1. ABSORPTION SPECTRA FOR THE 889 nm BAND OF METHANE DERIVED FROM INTRACAVITY LASER SPECTROSCOPY MEASUREMENTS MADE AS A FUNCTION OF LOW SAMPLE TEMPERATURES
SADASIVAN SHAJI and JIM O’BRIEN
Department of Chemistry & Biochemistry and Center for Molecular Electronics, University of Missouri, St. Louis, MO 63121;
LEAH O’BRIEN
Department of Chemistry, Southern Illinois University,
Edwardsville, IL,

2. Contents ILS Overview ILS setup
Cryogenic Chamber – design and features
Pressure – Temperature profiles and temperature calculated from pressure change
Determining temperature from rotational populations for 760 nm A band of oxygen
Methane spectra for 889 nm band at three temperatures: -112, -142 and -174 C.
Absorption coefficients for 889 nm band of methane at low temperatures
Conclusion
Acknowledgements

3. Overview
Methane is an important component in the atmospheres of giant planets and their major satellites
Laboratory spectral data at low temperatures are required to interpret the planetary data properly
Methane spectrum in near -IR region is intrinsically very weak and hence a very sensitive method like intracavity laser spectroscopy (ILS) is needed
Absorption lines appear superimposed on the output of the laser which is operated in a time-modulated fashion
ILS enhances the sensitivity with its tremendous 'effective pathlength' > 100 km being achieved easily
ILS is both ultrasensitive and quantitative method for acquiring weak absorption spectra

4. Schematic diagram for the intracavity laser spectrometer
FM=fold mirror; HR=high reflector;
OC=output coupler; AOM=acousto-optic modulator

5. Overview of the four walled Cryogenic chamber

6. Design of the four walled cryogenic chamber

10. Performance of the chamber for -112 °C

11. Performance of the chamber for -142 °C

12. Performance of the chamber for -174 °C

13. Temperature calculated using gas laws
Type of Gas
Set Temp. (oC)
Initial Pressure (torr)
Final Pressure (torr)
Calculated Temp. (oC)
Oxygen
-150
4.115
1.990
50.583
24.160
80.260
37.726
82.117
39.520
80.090
38.212
80.970
38.880
Helium
5.500
2.673
52.963
25.120
Argon
5.592
2.712
52.885
25.692
-130.2
Methane
5.129
2.464
51.035
25.145
-165
4.061
1.839
51.390
22.991
80.809
36.190
80.590
36.084
-141.4
80.324
35.932
5.534
2.412
58.353
26.591
5.538
2.484
-141.3
54.291
24.340

14. Temperature calculated using gas laws
Type of Gas
Set Temp. (oC)
Initial Pressure (torr)
Final Pressure (torr)
Calculated Temp. (oC)
Methane
-165
5.140
2.294
51.431
23.170
Oxygen
-196
5.574
1.878
52.407
17.903
79.815
27.322
79.840
27.265
80.735
26.938
-174.4
Helium
5.672
1.866
50.537
16.904
Argon
5.364
1.802
51.801
17.444
5.181
1.742
6.105
2.071
21.274
7.033
50.166
9.471
80.525
9.731

15. A section of the intracavity laser spectrum of oxygen A band at -174 °C showing the original and deconvolved data

16. Temperature checks – literature values
L R Brown and C Plymate, J. Molecular Spectroscopy 199, (2000)

17. Temperature checks – literature values
Roland Schermaul and Richard C.M.Learner, J. Quant. Spectrosc. Radiat. Transfer, 61 (6) (1999)

18. Temperature checks – literature values
Roland Schermaul and Richard C.M.Learner, J. Quant. Spectrosc. Radiat. Transfer, 61 (6) (1999)

19. Our experimental data
-112 °C

20. -142 °C
Our experimental data

21. Our experimental data
-174 °C

22. A section of intracavity laser spectrum of methane for three temperatures
The spectrum is normalized to methane pressure 2 torr and pathlength 5 km

23. A section of the methane spectrum in 889 nm band showing the original and deconvolved data. The spectrum is normalized to methane pressure 2 torr and pathlength 5 km.

24. Methane absorption coefficients for the 889 nm band derived
from ILS spectra for three temperatures averaged per Å

25. Conclusions
A four-walled cryogenic chamber that can be used for low temperature ILS studies is described
760 nm Oxygen A band is used to determine the rotational temperature.
The effective temperature in the chamber is in good agreement with that calculated from the gas laws.
Methane spectra is recorded for 889 nm band at different low sample temperatures.
Absorption coefficients for the 889 nm band at three low temperatures are determined.

26. Thank you Acknowledgements
Support from NASA’s Planetary Atmospheres Program (NAG ) is gratefully acknowledged.
Additional supplemental funding from National Science Foundation (CHE ) and University of Missouri Research Board for the Verdi laser is gratefully acknowledged.
Thanks to Norman (Ted) Windsor, UMSl Chemistry Machinist for fabricating the Brewster angle window mounts.
Thank you

27. Thermal transpiration check
Set Temperature (°C)
Initial Pressure (torr)
Final Pressure (torr)
Pressure change (torr)
Temperature calculated
(°C)
Gauge 1
-196
62.758
24.252
38.506a
Gauge 2
62.89
24.29
38.6
62.099
22.84
39.259b
62.26
22.86
39.4
-165
60.592
28.526
32.066c
60.72
28.56
32.16
a after 2 hours and 30 minutes
b after 4 hours and 30 minutes
c after 4 hours and 30 minutes
for Helium as sample gas
Leak rate is 10-4 torr/minute while the chamber envelope is under vacuum and 1.5 * 10-3 torr/minute while envelope is at atmospheric pressure