INDUCED ROTATIONAL TRANSITIONS OF METHANE MEASURED UP TO 26 THz AT SUBMHz ACCURACY WITH A cwTHz PHOTOMIXING SPECTROMETER Cédric BRAY 1 Arnaud CUISSET 1 Francis HINDLE ID: 633465
Download Presentation The PPT/PDF document "LINE POSITIONS OF CENTRIFUGAL DISTORSI..." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
LINE POSITIONS OF CENTRIFUGAL DISTORSION INDUCED ROTATIONAL TRANSITIONS OF METHANE MEASURED UP TO 2.6 THz AT SUB-MHz ACCURACY WITH A cw-THz PHOTOMIXINGSPECTROMETER
Cédric BRAY1, Arnaud CUISSET1, Francis HINDLE1, Gaël MOURET1, Robin BOCQUET1,Vincent BOUDON21Laboratoire de Physico-Chimie de l'Atmosphère, CNRS EA-4493 - Université du Littoral Côted'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, FranceSlide2
OverviewIntroductionI. ExperimentII. Measuring CH4 linesIII. CH4 global fit and database updatesIV. Conclusion and perspectivesSlide3
IntroductionSlide4
The CH4 rotational spectrum
0 cm–1
1310
cm
–1
1533
cm
–1
n
2
n
4
Methane is a non-polar molecule (spherical-top with
T
d
symmetry).
It has no permanent dipole moment.
But weak rotational transitions are induced by centrifugal
distorsion
and vibration.
Ground State (GS)
Bending
DyadSlide5
I. ExperimentSlide6
Optoelectronic conversion of
a THz
beatnote
into
photocurrant
Spatial superposition of
two
ECDL
800 nm
THz detection with a
helium-cooled bolometer (
T
= 4K
)
Disadvantages
Advantages
Various sample environments
may be used with this
spectrometer
Doppler limited rotational spectroscopy
A very large accessible frequency
range:
0,1 – 3,3 THz
No commercial solution for THz frequency metrology
Limited continuous
tunability
Low emitted
power
[
Hindle
et al., C.R. A. S., 9, 262, (2008
)]
THz spectroscopy by
photomixing
(ECDL: Extra-
Cavity
Diode Laser)Slide7
The photomixing cw-THz spectrometerSlide8
THz
= 1–2 and 1(775 nm) ≈ 2(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.
We use a turn-key commercial fs laser generating an ultra-stable frequency comb used as an optical rule.
=
[
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
THz metrology by means of a frequency combSlide9
500 MHz of continuous
tunability[Mouret et al., Opt. Express, 17, 22031, (2009)]THz frequency
transitions measured
with a kHz precision
Performances of the THz spectrometerSlide10
II. Measuring CH4 linesSlide11
CH4 lines below 3 THz (296 K)Slide12
Very weak lines!R(3) 4A2–3A1 line of
n4–n4Line position uncertainty:(n is the frequency of the baseline oscillation)Slide13
Ground state pure rotation lineSlide14
Ground state R(7) clusterSlide15
n4–n4 hot band lineSlide16
10 measured lines at high precisionFit 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)Slide17
III. CH4 global fit and database updatesSlide18
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)Slide19
HITRAN 2016 and MeCaSDa updatesThese 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.orgSlide20
IV. Conclusion and perspectivesSlide21
Rotation of non-polar moleculesPaper to
appear in the HITRAN Special issue of the Journal of Quantitative Spectroscopy and Radiative TransferPure rotation lines provide very accurate information about the vibrational ground state.Projects:CF4 (greenhouse gas)GeH
4 (Jupiter’s atmosphere)
[Boudon V., Carlos M., Richard C.,
Pirali
O., submitted
to J. Mol. Spectrosc
. (2017)]