1. A COMPREHENSIVE INTENSITY STUDY OF THE 4 TORSIONAL BAND OF ETHANE J. NOROOZ OLIAEE, N. Moazzen-Ahmadi Institute for Quantum Science and Technology Department of Physics and Astronomy, University of Calgary I. Ozier Department of Physics and Astronomy, University of British Columbia K. SUNG, T. CRAWFORD, L. R. BROWN Jet Propulsion Laboratory, California Institute of Technology E. H. WISHNOW Space Sciences Laboratory and Department of Physics, University of California, Berkeley V. M. DEVI Department of Physics, The College of William and Mary Canadian space agency

2.  Ethane is used as a tracer in the atmospheres of Jovian planets to understand the methane cycle.  To provide high quality line parameters for HITRAN and GEISA databases.  To study torsion mediated vibrational interactions Motivation

3. Resolution 0.0025 cm -1 Path Length 172 m Pressure 10 Torr Temperature 296 K Torsional bands N. Moazzen-Ahmadi et al., JMS (2001). Q-branch fundamental P-branch Q-branch 2 4 - 4

4. (a) Torsional fundamental 4 ; (b) 2 4  4 ; (c) 9 fundamental ; (d) 9 + 4  4 hot band; (e) 3 fundamental; (f) the forbidden 3 4 band, made bright by mixing with 9 ; and (g) the infrared-active difference band 12  9. L. Borvayeh et al., JMS 250, 51 (2008). 4-state analysis for normal ethane

5. Simulation of the torsional bands Integrated intensity Line strength N. Moazzen-Ahmadi and I. Ozier, 126, 99-112 (1987). N. Moazzen-Ahmadi, A.R.W. McKellar, J.W.C. Johns, and I. Ozier, J. Chem. Phys. 97, 3981 (1992).

6. Resolution 0.0025 cm -1 Path Length 172 m Pressure 10 Torr Temperature 296 K Torsional bands N. Moazzen-Ahmadi et al., JMS (2001). Q-branch of the fundamental P-branch Q-branch of 2 4 - 4

7. High-resolution Infrared Spectroscopy Lab. #1 #7 #6 #5 #8 #4 #3 #2 Cellspath lengthsTemperaturesWavelengths 10.02, 0.06, 0.15, 0.20, 0.25 m (fixed)T ~ 296 K (room)2 – 50 µm 26.6 – 32.5 m (variable)T ~ 296 K (room)0.7 – 50 µm 320 – 200 m (variable)T ~ 296 K (room)0.7 – 1.6 µm 44 m and 0.3 m cross cell (fixed)T ~ 296 K (room)IR and MW 50.204 cm (fixed) 80 < T < 297 K1.6 – 15 µm 624 m long (fixed) – Herriot cell 30 < T < 250 K1.6 – 7 µm 74 – 52 m (variable) 20 < T < 296 K15 – 500 µm 8up to a few meters (Emission port - TBD)296 < T < 400 K0.7 – 5 µm

8. SpectrumIIIIIIIVVVIVII T(K)166. 231.247.5248.2290.3292.1 p(Torr)84.5634.7647.48253.75125.2159.871.3 Resolution (cm -1 )0.01 0.02 0.01 Path length (m)52 Experimental conditions

9. P-branch 4 Q-branch 4 R-branch 4 Q-branch 2 4 - 4 P-branch 2 4 - 4 R-branch 2 4 - 4 Spectrum I T = 166 K P = 84.56 Torr Resolution = 0.01 cm -1 Absorption path = 52 m

10. Early attempts at intensity fits Residual The model underestimates the intensity in the P-branch The model overestimates the intensity in the R-branch A small dip in the residual near the band origin of the fundamental

11. RangeA(1)A(2)A(3)A(4)A(8)A(9) 249.9-258.70.99900.00160.28620.0453(1)-31.60(17)8.64 258.7-262.70.99890.00160.28620.0410(7)-32.35(12)8.64 262.7-266.91.00010.00170.28620.0411(5)-31.75(7)8.64 266.9-271.31.0022 0.00260.28620.0419(4)-31.35(6)8.64 271.3-275.51.0051 0.00120.28620.0424(4)-31.15(5)8.64 275.5-284.61.0058-0.00600.28620.0442(5)-30.44(8)8.64 284.6-291.91.0012 0.00000.28620.04047(1)-30.10(1)8.644(1) 293.7-300.70.9996-0.00110.28620.0438(8)-28.90(13)8.64 300.4-304.40.99920.00200.28620.0419(8)-28.07(11)8.64 304.4-308.01.00120.00100.28620.0432(6)-27.96(8)8.64 308.0-313.01.00050.00100.28620.0419(7)-27.21(9)8.64 313.0-319.21.00100.00200.28620.0432(11)-26.69(15)8.64 A(1) background level A(2) background tilt A(3) Constant A(4) Broadening parameter at P&T A(8) dipole moment A(9) dipole moment Piecewise fit

12. Spectrum II T = 166 K P = 34.76 Torr Resolution = 0.01 cm -1 Absorption path = 52 m J.K.G. Watson, J. Mol. Spectrosc. 153, 211-224 (1192). D. Papoušk, M.R. Aliev, Š. Urban, J. Mol. Spectrosc. 124, 285-305 (1985).

13. Extended QM model

14. Combined fit Slight dip Extended QM model Spectrum I T = 166 K P = 84.56 Torr Resolution = 0.01 cm -1 Absorption path = 52 m

15. Final fits Abundances: CH 3 -CH 3 at 97.871% 13 CH 3 -CH 3 at 2.129%

16. Final fits

18. Torsional spectrum of ethane were recorded at several temperatures between 293 K to 166 K, with the lower temperatures relevant to the stratosphere of Titan. The previously reported dipole moment expression proved to be inadequate and it had to be extended to include a Herman-Wallis type factor and two higher order terms. The intensity has been fitted to better than 1% for the lowest temperature spectra where interference from hot and difference bands is minimal. A consistent set of dipole moments have been obtained which would allow us to provide HITRAN and GEISA accurate line parameters for the 35 micron band of ethane. The results presented in this study will lead to an improved understanding of the methane cycle in planetary atmospheres; they will also permit other molecular features in the CIRS spectra to be identified. Conclusions