Pseudoline parameters to represent n-butane (n-C4H10) cross-sections measured in the 7–15 µm region for the Titan atmosphere

Abstract

We measured temperature-dependent cross-sections of n-butane (CH3CH2CH2CH3) in the mid-infrared (7–15 μm) region in support of remote sensing for the Titan's atmosphere. For this, 28 pure and N2-mixture spectra were obtained at 180–298 K using a high-resolution Fourier transform spectrometer (Bruker IFS 125 HR) at the Jet Propulsion Laboratory. The spectral resolutions were selected to be between 0.0039 and 0.06 cm−1, depending on the sample pressures. The observed spectra were fit simultaneously to generate one single set of pseudolines, which include line intensities and lower state energies at individual pseudoline positions. The observed spectra could be reproduced by the pseudolines to within 4% through line-by-line radiative transfer calculations. The integrated intensities at 296 K were measured to be 5.06(28), 7.18(27), 0.91(4), and 49.01(20) × 10−19 cm−1/(molecule.cm−2) in the 660–860, 860–1060, 1060–1200, and 1200–1538 cm−1 regions, respectively, by summing up the pseudoline intensity parameters. These results are observed to be significantly lower than that of the PNNL (Pacific Northwest National Laboratory) cross-sections obtained at low resolution. The pseudolines are electronically compiled in the HITRAN database format, which can be readily integrated with existing radiative transfer calculations. The measured cross-sections represented by the pseudolines could provide critical laboratory input in search of elusive species that might be captured in the Cassini/CIRS and the JWST/MIRI spectral observations.