or even strictly forbidden in the case of centrosymmetric species). It may thus seem useless to consider their far-infrared or THz spectrum. Nevertheless, this spectral region can provide invaluable information for these molecules. Firstly, in the case of tetrahedral species of type XY₄, pure rotation lines in the ground or in excited vibrational states can be induced through centrifugal distortion. Secondly, the strict selection rules for spherical-top molecules make some fundamental levels inaccessible though direct absorption. Here again, far-infrared studies can help to reach them through the study of low-lying difference bands. Thirdly, some larger and/or heavier species possess weak bands at low wavenumbers. In this talk, we will summarize some recent studies performed on the AILES beamline of the SOLEIL Synchrotron facility that illustrate these different cases with CH₄, CF₄, OsO₄, RuO₄, C₁₀H₁₆, C₆N₄H₁₀, C₈H₈ and SF₆.

In the 2016 meeting we reported our experimental linelist of NH₃ in 50-660 cm⁻¹ (See Paper FE08). The retrieved line positions and intensities were used as standards to validate HITRAN 2012 database and our empirical Hamiltonian models (Yu et al. 2010; Pearson et al. 2016). While the line position comparisons with HITRAN and our Hamiltonian models were excellent, the intensity comparisons were less satisfactory. During the past two years, we have updated our Hamiltonian model to improve the intensity prediction. In this presentation, we will report our significant improvement on intensity predictions, especially for the ∆K=3 forbidden transitions. We will also report comparisons of HITRAN 2016 with our existing experimental spectra.

The cyclic conformation of the formic acid dimer (HCOOH)₂ (FACD) is an elementary system under study to understand the concerted hydrogen transfer through equivalent hydrogen bonds. To this end, high-resolution molecular spectroscopy coupled to quantum chemical calculations is a powerful technique. So far molecular parameters have been reported for the ground state, the ν₂₂, ν₂₁ and ν₁₂+ν₁₄ vibrational levels, from spectra recorded using laser-based techniques in the 7.2 μm region M. Ortlieb and M. Havenith, J. Phys. Chem. A 111, 7355 (2007) ; K. G. Goroya, Y. Zhu, P. Sun, and C. Duan, J. Chem. Phys. 140, 164311 (2014) ; Y. Zhang, W. Li, W. Luo, Y. Zhu, and C. Duan, J. Chem. Phys. 146, 244306 (2017). Last year we reported the spectra associated with six rotationally resolved IR bands of FADC, recorded under jet-cooled conditions with the FTIR JET-AILES apparatus at synchrotron SOLEIL, and a QCL spectrometer at MONARIS. A special attention was paid to the analysis of the far-IR ν₂₄ fundamental band, associated with the intermolecular in-plane bending mode. Splittings due to vibration-rotation-tunneling motions were clearly observed and assigned. We will present the results of the analysis, using the model recently proposed by Zhang et al ,^a which is based on the inclusion of a c-type Coriolis-like coupling term to reproduce the rotation-tunneling interaction. This model was successfully used in the 7.2 μm region. The authors gratefully acknowledge the staff of the AILES beamline at synchrotron SOLEIL. The present work was funded by the French ANR Labex CaPPA through the PIA (contract ANR-11-LABX-0005-01), by the Regional Council Hauts de France, and by the European Funds for Regional Economic Development (FEDER).

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M. Ortlieb and M. Havenith, J. Phys. Chem. A 111, 7355 (2007) ; K. G. Goroya, Y. Zhu, P. Sun, and C. Duan, J. Chem. Phys. 140, 164311 (2014) ; Y. Zhang, W. Li, W. Luo, Y. Zhu, and C. Duan, J. Chem. Phys. 146, 244306 (2017)..

Theoretical rovibrational IR line lists computed on the empirically refined potential energy surfaces (PES) have excellent isotopologue consistency and reliablity to push the ongoing pursuit of “Best Theory + reliable High-resolution Experiment” (BTRHE) strategy to a higher level of prediction accuracy. The SO₂ benchmark uses experimental (Expt) data based Effective Hamiltonian (EH) models of a few SO₂ isotopologues and Ames-296K IR line lists of 30 SO₂ isotopologues. For microwave (MW) intensity, the Einstein A₂₁ coefficients demostrate isotopologue consistency better than 99.9%, which can help identify errors and inconsistencies in existing effective dipole moment (EDM) models or lab spectra analysis. For MW line position, the study goes from simple trial to systematic investigations on the convergence, uncertainties, higher order term effects, fixing EH parameters, mass coordinates, and other prediction scheme, etc. We confirm the feasibility of a two-orders-of-magnitude accuracy improvement over the original Ames IR line lists. By refining the rotational constants and quartic centrifugal distortion constatnts using the linear or quadratic extrapolations on their differences between the EH(Expt) and EH(Ames) IR list based parameter values, A₀ / B₀ / C₀ deviations can be as small as 0.01-0.02 MHz, and line position deviations can be reduced to 0-5 MHz for J < 30, K_a < 10-15 transitions. We report a microwave line set consisting of 644,636 transitions with reliable 296K IR intensity and Einstein A₂₁ coefficient for all 30 isotopologues of SO₂. The line position predictions are the best available, which will facilitate both the astronomical identification and lab MW analysis of those unobserved minor isotopologues. The procedure can be easily extended onto rovibrational bands and other molecular systems, while data precision higher than 0.003-0.03 MHz, or 1E-6 - 1E-7 cm⁻¹, is preferred.