Transcript MJ03_DiLauro_C2H6-7um_MSS-2010.ppt

High Resolution Investigation of the Ethane
Spectrum at 7 Micron (1430 cm-1)
Carlo di Lauro, Franca Lattanzi
Dipartimento di Chimica Farmaceutica e Tossicologia,
Universita di Napoli Federico II , I-80131 Naples, Italy
Keeyoon Sung, Linda R. Brown
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Jean Vander Auwera
Service de Chimie Quantique et Photophysique, Universite Libre de Bruxelles,
CP 160/09, 50 avenue F.D.Roosevelt, B-1050 Brussels, Belgium
Arlan W. Mantz
Dept. of Physics, Astronomy and Geophysics, Connecticut College, New London, CT 06320, USA
Mary Ann H. Smith
Science Directorate, NASA Langley Research Center, Hampton, VA 23681, USA
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65th Molecular Spectroscopy Symposium, Columbus, OH, June . 2010
Ethane is abundant in planetary atmospheres
Ethane Has 12 fundamentals
but only 5 are IR active.
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C2H6 ν8 Q branches are prominent in Titan:
► The spectrum of Titan at
0.5 cm–1 resolution (apodized)
recorded by the CIRS FTIR
on the Cassini spacecraft.
► Dotted line: calculated
spectra using CH4 (HITRAN
2004) and C2H6 (based on
PNNL cross sections).
► Our goal: obtain C2H6 line
positions and intensities to
create the first HITRAN-like
database for this region.
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New GOSAT-2009 line list in HITRAN format
65th Molecular Spectroscopy Symposium, Columbus, OH, June . 2010
C2H6 spectra
at cold and room temperatures
↔
2ν4+ν9
↑
◊
ν6
ν4+ν12
ν8
◊ ◊
◊
◊◊ ◊
◊
◊
◊
◊
◊
► Red: room temperature Brussels 294 K (0.063 hPa, 13.80 m, MOPD = 450 cm)
► Blue: cold temperature JPL
131 K (1.3 hPa, 0.204 m, MOPD = 324 cm)
Right: Expanded view provides direct visual indication of high J (↔) transitions
and the ν4 + ν8 - ν4 hot band (↑)
◊ The random strong features are from residual H2O inside the evacuated FTS.
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Expanding Prior Analyses of
12C H
2 6
States
e.g. F. Lattanzi et al. , J. Mol. Spectrosc. 248 (2008)
Vib.
Band
ν4+ν12
ν8
Type &
Vib Sym
Center
(cm-1)
┴
IR Eu
1480.61
┴
IR Eu
1471.76
ν11
Raman Eg
1468
6ν4
Raman E3s
Raman E3d
1429
1358
ν2
Raman A1g
1397
IR Eu
1388
2ν4+ν9
ν6
┴
║ IR A1u
1379.15
► The 7 μm region of ethane contains four fundamentals plus
two combination states and two components of an overtone.
► This results in a high density of transitions involving rotation-torsion structure,
further complicated by hot bands arising from the ν4 state at 289 cm-1.
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Experimental Details: high resolution FTIR
FTS and Gas conditions
Light Source
Beam Splitter
Detector
Resolution (unapodized)
Normal Samples
Sample Pressure (hPa)
Path length (cm)
Temperature (K)
Useable Band Pass (cm-1)
Calibration Standards
Position precisions
JPL: Cold Temp
Brussels: Room Temp
Bruker 125 HR
Bruker 120 HR
Glower
KBr
MCT
0.0028 cm-1
C2H6
1.3
20.4
131, 150
1240 – 1850
Glower
KBr
MCT
0.0021 cm-1
C2H6
0.065, 0.11, 0.30
1380
294
878 - 1755
Residual H2O
0.0003 cm-1
N2O
0.0003 cm-1
Over 12000 positions from room temperature data
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New modeling permitted further assignments
of greatly perturbed Q branches of ν8!
► Upper left: One hot band ν4 + ν8 - ν4 easily identified
► Lower left: level crossings at high J create band head in PQ1
► Right panels: O- and S-type forbidden lines
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New assignments at much higher J
↔
..
cm-1
► R-lines of ν8, with two perturbation-activated t-transitions to ν6
with ΔK=3 (underlined and marked by a star).
