Hyperfine-InducedUngerade-GeradeSymmetry Breaking in a Homonuclear Diatomic Molecule near a Dissociation Limit:I2127at theP322−P122Limit

A symmetry breaking of the ungerade-gerade character through hyperfine perturbation has been directly observed for the first time in a homonuclear diatomic molecule (${}_{}{}^{127}{}_{}{}^{}\mathrm{I}_2^{}{}_{}{}^{}$). An important consequence of this symmetry breaking is the mixing of ortho and para modifications. Evidenced by anomalies in the observed hfs of high-lying rovibrational levels in the $B{0_{\mathrm{u}}}^{+}$ state and extra lines, it is due to a hyperfine perturbation of this state by a 1g state sharing the same dissociation limit. Such a situation should be met in other molecules.