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-4493 - Université du Littoral Côte d'Opale, F-59140 Dunkerque, France, 2Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303, CNRS / Université Bourgogne Franche-Comté – 9 Av. A. Savary, BP 47870, F-21078 Dijon Cedex, France

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

Introduction

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

I. Experiment

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)]

The photomixing cw-THz spectrometer

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.

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

II. Measuring CH4 lines

CH4 lines below 3 THz (296 K)

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:

Ground state pure rotation line

Ground state R(7) cluster

n4–n4 hot band line

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, 024312 (2016) Champion J.-P., Hilico J.-C., Brown L. R., J. Mol. Spectrosc. 133, 244–255 (1989)

III. CH4 global fit and database updates

Global CH4 fit Amyay B., Louviot M., Pirali O., Georges R., Vander Auwera J., Boudon V., J. Chem. Phys. 144, 024312 (2016) Champion J.-P., Hilico J.-C., Brown L. R., J. Mol. Spectrosc. 133, 244–255 (1989)

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 : http://portal.vamdc.org

IV. Conclusion and perspectives

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)]