Saturation spectroscopy with a CO2 waveguide laser: Analysis of the ν6 bands of CH3 79Br and CH3 81Br

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Abstract
About 100 Doppler‐free hyperfine resonances arising from rovibrational transitions of methyl bromide have been observed and accurately measured by means of saturation laser spectroscopy using an absorption cell inside the cavity of a CO2 waveguide laser. Most of these transitions have been assigned to the ν6 fundamental bands of CH3 79Br and CH3 81Br. Assignments were deduced from computed spectra and checked with the resolved hyperfine quadrupole structure. An overall fit including recent microwave line frequency measurements, infrared inverted Lamb dip frequencies, infrared laser Stark measurements, and submillimeter emission frequencies yields a set of molecular parameters for the ν6 band of CH3 79Br and CH3 81Br. With respect to previous values, an improvement of one to two orders of magnitude is obtained for all the molecular parameters of the ν6 band. In particular, all the sextic centrifugal distortion constants are obtained significantly from the fit.