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. OzierDepartment of Physics and Astronomy, University of British Columbia

K. SUNG, T. CRAWFORD, L. R. BROWNJet Propulsion Laboratory, California Institute of Technology

E. H. WISHNOWSpace Sciences Laboratory and Department of Physics, University of

California, Berkeley

V. M. DEVIDepartment of Physics, The College of William and Mary

Canadian space agency

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

260.25 260.50 260.75 261.00

WAVENUMBER /cm-1

TRA

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MIT

TAN

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0.90

0.92

0.94

0.96

0.98

1.00

0.92

0.94

0.96

0.98

1.00

Resolution 0.0025 cm-1

Path Length 172 m

Pressure 10 Torr

Temperature 296 K

Torsional bands

N. Moazzen-Ahmadi et al., JMS (2001).

287.9 288.0 288.1 288.2 288.3 288.4 288.5 288.6 288.7

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*

**

** ** s

r rs

r

srs

rs

rsr ssr

Q-branch fundamental P-branch

Q-branch 24-4

(a) Torsional fundamental 4; (b) 24 4; (c) 9 fundamental ; (d) 9 +4 4 hot band;(e) 3 fundamental; (f) the forbidden 34 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

Simulation of the torsional bands

1 1

0 z

J J

J J

4 4( , ,v , )/ ( ', ,v ', )/4 4 4 4( , , v , ; ', , v ', ) ( , , v , ; ', , v ', ) E J K kT E J K kT

VTR

A J K J K S J K J K e eZ

Integrated intensity

2 2 2 22

4 4 4 4( , , v , ; ', , v ', ) g v , v ',PR

K m KS J K J K J

m

22 24 4 4 4

(2 1)( , , v , ; ', , v ', ) g v , v ', ( 1)

1Q

JS J K J K J J J K

J J

1J R branchm

J P branch

38 / 2N hc

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).

260.25 260.50 260.75 261.00

WAVENUMBER /cm-1

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TAN

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0.90

0.92

0.94

0.96

0.98

1.00

0.92

0.94

0.96

0.98

1.00

Resolution 0.0025 cm-1

Path Length 172 m

Pressure 10 Torr

Temperature 296 K

Torsional bands

N. Moazzen-Ahmadi et al., JMS (2001).

287.9 288.0 288.1 288.2 288.3 288.4 288.5 288.6 288.7

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r

srs

rs

rsr ssr

Q-branch of the fundamental P-branch

Q-branch of 24-4

High-resolution Infrared Spectroscopy Lab.

#1

#7

#6

#5

#8

#4

#3

#2

Cells path lengths Temperatures Wavelengths

1 0.02, 0.06, 0.15, 0.20, 0.25 m (fixed) T ~ 296 K (room) 2 – 50 µm

2 6.6 – 32.5 m (variable) T ~ 296 K (room) 0.7 – 50 µm

3 20 – 200 m (variable) T ~ 296 K (room) 0.7 – 1.6 µm

4 4 m and 0.3 m cross cell (fixed) T ~ 296 K (room) IR and MW

5 0.204 cm (fixed) 80 < T < 297 K 1.6 – 15 µm

6 24 m long (fixed) – Herriot cell 30 < T < 250 K 1.6 – 7 µm

7 4 – 52 m (variable) 20 < T < 296 K 15 – 500 µm

8 up to a few meters (Emission port - TBD) 296 < T < 400 K 0.7 – 5 µm

Spectrum I II III IV V VI VII

T(K) 166. 166. 231. 247.5 248.2 290.3 292.1

p(Torr) 84.56 34.76 47.48 253.75 125.2 159.8 71.3

Resolution (cm-1) 0.01 0.01 0.01 0.02 0.02 0.01 0.01

Path length (m) 52 52 52 52 52 52 52

Experimental conditions

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Inte

nsity

/ cm

-1

230 240 250 260 270 280 290 300 310 320Wavenumber/cm-1

P-branch 4

Q-branch 4

R-branch 4

Q-branch 24-4

P-branch 24-4

R-branch 24-4

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rma

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d I

nte

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cm

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263 264 265 266 267 268Wavenumber/cm-1

