1. d'Opale, F-59140 Dunkerque, France,
LINE POSITIONS OF CENTRIFUGAL DISTORSION INDUCED ROTATIONAL TRANSITIONS OF METHANE MEASURED UP TO 2.6 THz AT SUB-MHz ACCURACY WITH A cw-THz PHOTOMIXING SPECTROMETER
Cédric BRAY1, Arnaud CUISSET1, Francis HINDLE1, Gaël MOURET1, Robin BOCQUET1,
Vincent BOUDON2
1Laboratoire de Physico-Chimie de l'Atmosphère, CNRS EA Université du Littoral Côte
d'Opale, F Dunkerque, France,
2Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303, CNRS / Université Bourgogne Franche-Comté – 9 Av. A. Savary, BP 47870, F Dijon Cedex, France

2. Overview
Introduction
I. Experiment
II. Measuring CH4 lines
III. CH4 global fit and database updates
IV. Conclusion and perspectives

3. Introduction

4. The CH4 rotational spectrum
Bending Dyad
Methane is a non-polar molecule (spherical-top with Td symmetry).
It has no permanent dipole moment.
But weak rotational transitions are induced by centrifugal distorsion and vibration.
1533 cm–1 n2
1310 cm–1 n4
Ground State (GS)
0 cm–1

5. I. Experiment

6. THz spectroscopy by photomixing
Optoelectronic conversion of
a THz beatnote into photocurrant
Spatial superposition of
two ECDL 𝜆 ~ 800 nm
𝜈 𝑇𝐻𝑧 = 𝜈 1 − 𝜈 2
THz detection with a
helium-cooled bolometer (T = 4K)
(ECDL: Extra-Cavity Diode Laser)
Disadvantages
Advantages
Low emitted power
A very large accessible frequency range: 0,1 – 3,3 THz
Limited continuous tunability
Doppler limited rotational spectroscopy
No commercial solution for THz frequency metrology
Various sample environments may be used with this spectrometer
[Hindle et al., C.R. A. S., 9, 262, (2008)]

7. The photomixing cw-THz spectrometer

8. THz metrology by means of a frequency comb
𝜈THz= 𝜈1–𝜈2 and 𝜈1(775 nm) ≈ 𝜈2(775 nm) ≈ 387 THz
THz spectroscopic applications require a sub-MHz frequency metrology in order to be competitive with accuracy of electronic sources. A relative uncertainty better than 10-8 should be achieved.
𝜈 𝑇𝐻𝑧 =Δ𝑛. 𝜈 𝑟𝑒𝑝 + 𝜈 𝑃𝐿𝐿3
𝜈 𝑃𝐿𝐿1 = 𝜈 𝑃𝐿𝐿2
[Hindle et al., App. Phys. B, 104, 763, (2011)]
A reference frequency is fixed by the phase-locking of CW1 et CW2 onto 2 comb lines.
The continuous tunability is possible using CW3 phase-locked onto CW2.
The CW1/CW3 beatnote is converted in tunable CW-THz
We use a turn-key commercial fs laser generating an ultra-stable frequency comb used as an optical rule.

9. Performances of the THz spectrometer
[Mouret et al., Opt. Express, 17, 22031, (2009)]
THz frequency transitions measured with a kHz precision
500 MHz of continuous tunability

10. II. Measuring CH4 lines

11. CH4 lines below 3 THz (296 K)

12. Very weak lines! Line position uncertainty: R(3) 4A2–3A1 line of n4–n4
(n is the frequency of the baseline oscillation)
Line position uncertainty:

13. Ground state pure rotation line

14. Ground state R(7) cluster

15. n4–n4 hot band line

16. 10 measured lines at high precision
Fit residuals compared to previous works:
Amyay B., Louviot M., Pirali O., Georges R., Vander Auwera J., Boudon V., J. Chem. Phys. 144, (2016)
Champion J.-P., Hilico J.-C., Brown L. R., J. Mol. Spectrosc. 133, 244–255 (1989)

17. III. CH4 global fit and database updates

18. Global CH4 fit
Amyay B., Louviot M., Pirali O., Georges R., Vander Auwera J., Boudon V., J. Chem. Phys. 144, (2016)
Champion J.-P., Hilico J.-C., Brown L. R., J. Mol. Spectrosc. 133, 244–255 (1989)

19. HITRAN 2016 and MeCaSDa updates
These 10 new lines have been included in the HTRAN 2016 database version (paper submitted).
They have also been used for the MeCaSDa calculated line list database, accessible through the Virtual Atomic and Molecular Data Centre (VAMDC) portal :

20. IV. Conclusion and perspectives

21. Rotation of non-polar molecules
Paper to appear in the HITRAN Special issue of the Journal of Quantitative Spectroscopy and Radiative Transfer
Pure rotation lines provide very accurate information about the vibrational ground state.
Projects:
CF4 (greenhouse gas)
GeH4 (Jupiter’s atmosphere)
[Boudon V., Carlos M., Richard C., Pirali O., submitted to J. Mol. Spectrosc. (2017)]