Dipole moments of CH3F in the ν3 and ν6 vibrational excited states from the Stark effect

- The CH3F dipole moments in the ν3 and ν6 vibrational excited states are determined.
- The Stark second-order effects are eliminated through a difference approach.
- The standard Eurachem Guide method is used to evaluate the combined uncertainties.

Both the parallel and perpendicular rotational Stark components of the methyl fluoride J, k, l: 2, ±1, 0←1, ±1, 0 and 2, ±1, ∓1←1, ±1, ∓1 lines in the ν3 and ν6 excited vibrational states were measured in the microwave region of ca 100 GHz at the Stark fields in the range of ca 150-1100 V/cm. The analysis of the Stark components was complicated by unexpected hyperfine structure patterns spanning a few MHz, which were observed for MJ components with large Stark shifts (>300 MHz), most probably due to an enhancement through the Stark effect. The permanent electric dipole moments for these two vibrational states were derived from selected separated hyperfine components of the Stark components: μ3=1.90345(46) D, μ6=1.86394(32) D (the expanded uncertainties are given with the coverage factor k=2). The significant perturbational second-order Stark contributions were simply eliminated using a Stark component difference approach. Third-order Stark contributions were included in the analysis, and higher-order Stark contributions and polarizability contributions were neglected since they are inconsequential in this study.