Forbidden Linesλ2967.5andλ2269.8of Mercury HgI

The structure of the forbidden lines $\lambda 2967.5 (6^1{D}_2\to 6^{3}P_{}^{\mathrm{o}}{}_0{}^{})$ and $\lambda 2269.8 (6^{3}P_{}^{\mathrm{o}}{}_2{}^{}\to 6^1{S}_0)$ of neutral mercury was studied with a high resolving power in order to get evidence for their perturbational nature. It was predicted some time ago by several authors that these lines should be emitted only by the odd isotopes of mercury. The emission would be of the electric dipole type, the selection rule $\Delta J=0,\pm 1$ being partially invalidated by the weak interaction of the outer electrons with the nuclear magnetic moment. Only one line, namely $\lambda 2655.8$ ($6^{3}P_{}^{\mathrm{o}}{}_0{}^{}\to 6^1{S}_0$) of mercury was analyzed in the past and definitely shown to belong to this category. The structure of the line $\lambda 2967.5$ was fully resolved in the third order of a 30-foot Chicago grating. The structure of the line $\lambda 2269.8$ was obtained by means of an aluminized Farby-Perot etalon and a medium size quartz spectrograph. All components predicted for both cases were observed, except a very weak one for the line $\lambda 2269.8$. The distances and intensities are in a very good agreement with theoretical predictions, hence establishing beyond doubt the nuclear perturbational nature of the forbidden transitions in the spectra of unperturbed atoms of zinc, cadmium, and mercury.