Isotope Shifts in Erbium

An investigation of the erbium isotope shifts has been conducted in the region between 4000 and 7000A using the dispersion of a Fabry-Perot interferometer crossed with a Steinheil three-prism glass spectrograph. Structures were measured in 68 lines, and components due to the even isotopes ${\mathrm{Er}}^{166}$, ${\mathrm{Er}}^{168}$, and ${\mathrm{Er}}^{170}$ have been identified. The ratio of the isotope shifts $\frac{({\nu }_{168}-{\nu }_{170})}{({\nu }_{166}-{\nu }_{168})}$ is close to unity in nearly all lines measured, and it is concluded that the undisturbed value of the ratio probably does not differ from unity by more than 4 percent. Electron configurations have been proposed to describe the observed shifts, and in particular those lines, in which the component due to ${\mathrm{Er}}^{170}$ is shifted to larger wave numbers than the components due to the other even isotopes, probably arise from the two-electron transitions $4f^{11}6s6p$ to $4f^{11}5d^2$.