Determination of A0 and D0K of 13CH3F from the Δk = ±2 Forbidden Transitions to the v5 = 1 Vibrational Level

The Fourier transform spectrum of the vibration-rotational bands ν 2 and ν 5 of gaseous fluoromethane 13 CH 3 F was measured at an unapodized resolution 0.0041 cm −1 . Among the assigned lines, 102 belonged to the Δ k = ±2 perturbation-allowed transitions to the doubly degenerate vibrational state v 5 = 1. Combination differences were generated for the ground state of 13 CH 3 F from the wavenumbers of the allowed and forbidden vibration-rotational transitions to the v 5 = 1 level. They were used in a simultaneous least squares fit with the previously measured microwave, submillimeterwave, and far infrared transition frequencies of the pure rotational transitions in the ground state and with the combination differences generated from the allowed transitions to the v 3 = 1 state of 13 CH 3 F (Pracna, Klee, Winnewisser, Papousek, Yen-Chu Hsu, and Hann-Sen Chen, J. Mol. Spectrosc. 156, 217-226 (1992). This made it possible to determine precisely for the first time A 0 and D 0 K for 13 CH 3 F: A 0 = (5.1824246 ± 0.0000015) cm −1 , D 0 K = (70.912 ± 0.033) × 10 −6 cm −1 .