Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Gas phase rovibrational spectroscopy of DMSO, Part II: « Towards a THz observation of 4-fold clusters » A.Cuisset, M.A Martin-Drumel, F. Hindle, G. Mouret, D. A. Sadovskii Laboratoire de Physico-Chimie de l’Atmosphère, Maison de la Recherche en Environnement Industriel, Dunkerque, France

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Development of spectrometers using optoelectronic and electronic sources for gas phase THz analysis of molecular systems with environmental interest Line position and Line profile analysis of atmospheric compounds from their rotational signatures in the THz range Detection, quantification and kinetic studies of pollutants in realistic media Gas phase THz rovibrational spectroscopy of weakly volatil compounds at low and high resolution The topics of our research group

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Sub-THz analysis of pure rotational transitions in the lowest energy vibrational modes of DMSO  At room temperature, we are able to probe and assign pure rotational transitions of DMSO in the six lowest frequency vibrational modes ν 24, ν 13, ν 12, ν 11 & ν 23  ν 24, ν 13, and ν 12 were not observed on the FT-FIR spectra ν 24 (190cm -1 )ν 13 (232cm -1 ) ν 12 (283cm -1 ) ν 11 (323cm -1 ) ν 23 (376cm -1 )

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Sub-THz analysis of pure rotational transitions in the lowest energy vibrational modes of DMSO ☞ The pitchfork bifurcation leading to the X states or 4-clusters emergence is observed for ν 23 not for ν 11 with: ☞ The ν 23 bending vibration “softens” the C 2 SO frame, induces specific deformations of its averaged geometry, and amplifies specific dynamical responses to the centrifugal forces. How is the rotational dynamics of the others low-energy vibrational modes? Is ν 23, the only mode to present a gyroscopic destabilization?

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al A versatile solid state subTHz source for gas phase spectroscopy

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Sub-THz DMSO high resolution spectra Spectral domain70 – 700 GHz Time constant100 ms Scanning rate5 GHz/hour Spectral resolution 100 kHz Spectral purityfew kHz Optical pathlength 1,25 m DetectorInSb – bolometer Frequency modulation 50 kHz Pressure range10 -4 – mbar S/N ratio> 20 frequency precision < 30 kHz ☞ Around 4000 newly assigned transitions have been obtained in addition to the ground state mw measurements of Dreizler et al. (Z naturfosch, 1965) and submm of Margulès et al (JMS, 2010). ☞ The majority of those new lines are associated to rotational transitions in the 5 lowest energy fundamental vibrational bands

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Sub-THz DMSO high resolution spectra: Results of the fit ☺ A global fit have been performed including mw, THz and FIR frequencies; around 500 rotational transitions (J max =60) in each vibrationnally excited states are assigned. ☺ The molecular parameters of the effective hamiltonian developped to the octic constants for the ground state and the 5 lowest energy vibrational states have been obtained ☹ For ν 13 & ν 24 we, actually, ignored in the fit the splittings due to internal rotations of the 2 methyls groups in the case of a 1000 cm -1 barrier. ☺ A standard error close to 1 MHz (2σ) is obtained for submm frequencies of ν 11, ν 23, ν 13 & ν 24 ☹ 10 MHz (2σ) is obtained for the standard error of submm frequencies of ν 12 (a clear strong level crossing pattern is observed near J = 35 … 40 ) ☹ The frequency of the band centers for ν 12, ν 13 & ν 24 are not determined in this work

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Sub-THz DMSO high resolution spectra: Results of the fit 15 GHz (0,5 cm -1 ) part of DMSO spectrum 15 GHz (0,5 cm -1 ) part of DMSO spectrum

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Sub-THz DMSO high resolution spectra: Results of the fit 1 GHz (0,03 cm -1 ) part of DMSO spectrum 1 GHz (0,03 cm -1 ) part of DMSO spectrum Overtone 2ν 24 is also observed and is not assigned. A complex absorption pattern is observed due to the internal rotation of 2 methyl groups.

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Rotational structure of DMSO in the lowest energy vibrational modes ☞ The scale of the total splitting of the rotational multiplet of DMSO does not allow to assess directly the differences in the rotational structures of different vibrational states

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Rotational structure of DMSO in the lowest energy vibrational modes ☞ To understand these differences, for each vibrational states “ν”, and for each principal axes “s”, we consider the classical energy in excited states relative to the energy in the ground state: ☞ The differences for J→0 reflect the change in the averaged geometry of the molecule ☞ Large variations of H ν, s with J signal essential vibration-rotation interactions and significant perturbations in the rotational levels. ☺ Except H ν23, A (J), these functions are nearly constant at low J and some varying notably for J>25 ☺ H ν23, A (J) presents a qualitatively different, strong, immediate non linear variation at low J

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Rotational structure of DMSO in the lowest energy vibrational modes ☞ Another quantity that can help to visualisize the exceptional properties of ν 23 is the dynamic asymmetry: ☞ Except for ν 23 the dynamic asymmetry remain nearly constant, equal to the static asymmetry parameter ☞ DMSO is more asymmetric in ν 12 and ν 13 compared to the other states. ☞ ν 23 has an exceptional behavior: ε drops quickly and immediately with J and DMSO approaches the symmetric top configuration ☞ H ν23, A (J) is in the anomalously large tensorial corrections of order J 4 with phenomenological parameters d 1 and d 2.

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Conclusion: the physical meaning of gyroscopic destabilization and emergence of 4-clusters in DMSO DMSO, due to its exceptional near-symmetric-top geometry, is a molecule with potentially complex and interesting rotational dynamics. Using the FIR data on the ν 23 state, we have succeeded to uncover such dynamics. The phenomenon of the gyroscopic destabilization of the A axis of DMSO is very similar to that predicted and found in the AB2 molecules in the 1990s. 4-fold clusters are experimentally observed at high J values. DMSO is the first known large molecule presenting this universal phenomenon. Using subTHz data, we demonstrate that, except ν 23, low energy vibrational states of DMSO have standard, relatively unperturbed rotational structure, similar to that in the ground state. The analysis of the ν 23 ν 23 rotational sideband confirmed the unusual rotational dynamics and the emergence of 4 clusters due to a breakdown of the rotation-vibration separation. If we exclude the possibility of a 1:1 resonance between ν 23 and an other fundamental low-frequency vibrational state, we have still to examine the overtone and combination states of the torsional modes. An analysis of splitting due to the internal rotation of the 2 methyl groups has to be considered.

Friday, June 21, th OSU SYMPOSIUM MOLECULAR SPECTROSCOPY FB06: Cuisset & al Dimitrii Sadovskii Sophie Eliet Marie Aline Martin Abdelazziz Omar Gaël Mouret Francis Hindle Robin Bocquet Arnaud Cuisset