Quantum defects and specific-isotopic-shift measurements innsandnphighly excited states of lithium: Exchange effects between Rydberg and core electrons

By high-resolution double-resonance spectroscopy experiments in the millimeter-wavelength domain, we have determined precise values of the quantum defects in the s and p Rydberg levels of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ (principal quantum number n ranging from 18 to 40). A detailed comparison of the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ data has yielded the value of the specific isotopic shift in the np series, which is the first evidence to our knowledge of an exchange effect between a tightly bound core electron and a very weakly bound Rydberg electron in an alkali-metal atom.