Core polarization and quantum defects in high-angular-momentum states of alkali atoms

We discuss the systematics of energy levels of high-angular-momentum Rydberg states of alkali-metal atoms, and show that they are well described by a quantum-defect model. Polarization of the core electrons by the valence electron is shown to be the dominant contribution to the quantum defect. As an example, recently observed splittings between the $l=3,4,\mathrm{and} 5$ levels in various Rydberg states of sodium are analyzed to yield values for the core polarizabilities. Effective values of the dipole and quadrupole polarizability of 1.0015(15) and 0.48(15) a.u., respectively, are obtained. Implications for the precise determination of the structure of highly excited Rydberg states in alkali atoms are discussed.