Rotational parameters of the first torsional state of ethane from lower state combination differences in ν9 + ν4 − ν4

Abstract Using interferometric spectra obtained on the McMath Solar Telescope FTS spectrometer in double pass configuration (0.0025 cm −1 resolution), the ν 9 + ν 4 − ν 4 hot band of ethane has been assigned. The torsional splittings, on the order of 0.1 cm −1 , are clearly resolved. A total of 556 lower state combination differences yield the B 4 σ values 0.6605099(73), 0.6605005(44), 0.6604929(76), and 0.6604846(54) cm −1 for σ = 0, 1, 2, 3, respectively. D 4 σ J is determined to be 1.0261(68), 1.0214(37), 1.0158(49), and 1.0147(74) × 10 −6 cm −1 , respectively. D 4 σ JK yields 2.741(15), 2.724(18), 2.725(13), and 2.677(18) × 10 −6 cm −1 . Analysis of the individual σ-state results is presented. An expression is presented which characterizes the torsional dependence of the rotational constants; coefficients in the expression are determined by least-squares analysis of the rotational constants determined from analysis of the ν 9 + ν 4 − ν 4 band and the rotational parameters of the ground state. The B -value for 3 ν 4 , σ = 0, is predicted to be 0.656007 cm −1 compared to a value of 0.656090(24) cm −1 in the literature. Predictions of higher excited torsional state rotational parameters are presented.