Isomer Shifts of the First and Second Excited Levels of the Ground-State Rotational Band inYb171

The isomer shifts of the 66.7-keV ${\frac{3}{2}}^{-}\to {\frac{1}{2}}^{-}$ and the 75.9-keV ${\frac{5}{2}}^{-}\to {\frac{1}{2}}^{-}$ $\gamma$ transitions of the ½[521] ground-state rotational band in ${}_{}{}^{171}{}_{}{}^{}\mathrm{Yb}$ have been investigated, using the Mössbauer effect. From the observed shifts, the ratio of change in mean square charge radii for the two excited states was found to be $\frac{\Delta 〈r^2〉\mathrm{}\left(76\right)\mathrm{}}{\Delta 〈r^2〉\mathrm{}\left(67\right)\mathrm{}}=+0.42\mathrm{}\pm \mathrm{0.20.}$ The decrease of $\Delta 〈r^2〉$ for higher rotational excitation is in disagreement with simple collective theory, and is discussed in terms of nonadiabatic effects.