Hyperfine-Induced Lifting of Parity Degeneracy in Noninverting Molecules

We report experimental evidence for the splitting of $E$ levels in ${}^{28}{\mathrm{SiF}}_4$, which are degenerate in parity as far as the rovibrational Hamiltonian solely is concerned. The parity degeneracy of rovibrational levels is in fact lifted by hyperfine interactions, even though the associated total spin is $I=0\mathrm{}$. We compare this situation with that in other molecules (S${\mathrm{F}}_6$ and P${\mathrm{H}}_3$) for which similar effects have already been reported. We also predict the splitting of the $P\left(7\right)E^0(\pm )$ line of ${}^{12}$C${\mathrm{H}}_4$, a resonance used currently as a frequency standard, assumed up to now to be perfectly degenerate.