Avoided Crossings in Molecular-Beam Electric-Resonance Spectroscopy: The Observation of Forbidden ($\Delta K=\pm 1, \pm 2, \pm 3$) Transitions in Phosphoryl Fluoride (OP${\mathrm{F}}_3$)

Normally forbidden transitions obeying the selection rules $\Delta K=\pm 1, \pm 2, \mathrm{and} \pm 3$ have been observed in OP${\mathrm{F}}_3$ in the ground vibronic state by a new avoided-crossing technique based on the molecular-beam electric-resonance method. It is shown how this technique can be used in suitable symmetric rotors to study the $K$-dependent terms in the rotational Hamiltonian, the effects of centrifugal distortion on the total electric dipole moment, and the nuclear hyperfine effects off-diagnoal in $K$.