Spectrum IT = 166 KP = 84.56 TorrResolution = 0.01 cm-1

Absorption path = 52 m

Early attempts at intensity fits

0.6

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d I

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230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315Wavenumber/cm-1

Residual

2 2 2 22

4 4 4 4( , , v , ; ', , v ', ) g v , v ',PR

K m KS J K J K J

m

22 24 4 4 4

(2 1)( , , v , ; ', , v ', ) g v , v ', ( 1)

1Q

JS J K J K J J J K

J J

The model underestimates the intensity in the P-branchThe model overestimates the intensity in the R-branchA small dip in the residual near the band origin of the fundamental

0.6

0.7

0.8

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d I

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255 260 265 270 275 280 285 290 295 300 305 310 315Wavenumber/cm-1

Range A(1) A(2) A(3) A(4) A(8) A(9)249.9-258.7 0.9990 0.0016 0.2862 0.0453(1) -31.60(17) 8.64258.7-262.7 0.9989 0.0016 0.2862 0.0410(7) -32.35(12) 8.64262.7-266.9 1.0001 0.0017 0.2862 0.0411(5) -31.75(7) 8.64266.9-271.3 1.0022 0.0026 0.2862 0.0419(4) -31.35(6) 8.64271.3-275.5 1.0051 0.0012 0.2862 0.0424(4) -31.15(5) 8.64275.5-284.6 1.0058 -0.0060 0.2862 0.0442(5) -30.44(8) 8.64284.6-291.9 1.0012 0.0000 0.2862 0.04047(1) -30.10(1) 8.644(1)293.7-300.7 0.9996 -0.0011 0.2862 0.0438(8) -28.90(13) 8.64300.4-304.4 0.9992 0.0020 0.2862 0.0419(8) -28.07(11) 8.64304.4-308.0 1.0012 0.0010 0.2862 0.0432(6) -27.96(8) 8.64308.0-313.0 1.0005 0.0010 0.2862 0.0419(7) -27.21(9) 8.64313.0-319.2 1.0010 0.0020 0.2862 0.0432(11) -26.69(15) 8.64

A(1) background levelA(2) background tiltA(3) ConstantA(4) Broadening parameter at P&TA(8) dipole momentA(9) dipole moment

Piecewise fit

0.75

0.80

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1.00

1.05

No

rma

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d I

nte

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-1

256 258 260 262 264 266 268 270 272 274 276 278 280 282 284 286 288 290 292 294 296 298 300 302 304 306 308 310 312Wavenumber/cm-1

Spectrum IIT = 166 KP = 34.76 TorrResolution = 0.01 cm-1

Absorption path = 52 mJ.K.G. Watson, J. Mol. Spectrosc. 153, 211-224 (1192).D. Papoušk, M.R. Aliev, Š. Urban, J. Mol. Spectrosc. 124, 285-305 (1985).

2 21 1 1 1 1

1, sin 6 ,

2 2J K

z z

iJ J b J J J J J J J J

2 30

J Kz z zJ J J J J J J

2 2 22

4 4 4 4

22 2

( 1)( , , v , ; ', , v ', ) g v , v ',

1

3 / 2 ( 1)

R

J J K K

K J KS J K J K J

J

b J J K

2 2 22

4 4 4 4

22 2

( , , v , ; ', , v ', ) g v , v ',

1/ 2

P

J J K K

K J KS J K J K J

J

b J J K

2

4 4 4 4

2 2

2

2

(2 1)( , , v , ; ', , v ', ) g v , v ',

1

1( 1)

2

1( 1) ( 1)

2

Q

J J K K

J K

JS J K J K J

J J

b J J K K

b J J K J J

Extended QM model

4 4 4 4v , sin 6 v ', / v , v ',J

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d I

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225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320Wavenumber/cm-1

Combined fit

Slight dip

Extended QM model

Spectrum IT = 166 KP = 84.56 TorrResolution = 0.01 cm-1

Absorption path = 52 m

Final fits

Abundances:CH3-CH3 at 97.871% 13CH3-CH3 at 2.129%

Final fits

Final fits

Spectrum I II

-28.78(4) -29.16(8)

8.647(2) 8.648(6)

b -1.27(2) -1.45(4)

J J 10-3 0.87(13) 1.27(32)

K K 10-3 -13.8(4) -9.6(8)

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