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

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

CH4 POLYADS

S. Albert et al. 2009, Chem. Phys. 356, 131

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

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)

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

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

EMISSION EXPERIMENTAL SETUP

Hargreaves et al. 2012, ApJ 757, 46

PREVIOUS RESULTS

Dyad

PentadOctad

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

LOWER STATE ENERGIES

Dyad (ν4) Octad (ν3 + ν4)Pentad (ν3)

Empirical

HITRAN 2008

PROBLEMS WITH EMISSION

Intensiti es are notoriously diffi cult to calibrate

Self absorpti on Particularly for octad region

Self absorption: T

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

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

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

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

LOWER STATE ENERGIES

Old method New method

Pentad

ν3

ν3+ν4-ν4

Octad Pentad Octad

ν3+ν2

ν3+ν1

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

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

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).