Mid-infrared cross-sections and pseudoline parameters for trans-2-butene (2-C4H8)

We measured cross-sections for trans-2-butene (trans-2-C4H8: CH3-CH=CH-CH3) in the mid-infrared (800-1533 cm-1) in support of remote sensing of Titan's atmosphere. Trans-2-butene is one of many C4 hydrocarbons that is predicted to be present in Titan's atmosphere in detectable abundance, but it has also eluded detection thus far. We collected 20 pure and N2-mixture spectra at temperatures between 180-297 K using a high-resolution Fourier transform spectrometer (Bruker IFS 125 HR) at the Jet Propulsion Laboratory. Spectral resolutions were selected between 0.0039 and 0.062 cm-1, depending on sample pressures. We observed several fundamental modes of vibration and compared their updated band centers against those previously reported in the literature. Our observations span the 800-1533 cm-1 range, which contains multiple vibrational bands and hot band features, and we report temperature-dependent cross-sections in this spectral range. To ensure that our cross-section results are reliable measures of band intensities, we also collected an additional set of 8 room temperature N2-mixture spectra, investigating the linearity between integrated absorbance and optical burden. We fit all of the observed spectra simultaneously to derive a single set of pseudoline parameters, which include intensities and lower-state energies at separate frequency bins, which are considered as pseudoline positions. The pseudolines are observed to be able to reproduce the observed spectra via line-by-line radiative transfer calculations to within a few percent across the spectral region. The pseudoline list is compiled in HITRAN database format, making it readily integrable in existing radiative transfer codes. The absorption cross-sections represented by the pseudolines provide important laboratory input to the search for elusive molecules in future observations with JWST/MIRI or ground-based observatories such as NASA's Infrared Telescope Facility, as well as in the Cassini/CIRS data archive.

Automated summary

We measured cross-sections for trans-2-butene in the mid-infrared range to support remote sensing of Titan's atmosphere. The results provide laboratory input for future observations and data analysis. The study achieved a high-resolution characterization and compiled the data in a readily integrable database format.