Precision FT-IR laboratory measurements of atmospheric molecule

Abstract:

Spectroscopic information is crucial a priori input to interpret atmospheric spectroscopic observations through radiative transfer modeling. The spectroscopic observations lead us to determine the physical and chemical conditions (e.g., atmospheric pressure, temperatures, composition, abundances …). In order to avoid false interpretations of the observed spectra, the molecular spectroscopic information (either line parameters or cross sections) must be sufficiently accurate and complete.

 To achieve this goal, we employ a broad-band Fourier transform spectrometer, Bruker IFS-125HR equipped with multiple detectors (He-cooled bolometers, N₂-cooled MCT and InSb, warm InGaAs, Boron-doped silicates, photodiode) and beamsplitters (Mylars, KBr, CaF₂, Quartz) sufficient to cover entire infrared spectra region from 20 to 15000 cm^-1. We vacuum-couple up to 15 different absorption cells to the FT-IR, including five coolable cells with optical path lengths ranging from 0.02 m to 52 m; three of them are capable of reaching temperatures down to 50 K. We also have one heatable cell reaching 400 K. Finally, an extra vacuum chamber newly implemented to the Bruker 125HR enables both emission and absorption spectroscopy utilizing the emission port of the FT-IR.

Recently, several studies of atmospheric molecules have been completed using the FT-IR at JPL in support of the Earth, terrestrial and planetary atmospheric remote sensing. These include analyses of C₃H₈ (propane) and ¹⁶O¹²C¹⁷O mid- and near-infrared regions, ¹³C¹²CH₆ and ¹²C₂H₆ at longer wavelengths. In addition, we are studying the O₂(A) line mixing and collision-induced absorption in the O₂ A-band at 0.76 µm, as well as temperature-dependent cross sections of C₆H₆ (benzene) and C₃H₆ (propene) in the mid-infrared region. We will present an overview of results and work in progress.

[Research described in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, Connecticut College, and NASA Langley Research Center, under contracts and cooperative agreements with the National Aeronautics and Space Administration.]