Second-order magnetic contributions to the hyperfine splitting of the 5snd ¹D₂ states in ⁸7Sr

The hyperfine structure of 5snd ^1D_2 states in ^{87}Sr was studied for principal quantum numbers 11 <= n <= 25 by Doppler-free two-photon excitation combined with thermionic detection. Based on the more accurate data, compared to previous results, the hyperfine structure was analysed by second order perturbation theory using MQDT wavefunctions. As a result we find that the surprisingly large B-factors obtained by fitting the data with the Casimir formula do not reflect a quadrupolar interaction but rather consist predominantly of second order magnetic dipole contributions. Further, inconsistencies in the King plot of odd-even isotope shifts were removed by this type of analysis. The result demonstrates the importance of higher order effects in hyperfine splittings of excited states in atoms with two valence electrons, even when the fine structure splitting is large compared to the hyperfine splitting.