N2 and H2 broadened isobutane infrared absorption cross sections and butane upper limits on Titan

Abstract

High resolution infrared absorption cross sections of isobutane, (CH3)3CH, were measured in the 1050–1900 cm−1 spectral range by Fourier transform spectroscopy. Four sample temperatures (210, 234, 265 and 296 K) were used with three pressures (10, 30 and 100 Torr) of N2 and H2 broadening gas. Spectra of pure isobutane samples were also recorded. These cross sections are useful for the interpretation of infrared spectra of the Giant Planets and Titan. The isobutane cross sections give an upper limit of 3.91e-8 for its fractional abundance on Titan based on CIRS/Cassini infrared spectra. Using n-butane, CH3(CH2)2CH3, cross sections from the literature, we obtain an upper limit of 5.13e-7 for its abundance on Titan. The combined upper limit for both isomers of butane, C4H10, on Titan of 5.52e-7 is consistent with the lower limit of recent model predictions.