Avoided-crossing molecular-beam spectroscopy of symmetric tops. I. Phosphoryl fluoride (OPF3)

A new avoided-crossing technique using a conventional molecular beam electric resonance spectrometer has been developed for studying symmetric rotors. By means of an external electric field, two levels with different values of K are made nearly degenerate and normally forbidden electric-dipole transitions between the interacting levels are observed. Mixing matrix elements ηST with ΔK = ± 3 arise from the centrifugal distortion dipole moment μD and mixing terms ηHYP, with ΔK = ± 1, ± 2 arise from the nuclear hyperfine Hamiltonian. Explicit expressions for ηHYP are given in an Appendix. Many of these terms break the symmetry of both the rotational and nuclear spin parts of the wave functions. The avoided-crossing method is discussed in detail, with emphasis on its application to the measurement of (A0–B0). It is shown how the technique can be used to determine the perpendicular moment μD, as well as μJ, and μK, the constants which characterize the dependence of the parallel dipole moment μ on J and K, respectively. Other applications include the experimental investigation of the selection rules for the individual terms in ηHYP and the determination of the sign of the rotational g-factors [Formula: see text] and [Formula: see text].∙The method has been applied to phosphoryl fluoride (OPF3). It has been determined that (A0–B0) = 217.4987(44) MHz, μD = 5.856(20) × 10−6 D, μJ = −3.38(10) × 10−6 D, and both [Formula: see text] and [Formula: see text] are negative.