Isotope Shift in the Spectrum of Neutral Lead

The isotope shift and the ${\mathrm{Pb}}^{207}$ h.f.s. in nine of the strongest PbI lines in the violet and ultraviolet regions have been measured, using material enriched in the 204 isotope. For all lines with measurable structure the 204-206 interval is less than the 206-208 interval. In the resonance line $\lambda 2833$ and in $\lambda 3639$, for both of which the measurements are almost completely unaffected by superposed h.f.s., the ratio of the 204-206 to the 206-208 interval is 0.89 and 0.82 respectively. The average of our observations and others for the staggering of the center of gravity of the 207 components for all lines is about 0.63 for the ratio of the 206-207 and 207-208 intervals. Isotope splitting of about 0.050 ${\mathrm{cm}}^{-1\mathrm{}}$ in the $6p6d$ levels contributes further evidence of perturbations by $6p7s$ and $6p8s$ levels. The staggering is discussed in terms of Breit's theory of polarizability of nuclei by electrons. These Pb isotope shifts indicate decreasing nuclear stability in the progression 204 to 208.