Hightemperature methane absorption with a dual frequency comb

High-temperature methane absorption with a dual frequency comb spectrometer Nathan Malarich 1, David Yun 1, Sean Coburn 1, Keeyoon Sung 2, Brian Drouin 2, Greg Rieker 1 18 June 2019 1 Mechanical Engineering, University of Colorado at Boulder 2 Jet Propulsion Laboratory

Absorption database for exoplanets Absorption database for every possible chemical in atmosphere Telescope data of exoplanets transecting host star Data processing Planet temperature, which molecules in atmosphere What/where is life? NASA Goddard Wong, Bernath, Rey, Nikitin, Tyuterev The Astrophysical Journal 2019 2

Outline • • Accurate high-temperature facility Review new laboratory data Compare existing databases Introduce Smax(E”) plot – filter weak features – Identify dubious features 3

Engineering a high-temperature facility end-cap mirror Dual frequency comb laser Dry-air purge tube Methane photodetector • Design considerations for accurate database – Temperature uniformity vs natural convection – Avoid species decomposition 4

Temperature uniformity 793 782 776 767 783 Dual frequency comb laser Dry-air purge tube 772 763 photodetector 777 773 Methane 687 780 1 Thot • Natural convection loops – In situ thermocouples near laser path – Laser path skims top of cell 1 Leong end-cap mirror Tcold “Three-dimensional natural convection in a horizontal cylinder” 1998 5

Methane purity end-cap mirror Dual frequency comb laser Dry-air purge tube Methane photodetector Less absorbance CH 4 Cx. Hy + H 2 C(solid) + H 2 Wall deposits, loss of laser signal Cantera simulation, P = 1 atm 6

Dual frequency comb laser • 105 equally-spaced, 1 Hz linewidth lasers • Customizable bandwidth • . 0067 cm-1 (200 MHz) resolution – Frequencies referenced to NIST maser and GPS clock Raw spectrum 296 K, 300 Torr 7

Furnace spectra inset 8

Predicting high-temperature spectra • HITRAN 2016 – 70 K and 296 K empirical linelist 1 • Exomol – Ab initio calculations “ 10 to 10 linelist” 2 296 K 100 Torr CH 4 1 Wang, Mondelain, Kassi, Campargue JQSRT 2012 2 Yurchenko, Tennyson Mon. Not. R. Astron. Soc. 2011 9

Differences at elevated temperature 296 K 100 Torr CH 4 473 K 100 Torr CH 4 10

Predicting high-temperature spectra 296 K 11

Predicting high-temperature spectra 296 K cavity-ringdown detection limit 1 1 Wang, Mondelain, Kassi, Campargue JQSRT 2012 12

Predicting high-temperature spectra 296 K 1 Wang, Mondelain, Kassi, Campargue JQSRT 2012 2 Authors’s work 13

Can calculate maximum linestrength Smax d. S/d. T = 0 Smax Tmax 14

Filtered Exomol linelist Only 8000 lines above this linestrength filter we can see on our spectrum. so reducing Exomol lines in this region from ~1 B to ~10 K with this filter 15

Update anomalous HITRAN features Hitran updated filtered 473 K 100 Torr 16

Conclusions • Dual-frequency comb CH 4 measurements – Engineering a high-temperature facility – Updating Hitran 2016 linelist – Future work: lineshape parameters • Use Smax(E”) plots to filter weak lines and determine incorrect strong lines – Applicable to other molecules – Theoretical foundation? Funding from NASA 17