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, 62026-1652

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

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

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

Overview of the four walled Cryogenic chamber

Design of the four walled cryogenic chamber

Performance of the chamber for -112 °C

Performance of the chamber for -142 °C

Performance of the chamber for -174 °C

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 -130.95 50.583 24.160 -133.037 80.260 37.726 -133.946 82.117 39.520 -130.575 80.090 38.212 -131.66 80.970 38.880 -132.58 Helium 5.500 2.673 -130.58 52.963 25.120 -133.92 Argon 5.592 2.712 -130.687 52.885 25.692 -130.2 Methane 5.129 2.464 -132.127 51.035 25.145 -128.32 -165 4.061 1.839 -140.94 51.390 22.991 -142.18 80.809 36.190 -141.42 80.590 36.084 -141.4 80.324 35.932 -141.29 5.534 2.412 -145.94 58.353 26.591 -140.173 5.538 2.484 -141.3 54.291 24.340 -141.77

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 -141.89 51.431 23.170 Oxygen -196 5.574 1.878 -174.045 52.407 17.903 -172.596 79.815 27.322 -173.416 79.840 27.265 -173.04 80.735 26.938 -174.4 Helium 5.672 1.866 -176.445 50.537 16.904 -174.76 Argon 5.364 1.802 -174.57 51.801 17.444 -174.33 5.181 1.742 -174.28 6.105 2.071 -173.34 21.274 7.033 -175.44 50.166 9.471 80.525 9.731

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

Temperature checks – literature values L R Brown and C Plymate, J. Molecular Spectroscopy 199, 166-179 (2000)

Temperature checks – literature values Roland Schermaul and Richard C.M.Learner, J. Quant. Spectrosc. Radiat. Transfer, 61 (6) 781-794 (1999)

Temperature checks – literature values Roland Schermaul and Richard C.M.Learner, J. Quant. Spectrosc. Radiat. Transfer, 61 (6) 781-794 (1999)

Our experimental data -112 °C

-142 °C Our experimental data

Our experimental data -174 °C

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

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.

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

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.

Thank you Acknowledgements Support from NASA’s Planetary Atmospheres Program (NAG5-12013) is gratefully acknowledged. Additional supplemental funding from National Science Foundation (CHE-0213356) 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

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 -159.286 Gauge 2 62.89 24.29 38.6 -159.347 62.099 22.84 39.259b -164.37 62.26 22.86 39.4 -164.559 -165 60.592 28.526 32.066c -134.573 60.72 28.56 32.16 -134.701 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