Microwave fourier transform spectroscopy of pure rotational ν4 ↔ ν4 transitions and reanalysis of the [formula omitted] dyad of methane-d 4

Abstract

Pure rotational transitions of a spherical top in a degenerate vibrational state have been observed directly for the first time with the help of pulsed microwave Fourier transform (MWFT) spectroscopy. Twelve rotational transitions in the v 4 = 1 vibrational excited state of CD 4 have been identified. The theoretical basis for the transition moments has been developed and the line strengths of the rotational transitions have been estimated. The measured rotational transition frequencies have been included in a reanalysis of the data from a previously recorded high-resolution FTIR spectrum of the ν 2 and ν 4 bands. The v 4 = 1 state of CD 4 is strongly coupled to the v 2 = 1 state by Coriolis interaction. Thirty molecular parameters of the ν 2 ν 4 dyad have been fitted finally from the combined microwave and infrared data. The microwave data are reproduced with a standard deviation of 42 kHz, and the infrared data with a standard deviation of 0.0002 cm −1; in each case, this is close to the estimated experimental prescision.