High resolution analysis of the ν26 and 2ν9-ν9 bands of propane: modelling of Titan's infrared spectrum at 13.4 µm

Abstract

Using high-resolution Fourier transform spectra of propane recorded in the 13.4 µm (resolution 0.002 cm^-1) and in the 27.4 µm (resolution 0.0011 cm^-1) spectral regions, it was possible to perform a thorough assignment of both the ν₂₆ band at 745 cm^-1 and the 2ν₉-ν₉ band at about 370 cm^-1. This simultaneous assignment was necessary since the 26¹ and 9² rotational levels interact strongly through an A-type Coriolis interaction. The experimental upper-state rotational levels were fit using a Hamiltonian matrix which takes into account explicitly this strong A-type Coriolis interaction. With this model the experimental energy levels were fit to within their experimental uncertainties and accurate band centres, rotational, centrifugal distortion and coupling constants were determined. The following band centres were derived: ν_o(ν₂₆) = 748.530882(80) cm^-1 and ν_o(2ν₉) = 740.29213(10) cm^-1. Finally, it is shown how the use of these new spectral parameters greatly improves the accuracy of modelling of Titan's infrared spectrum at 13.4 µm, relative to the currently available GEISA atlas for the same band.