► ↔ anomalous J- pattern of rQ16 and its eventual turn
is caused by the same interaction, with K=19 in ν6 and 17 in ν8.
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pQ
3
of ν8 split into 4 components by interaction
► arises from nearly
constant torsional
splitting and K-splitting
(increasing with J).
.
► the (–l), K=2 levels are
split by the l(2,–1)
interaction with the (+l),
K=1 levels [also affected
by l(2,2)-doubling].
► K- components: marked above spectrum
► K+ components: marked below spectrum
►The J-spacing decreases with increasing J
leading to a bandhead in the K– subset.
(h = hot transition, w = water line)
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► The split components
K± correspond to upper
state wavefunctions
√2 (ΨJKl ± ΨJ–K–l)
with K=2 and l = –1.
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Interaction schemes at higher K
► left: level crossings between ν8 and ν6
► right: no transitions of 3-quanta band assigned (yet)
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Effects of perturbation on rotational constant B
Effective B-values for ν8
►The strong perturbation
within ν8 levels is illustrated
using the effective rotational
constants B, as determined
from ν8 transitions.
► These points should fall on
one smoothly varying line.
► The values for levels linked
by the l (2,–1) interaction
(connected by straight lines)
show opposite deviations
from the patterns at high
| Kl |.
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Global Hamiltonian (so far)
Vibs.
ν6
ν8
ν6
ν8
E
C
C
ν4+ν12
2ν4+ν9
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ν4+ν12
2ν4+ν9
C
E
F
l(2,2)
l(2,-1)
F
E
l(2,-1)
l(2,-1)
l(2,2)
l(2,-1)
E
C
l(2,-1)
l(2,2)
65th Molecular Spectroscopy Symposium, Columbus, OH, June. 2010
Assigned IR lines of C2H6 at 7 microns
Band
cm-1
K max
J max
Assigned
ν4+ν12
1480.749
4
30
371
ν8
1471.837
20
39
3415
2ν4+ν9
1388 ?
ν6
1379.150
10
31
613
ν4+ν8 - ν4
1471.915
11
998
Cold
bands:
0.0038
4399
Hot
band
0.012
998
rms
cm-1
rms
cm-1
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ΔK=-2
ΔK=+2
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ΔK=3
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Calculated vs observed ethane spectra
► Red: observed at 294 K (0.065 hPa)
► Blue: predicted ν8, ν6, ν4 + ν12
► green: hot band: ν4 + ν8 - ν4
The random strong features are residual H2O
inside the evacuated FTS.
►transitions moments:
ν8 and ν6 from selected line intensities,
ν4 + ν12 (solely from borrowing)
ν4 + ν8 - ν4 Boltzmann scaling
► Preliminary linelist will have some
calculated positions replaced with
observed line centers.
► Line intensities (still in progress).
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Importance of ethane isotopologues
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CONCLUSION
These new linelists with lower states energies
► Characterize the line-by-line spectrum
► Improve molecular databases for planetary remote sensing
Future work
► Additional experimental efforts and theoretical analyses
to understand the whole spectrum and
provide reliable prediction of molecular line parameters
for remote sensing of planetary atmospheres.
► This study is one important step toward this ultimate goal.
JVDA acknowledges financial support from the Fonds de la Recherche Scientifique (FRS-FNRS,
Belgium, contracts FRFC and IISN), and the Actionde Recherches Concertées of the
Communauté française de Belgique. Part of the research described in this paper was performed at
the Jet Propulsion Laboratory, California Institute of Technology, Connecticut College, and NASA
Langley under contracts and grants with the National Aeronautics and Space Administration.
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Interaction at low K-values.
► Fermi resonance of ν8 and ν4+
ν12, shown for +l.
► l(2,-1) resonance within ν8,
dashed lines; shown for those pairs
where lines with ΔK=±2.
The mechanism by which the l (2,2)
splitting of K=1,+l is transmitted (by
l(2,-1) resonance to K=2,-l, (in PQ3).
► x,y-Coriolis coupling of
ν6 and 2ν4+ ν9: large torsional splitting
(~ 3 cm-1) causes a detectable
splitting in ν6 for K= 3, 4, 5.
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