Hyperfine structure of the 4f75d26sF11term of155,157Gd i by laser-rf double resonance

The atomic-beam laser-rf double-resonance method has been used to measure the hyperfine structure (hfs) of the J=2–8 levels of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{F}$ term of 4$f^7$5$d^2$6s in ${}_{}{}^{155}{}_{}{}^{}$,157Gd. Results are also reported for the J=2,5 levels of the ${}_{}{}^7{}_{}{}^{}\mathrm{D}$ term of 4$f^7$5d6$s^2$. Less precise hfs results are given for ten highly excited even-parity levels, and isotope shifts are given for 16 optical lines. L-S limit effective-operator expressions are given for the hfs A and B values of arbitrary states built from three open electron shells. When specialized to the ${}_{}{}^{11}{}_{}{}^{}\mathrm{F}$ term in Gd i, these expressions reveal a marked perturbation between the J=5 levels of 4$f^7$5$d^2$6s ${}_{}{}^{11}{}_{}{}^{}F$ and 4$f^7$5d6$s^2$ ${}_{}{}^7{}_{}{}^{}D$. The perturbation can also be detected from the fine-structure intervals.