68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 SF 6 THE FORBIDDEN BAND UNVEILED V. BOUDON, Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 5209 CNRS-Université de Bourgogne, 9. Av. A. Savary, BP 47870, F Dijon Cedex, France L. MANCERON, Ligne AILES – Synchrotron SOLEIL, L’Orme des Merisiers, F Gif-sur-Yvette, France and Institut des Sciences Moléculaires d’Orsay, UMR 8214 CNRS-Université Paris-Sud, Bât. 210, 91405, Orsay Cedex, France F. KWABIA-TCHANA, Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Université Paris-Est Créteil et Université Paris- Diderot, 61 Avenue du Général de Gaulle, Créteil Cedex, France P. ROY, Ligne AILES – Synchrotron SOLEIL, L’Orme des Merisiers, F Gif- sur-Yvette, France

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Contents I.The SF 6 molecule and its ν 6 mode II.Far-IR spectroscopy at SOLEIL III.Theoretical model IV.Band analysis and simulation V.Perspectives for SF 6 hot bands

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 I. The SF 6 molecule and its ν 6 mode

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Sulfur hexafluoride (SF 6 ) C3C3 C4C4 Point group: O h ν1ν1 ν2ν2 ν3ν3 ν4ν4 ν5ν5 ν6ν6 A 1g EgEg F 1u F 2g F 2u Stretching Bending Raman IR RamanInactive 775 cm cm cm cm cm cm -1 Normal modes of vibration : Small rotational constant: B 0 ≈ cm –1 v 6 = 1 is the lowest vibrational level and thus generates hot bands. But the ν 6 fundamental band is, in first approximation, inactive.

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Vibrational level population

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Observing ν 6 ? Hypothesis: ν 4 / ν 6 Coriolis interaction Prediction for ν 6 intensity: 6 orders of magnitude lower than ν 3

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 II. Far-IR spectroscopy at SOLEIL

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 AILES beamline at SOLEIL High Resolution Absorption Spectroscopy in the Far-IR Synchrotron beam entrance Interferometer Multipass cell Bolometer detectors Maximum spectral resolution = cm -1 Spectral range= cm -1

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Coolable log-path gas cell Adapted optics for long path ( < 150m) Four compartment chamber: Sample gas Convection-cooling gas (He) Liquid Nitrogen reservoir Insulating vacuum 1m Accurate far infrared high resolution spectra of diluted gas samples and mixture at temperatures : 400 – 90K

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 III. Theoretical model

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Theoretical model – Tensorial formalism Systematic expansion of effective Hamiltonian and transition moment up to any order and for any polyad scheme, thanks to group theory and tensorial methods  All interactions are automatically included  Vibrational extrapolation  Global analyses ParametersRotationVibration

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Perturbation-induced dipole moment Coriolis interaction between ν 6 and ν 4 does not give convincing results There must be a more complicated interaction pattern Anyway, contact transformation induces new terms in the dipole moment We can explain the ν 6 structure with a degree one rotational dipole moment operator (Herman-Wallis type), then considering the band as isolated: This term leads to a complex branch structure, which we do observe!

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 The ν 6 complex branch system

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 IV. Band analysis and simulation

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 The ν 6 band at high-resolution: overview

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 The ν 6 band at high-resolution: P branch

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 The ν 6 band at high-resolution: Q branch

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 The ν 6 band at high-resolution: R branch

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Fit results: seven ν 6 parameters determined

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Detail in the branch structure (I)

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Detail in the branch structure (II)

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Energy levels Good sampling of all branches

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 V. Perspectives for SF 6 hot bands

68th Ohio State University Symposium on Molecular Spectroscopy June 17–21, 2013 Toward hot band elucidation The ν 3 +ν 6 level is not accessible (too weak Raman band) ν 3 +ν 6 –ν 4 difference band could be observed (and ν 4 is well known), but is very weak in a difficult spectral region ν 3 +ν 2 –ν 2 and ν 3 +ν 1 –ν 1 already done!