EXPERIMENTAL ABSORPTION SPECTRA OF HOT CH 4 IN THE PENTAD AND OCTAD REGION ROBERT J. HARGREAVES MICHAEL DULICK PETER F. BERNATH MONDAY 16 TH JUNE 2014

CH 4 POLYADS ModeDegeneracyBand Origin (cm -1 )Type ν 1 (a 1 )12914Symmetric C-H stretch ν 2 (e)21526Bend ν 3 (t 2 )33020 a Asymmetric C-H stretch ν 4 (t 2 )31306 a Bend ν2ν2 ν3ν3 ν4ν4 ν1ν1 T d symmetry a infrared active ν 1 ≈ ν 3 ≈ 2ν 2 ≈ 2ν 4

CH 4 POLYADS S. Albert et al. 2009, Chem. Phys. 356, 131

MOLECULAR ATMOSPHERES The Sun K (e.g., CN, OH, CH, NH) Sunspots K (e.g., H 2 O, TiO) Brown Dwarfs Dwarf Stars Stars Exoplanets H+H+ Diatomic Molecules Polyatomic Molecules Temperature / K EARTH – 296 K HITRAN database H 2 O NH 3 CH 4 0

MOLECULAR ATMOSPHERES Brown Dwarfs Dwarf Stars Stars Planets H+H+ Diatomic Molecules Polyatomic Molecules Temperature / K H 2 O NH 3 CH 4 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. Hurt (Caltech/IPAC)

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  Lower state energy, E’’  Partition function, Q T

EMISSION EXPERIMENTAL SETUP Hargreaves et al. 2012, ApJ 757, 46

PREVIOUS RESULTS Dyad Pentad Octad

 From line strength equation ( S’ ):  Rearranging to give:  All lines were wavenumber and intensity calibrated to HITRAN 2008 (Rothman et al. 2009) EMPIRICAL LOWER STATE ENERGIES

LOWER STATE ENERGIES Dyad (ν 4 ) Octad (ν 3 + ν 4 )Pentad (ν 3 ) Empirical HITRAN 2008

PROBLEMS WITH EMISSION  Intensities are notoriously difficult to calibrate  Self absorption  Particularly for octad region Self absorption: T

Halogen Lamp NEW ABSORPTION CELL CH 4 pumped out CH 4 flows in Furnace Heating elements Quartz holder 75 cm 12 cm 15 cm45 cm FTS spectrometer  New method requires four spectra

 Same method carried out for each spectrum 1.Hot CH 4 + Lamp (hot absorption) 2.No CH 4 + Lamp (absorption baseline) 3.Hot CH 4, no lamp (hot emission) 4.No CH 4, no lamp (background baseline)  4 times 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 decomposition of CH 4  23°C, 200°C, …, 1000°C RESULTS SUMMARY

NEW CH 4 SPECTRA 1: Hot CH 4 + Lamp (+ background T) 3: No sample + Lamp (+ background T) 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) Demonstrates Kirchhoff’s Law of thermal radiation for optically thick lines ε = 1 - α

LOWER STATE ENERGIES Old method New method Pentad ν3ν3 ν3+ν4-ν4ν3+ν4-ν4 Octad PentadOctad ν3+ν2ν3+ν2 ν3+ν1ν3+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…  Still requires the addition of HITRAN  Not a problem as these are the strong lines  Intensities improvements need investigating  Continue investigating the new absorption 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 fitting  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).