Analysis of the ν2 + ν3 band of 12CH 4 and 13CH 4

Abstract

The ν 2 + ν 3 bands of 12CH 4 and 13CH 4 occurring in the region 4400–4650 cm −1 have been studied from spectra recorded with a high-resolution Fourier transform spectrometer (resolution better than 0.01 cm −1). Champion's Hamiltonian expansion, Canad. J. Phys. 55, 1802 (1977), is applied to the problem of the two interacting F 1 and F 2 vibrational sublevels of this type of a band. As the P branch of ν 2 + ν 3 is strongly overlapped by neighboring bands, a combination-difference method, adapted to tetrahedral XY 4 molecules has been developed to help assignments of lines. A fit of 700 transitions has been performed using 13 new effective constants in the case of 12CH 4. In the case of 13CH 4, 532 transitions have been fit to 18 constants. The known parameters, relative to the vibrational ground state and the ν 3 state for both methanes, and the ν 2 state for 12CH 4 were fixed throughout. Most of the perturbed levels, up to J′ = 12, are well reproduced and the general agreement between experimental and calculated transitions is satisfactory with standard deviations of 0.047 cm −1 ( 12CH 4) and 0.041 cm −1 ( 13CH 4). The results (order of magnitude of obtained ( ν 2 + ν 3) parameters and comparison of observed and computed intensities) indicate that the ν 2 + ν 3 band is perturbed by many other bands.