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 Er¹⁶⁶, Er¹⁶⁸, and Er¹⁷⁰ have been identified. The ratio of the isotope shifts (ν₁₆₈-ν₁₇₀)(ν₁₆₆-ν₁₆₈) 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 Er¹⁷⁰ is shifted to larger wave numbers than the components due to the other even isotopes, probably arise from the two-electron transitions 4f¹¹6s6p to 4f¹¹5d².