HIGH-RESOLUTION ANALYSIS OF VARIOUS PROPANE BANDS:

MODELING OF TITAN'SINFRARED SPECTRUM

J.-M. Flaud

Propane - Historical Perspective

• First identification of C3H8 on Titan came from Voyager IRIS (Maguire et al. Nature,1981)

• Although multiple bands identified, the S/N was poor,

• Only the 26 band at 721 cm-1 was ever used for VMR determination (papers by Coustenis et al.)

CIRS Titan Spectrum

• Instrument and mission

• Titan spectrum

• CIRS composition - overview

Cassini Composite Infrared Spectrometer (CIRS)

CIRS

When starting the study:

-line data only publicly available (GEISA 1992 and later) for the v26 mode at 748 cm-1, based on unpublished measurements by S. Daunt.

- Medium resolution lab absorption spectra courtesy of S. Sharpe, PNNL recorded at room temperature

• Propane has 27 IR modes with low energy modes: v14 (216 cm-1), v27 (268 cm-1), v9 (369cm-1)

lots of strong hot bands

Propane: spectroscopic data

CIRS Propane Band Detections:13-11 μm

CIRS Propane Band Detections:11-9 μm

CIRS Propane Band Detections:8-6 μm

We included in our study a new set of propane lines for

several bands v19(1338), v18 (1376),),{v24 ,v4}(1472)in the region 1300-1500 cm-1, as measured and modeled byFlaud, Lafferty and Herman, J. Chem. Phys(2001).

We checked the fundamental mode v26 intensities in

GEISA and found that they are about x 2.38 too high; probably because the band sum was scaled to lab spectra that includes hot bands

These enable an independent measurement of the propane abundance from a different CIRS focal plane (FP4) to the v26 (FP3).

Propane: New spectroscopic data

Need of line by line list to model the spectra

CIRS Propane Band Detections: 8-6 μm

241 51 171 41

241 HW

51 CA HW

171 CB CC HW

41 CA F CC HW

Hamiltonian matrix used to calculate the

{241, 51 ,171, 41 } interacting states of propane.

HW : Watson-type Hamiltonian. CA , CB , CC : a-, b-, c-type Coriolis interactions. F: Fermi-type interaction

Mixing coefficients of the J0,J and J1,J levels of the 41 state into the 241 state of propane. The different mixing coefficients lead to an inversion of the J0,J and J1,J states starting at J=15.

Central region of the C-type jet-cooled ν24 band of propane. Because of the strong A-type Coriolis interaction with the levels of 41, the Rq

0 and pQ1 lines of ν24 are highly perturbed.

Region Coustenis et al.(2007)

Vinatier et al.(2007)

This work

ν26(748 cm-1) 5.0(1.0) 4.5(1.5) 4.2(0.5)

ν18(1376 cm-1) 5.7(0.8)

ν24(1472 cm-1) 16.4(0.8)

Propane abundances on Titan

Propane – Summary

All four bands of propane tentatively identified by IRIS are now clearly seen by CIRS at much higher S/N.

In addition 3-4 further bands have now been detected.

Abundances retrieved here agree well with previous results for ν26, and with new ν18 measurement.

ν24 measurement in very poor agreement: probably due to continuum fitting and/or aliasing.

v26 needs to be re-measured for: - Better modeling and better accuracy- Missing hotbands,.

Hamiltonian matrix used to calculate the

{261, 92} interacting states of propane.

261 92

261 HW CA

92 CA HW

Vibrational State 261 92

Number of levels 1062 741

Jmax 58 46

Kmax 31 26

0.0000≤δ<0.0003 78.5% 70.6%0.0003≤δ<0.0006 13.3% 18.1%0.0006≤δ<0.0012 6.4% 7.3%0.0004≤δ<0.0035 1.8% 4.0%

Std, Deviation (10-3 cm-1)

0.41

The lines marked with a ‘*’ belong to the 9 cold band

Titan spectra average, 30S–30N latitude, 100–150km,(black line) compared to three models: (i)C2H2, C2H6 and HCN only (no propane); (ii) GEISA 2003 propane atlas; (iii) This work

Residuals

Spectra

(χ2 reduces from 6.9 to 2.4)

Region Coustenis et al.(2007)

Vinatier et al.(2007)

This work

ν26(748 cm-1) 5.0(1.0) 4.5(1.5) 4.2(0.5)

6.6(0.7)

ν18(1376 cm-1) 5.7(0.8)

Propane abundances on Titan

Propane – Next Steps

Need further theoretical work and lab spectroscopy to measure line positions and intensities and model the remaining bands:

ν8 – 860 cm-1

v21 – 922 cm-1

v20 – 1054 cm-1

v7 – 1157 cm-1

PROPANE AND TITAN

I would like to thank:

W. J. Lafferty, F. Kwabia, C. A. Nixon, D. E. Jennings, B. Bézard, N. A. Teanby, P. G. J. Irwin, T. M. Ansty, A. Coustenis, F. M. Flasar.

High resolution analysis of the ethylene-1-13C spectrum in the 8.4–14.3-μm region

J.-M. Flaud , W.J. Lafferty, Robert Sams, V. Malathy Devi, Journal of Molecular Spectroscopy 259 (2010) 39–45

Hamiltonian matrix used to calculate the {101, 81, 71, 41, 61} interacting states of ethylene -1-13C

101 81 71 41 61

101 HW Herm conj Herm conj Herm conj

81 CA HW Herm conj Herm conj

71 CA F HW Herm conj Herm conj

41 CB CC HW Herm conj

61 CA CA CB HW

Range of quantum numbers observed for experimental energy levels and a statistical analysis of the results of the

energy level calculation for the 101, 81, 71 and 41 ro-vibrational levels of ethylene -1-13C

Vibrational State 101 81 71 41

Number of levels 205 241 708 57

JMax 27 28 44 29

KMax 11 8 18 7,8 series mainly

0.000≤δ<0.001 88.3% 97.5% 95.6% 86%

0.001≤δ<0.002 11.2% 2.5% 4.2% 11.3%

0.002≤δ<0.0033 0.5% 0.2% 1.8%

Std, Deviation (10-3 cm-1)

0.48

Mixing coefficients of the Ka=7 and 8 rotational levels of 41 onto the 71 state for mono-13C ethylene

.A portion of the mono-13C ethylene spectrum showing absorption lines of the forbidden ν4 band.

Lines involving the Ka = 8 upper state levels of 41 are seen only because they borrow their intensity from the pQ10 line of the strong ν7 band. Unlabeled strong lines are the ν7 transitions.

877.3 877.4 877.5 877.6 877.7

J=30

24

Wavenumber (cm-1)

Ab

sorp

tion

Obs.

Calc.

Forbidden oQ10

branch of 4

1023

pQ10

branch of 7

25

10

Mono-13C ethylene in Titan

Problem: Discrepancy of ~40% in measured absolute intensities between high and low resolution spectra!!!

CONCLUSION

More high resolution spectroscopic work is needed for a lot of “difficult” molecules (Lots of vib-rot interactions, tunneling effects,…) in order to provide line by line lists (including hot bands)Or For “heavy” molecules cross sections measurements (Various P and Ts)

PROBLEM: BOTH require a lot of work!!