Hyperfine lifting of parity degeneracy and the question of inversion in a rigid molecule

This Letter demonstrates that the parity degeneracy of isolated vibration-rotation states of a rigid molecule can be removed by hyperfine interactions. Hyperfine structures of infrared transitions in ${\mathrm{PH}}_3$ are obtained, by saturation spectroscopy around 10 μm. One of these is used to demonstrate unequivocally that degenerate states of opposite parity are separated by the proton hyperfine interactions. We show that alternative explanations involving the physical inversion of the molecule are completely inconsistent with the details of the spectra obtained. In addition to the splitting conclusion, which has very wide generality, new confidence limits are placed on the possible inversion splitting.