experimental absorption spectra of hot ch 4 in the pentad and octad region robert j. hargreaves...
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EXPERIMENTAL ABSORPTION SPECTRA OF HOT CH4 IN THE
PENTAD AND OCTAD REGION
ROBERT J. HARGREAVE
Srha rg rea@odu .edu
MICHAEL DULICK
mdu l i ck@odu .edu
PETER F. BERNATH
pberna th@odu .edu
MONDAY 16 T H JUNE 2014
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CH4 POLYADS
Mode Degeneracy Band Origin (cm-1) Type
ν1 (a1) 1 2914 Symmetric C-H stretch
ν2 (e) 2 1526 Bend
ν3 (t2) 3 3020a Asymmetric C-H stretch
ν4 (t2) 3 1306a Bend
ν2 ν3 ν4ν1
Td symmetry
a infrared active
ν1 ≈ ν3 ≈ 2ν2 ≈ 2ν4
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CH4 POLYADS
S. Albert et al. 2009, Chem. Phys. 356, 131
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MOLECULAR ATMOSPHERES
10002000300040005000600070008000
The Sun - 5800 K (e.g., CN, OH, CH, NH)
Sunspots - 3200 K (e.g., H2O, TiO)
Brown Dwarfs
Dwarf Stars
Stars
Exoplanets
H+
Diatomic MoleculesPolyatomic Molecules
Temperature / K
EARTH – 296 KHITRAN database
H2O NH3 CH4
0
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MOLECULAR ATMOSPHERES
10002000300040005000600070008000
Brown Dwarfs
Dwarf Stars
Stars
Planets
H+
Diatomic MoleculesPolyatomic Molecules
Temperature / K
H2O NH3 CH4
0
Brown dwarfs Not planets <0.08 M
H fusion cannot occur deuterium burning (not planets)
L dwarfs characterised by FeH and CrH ( in near IR)
T dwarfs have strong H 2 O and CH 4 (overtones)
R. H
urt (
Calte
ch/I
PAC)
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CH 4
Most abundant molecule in Jupiter and Saturn
Major feature of exoplanets (hot Jupiters)
BROWN DWARFS & EXOPLANETS
~1400 K
~800 K
Cushing et al., 2006, ApJ 648, 614
Swain et al., 2008, Nature 452, 329
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ASTRONOMICAL REQUIREMENTS
If HITRAN (Rothman et al. 2012) is not appropriate we need: CH4 spectra for direct comparison
T = 500 – 2000 K Mid IR up to visible
Calculated line list
In order to obtain the column densiti es (Nl), what is needed?
From Beer-Lambert law:
Line strength:
Therefore we need to know Line positi on, ν Square of transiti on dipole moment, S J ’ J ’ ’
𝑺′=2𝜋 2𝝂𝑺 𝑱 ′ 𝑱 ′ ′
3 𝜀0h𝑐𝑄𝑇
exp(− 𝑬 ′ ′
𝑘𝑇 )[1−exp (− h𝝂𝑘𝑇 )] Lower state energy, E’’ Parti ti on functi on, QT
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EMISSION EXPERIMENTAL SETUP
Hargreaves et al. 2012, ApJ 757, 46
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PREVIOUS RESULTS
Dyad
PentadOctad
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From line strength equati on (S’):
Rearranging to give:
All l ines were wavenumber and intensity calibrated to HITRAN 2008 (Rothman et al. 2009)
EMPIRICAL LOWER STATE ENERGIES
𝑆 ′
𝑆0′ =
𝑄0
𝑄exp ( 𝐸
′ ′
𝑘𝑇 0
−𝐸 ′ ′
𝑘𝑇 )[ 1−exp (− h𝜈𝑘𝑇 )1− exp(− h𝜈𝑘𝑇 0
) ]ln ( 𝑆𝑄 𝑅0
𝑆0𝑄0𝑅 )=− 𝑬 ′ ′
𝑘𝑇+𝐶0
𝑅=1−exp (− h𝜈𝑘𝑇 )𝐶0=𝐸 ′ ′
𝑘𝑇 0
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LOWER STATE ENERGIES
Dyad (ν4) Octad (ν3 + ν4)Pentad (ν3)
Empirical
HITRAN 2008
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PROBLEMS WITH EMISSION
Intensiti es are notoriously diffi cult to calibrate
Self absorpti on Particularly for octad region
Self absorption: T
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Halogen Lamp
NEW ABSORPTION CELL
CH4 pumped out
CH4 flows in
Furnace
Heating elements
Quartz holder
75 cm
12 cm
15 cm 45 cm
FTS spectrometer
New method requires four spectra
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Same method carried out for each spectrum1. Hot CH4 + Lamp (hot absorpti on)
2. No CH4 + Lamp (absorpti on baseline)
3. Hot CH4, no lamp (hot emission)
4. No CH4, no lamp (background baseline)
4 ti mes longer than previous method!
60 Torr of CH 4
600 scans (~4 hours) 0.02 cm - 1 10 Temperatures
Limited to below 1000°C due to decompositi on of CH 4
23°C, 200°C, …, 1000°C
RESULTS SUMMARY
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NEW CH4 SPECTRA
1: Hot CH 4 + Lamp (+ background T )
3: No sample + Lamp (+ background T )
τ=1−23−4
2: Hot CH 4 + no lamp (+ background T )
4: No sample + no lamp (+ background T )
500°C
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EQUILIBRIUM
1: Hot CH 4 + Lamp (+ background T )
2: Hot CH 4 + no lamp (+ background T )
DemonstratesK i rchhoff ’s Law of thermal rad iati on for opti ca l ly th ick l ines
ε = 1 - α
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LOWER STATE ENERGIES
Old method New method
Pentad
ν3
ν3+ν4-ν4
Octad Pentad Octad
ν3+ν2
ν3+ν1
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Comparisons at 800°C for pentad and octad
1 to 1 ratio maintained between old and new method
Comparisons with HITRAN are underway
INTENSITY COMPARISON
Old vs new
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Analysis is ongoing… Sti ll requires the addition of HITRAN
Not a problem as these are the strong lines Intensities improvements need investigating
Conti nue investi gati ng the new absorpti on method Further into the near IR Tetradecad (5000 – 6500 cm -1) Region can only be studied in absorption
Spectra will help with further assignments Multispectral fi tti ng LabFit (developed by D. C. Benner)
FUTURE WORK
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THANKS FOR LISTENING
This work has been funded by a NASA laboratory astrophysical grant.
Previous work was carried out at the University of York (UK).