Direct observation of the ΔJ=2 forbidden transition in Nd i induced by the second-order hyperfine perturbation

The fluorescence of a forbidden transition from the ground state ${(}^5$I, J’ ’=4) to the 18 249.275-${\mathrm{cm}}^{\mathrm{-}1}$ (J=6) level in Nd i was directly observed by atomic-beam laser spectroscopy. This transition was found to be caused by the second-order hyperfine perturbation from the close-lying 18 249.784-${\mathrm{cm}}^{\mathrm{-}1}$ (J’=5) level. Thus the fluorescence peaks were observed only for odd isotopes of ${}_{}{}^{143}{}_{}{}^{}\mathrm{Nd}$ and ${}_{}{}^{145}{}_{}{}^{}\mathrm{Nd}$, and no peaks for even isotopes. Both the relative frequencies and the relative intensities of all the observed forbidden transitions agree well with the theoretical calculation.