Polarization, penetration, and exchange effects in the hydrogenlike nf and ng terms of cesium

Abstract

We have made precision measurements of the 4f–ng transitions (n = 5–11) of cesium observed in emission by using high-resolution Fourier spectroscopy. The 2G fine-structure intervals are found to have normal ordering and a slightly greater than hydrogenic splitting. Each of the 2F and 2G series of levels can be represented by a polarization formula, but the effective dipole and quadrupole polarizabilities derived from the two series differ widely. By a series of calculations using Hartree–Fock wave functions, we show that penetration and core–valence exchange effects, which are neglected in the polarization formula, contribute up to 20% of the departure from the hydrogenic nf term value. To a good approximation this explains the observed discrepancy in polarizabilities. The effective polarizabilities we obtained are compared with values measured by other experimental techniques. The role of nonadiabatic effects in limiting the accuracy and utility of polarizabilities derived from spectral data is discussed. Based on our 2G data and the best available values for the 2S and 2F levels, we find the Cs i ionization energy to be 31406.4556(20) cm−1.