HIGH-RESOLUTION ANALYSIS
OF VARIOUS PROPANE BANDS:
MODELING OF TITAN'S
INFRARED 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
Propane: spectroscopic data
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unpublished measurements by S. Daunt.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
CIRS Propane Band Detections:13-11 μm
CIRS Propane Band Detections:11-9 μm
CIRS Propane Band Detections:8-6 μm
Propane: New spectroscopic data
Need of line by line list to model the spectra
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 by
Flaud, 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).
CIRS Propane Band Detections: 8-6 μm
Hamiltonian matrix used to calculate the
{241, 51 ,171, 41 } interacting states of propane.
241
51
171
241
HW
51
CA
HW
171
CB
CC
HW
41
CA
F
CC
41
HW
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 Rq0 and pQ1 lines of ν24 are highly perturbed.
Propane abundances on Titan
Region
Coustenis Vinatier et
et al.(2007) al.(2007)
This work
ν26(748 cm-1)
5.0(1.0)
4.2(0.5)
4.5(1.5)
ν18(1376 cm-1)
5.7(0.8)
ν24(1472 cm-1)
16.4(0.8)
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
Number of levels
Jmax
Kmax
0.0000≤δ<0.0003
0.0003≤δ<0.0006
0.0006≤δ<0.0012
0.0004≤δ<0.0035
Std, Deviation
(10-3 cm-1)
261
1062
58
31
78.5%
13.3%
6.4%
1.8%
92
741
46
26
70.6%
18.1%
7.3%
4.0%
0.41
The lines marked with a ‘*’ belong to the 9 cold band
Spectra
Residuals
(χ2 reduces
from 6.9 to 2.4)
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
Propane abundances on Titan
Region
Coustenis Vinatier et
et al.(2007) al.(2007)
This work
ν26(748 cm-1)
5.0(1.0)
4.2(0.5)
6.6(0.7)
5.7(0.8)
ν18(1376 cm-1)
4.5(1.5)
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
101
HW
Herm conj
Herm conj
Herm conj
81
CA
HW
Herm conj
71
CA
F
HW
41
CB
61
CA
61
Herm conj
Herm conj
Herm conj
CC
HW
Herm conj
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
Mixing coefficients (%)
50
40
30
20
Ka =8
10
Ka =7
0
5
10
15
20
25
J
30
35
40
.A portion of the mono-13C ethylene spectrum showing absorption lines
of the forbidden ν4 band.
o
Forbidden Q10 branch of 4
p
Q10 branch of 7
10
23
Absorption
24
10
Obs.
25
J=30
Calc.
877.3
877.4
877.5
877.6
877.7
-1
Wavenumber (cm )
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.
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!!