68th ohio state university symposium on molecular spectroscopy june 17–21, 2013 sf 6 the forbidden...

68th Ohio State University Symposium on Molecular Spectroscopy • June 17–21, 2013

SF6THE FORBIDDEN BAND UNVEILED

V. BOUDON, Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 5209 CNRS-Université de Bourgogne, 9. Av. A. Savary, BP 47870, F-21078 Dijon Cedex, France

L. MANCERON, Ligne AILES – Synchrotron SOLEIL, L’Orme des Merisiers, F-91192 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, 94010 Créteil Cedex, France

P. ROY, Ligne AILES – Synchrotron SOLEIL, L’Orme des Merisiers, F-91192 Gif-sur-Yvette, France


Contents

I. The SF6 molecule and its ν6 mode

II. Far-IR spectroscopy at SOLEIL

III. Theoretical model

IV. Band analysis and simulation

V. Perspectives for SF6 hot bands


I. The SF6 molecule and its ν6 mode


Sulfur hexafluoride (SF6)

C3

C4

Point group: Oh

ν1 ν2 ν3 ν4 ν5 ν6

A1g Eg F1u F1u F2g F2u

Stretching Stretching Stretching Bending Bending Bending

Raman Raman IR IR Raman Inactive

775 cm-1 643 cm-1 948 cm-1 615 cm-1 523 cm-1 347 cm-1

Normal modes of vibration :

Small rotational constant: B0 ≈ 0.091 cm–1

v6 = 1 is the lowest vibrational level and thus generates hot bands.

But the ν6 fundamental band is, in first approximation, inactive.


Vibrational level population


Observing ν6?

Hypothesis:

ν4 / ν6 Coriolis interaction

Prediction for ν6 intensity:

6 orders of magnitude lower than ν3


II. Far-IR spectroscopy at SOLEIL


AILES beamline at SOLEILHigh Resolution Absorption Spectroscopy in the Far-IR

Synchrotron beam entrance

Interferometer

Multipass cell

Bolometer detectors

Maximum spectral resolution = 0.001 cm-1

Spectral range= 7-1000 cm-1


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


III. Theoretical model


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

Parameters Rotation Vibration


Perturbation-induced dipole moment• Coriolis interaction between ν6 and ν4 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 ν6 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!


The ν6 complex branch system


IV. Band analysis and simulation


The ν6 band at high-resolution: overview


The ν6 band at high-resolution: P branch


The ν6 band at high-resolution: Q branch


The ν6 band at high-resolution: R branch


Fit results: seven ν6 parameters determined


Detail in the branch structure (I)


Detail in the branch structure (II)


Energy levels

Good sampling of all branches


V. Perspectives for SF6 hot bands


Toward hot band elucidation

• The ν3+ν6 level is not accessible (too

weak Raman band)

• ν3+ν6–ν4 difference band could be

observed (and ν4 is well known), but is

very weak in a difficult spectral region

• ν3+ν2–ν2 and ν3+ν1–ν1 already done!

68th ohio state university symposium on molecular spectroscopy june 17–21, 2013 sf 6 the forbidden...

Documents

molecular spectroscopy

modefarir spectroscopy

umr cnrs

cnrsuniversit parissud

ligne ailes synchrotron

contentsthe sf6 molecule

crteil cedex

ailes beamline