Rydberg series of alkaline-earth atoms Ca through Ba. the interplay of laser spectroscopy and multichannel quantum defect analysis

Abstract

The purpose of this paper is to show how the strong interplay between the laser spectroscopy measurements and theoretical analyses performed using the multichannel quantum defect theory (MQDT) has considerably enlarged our knowledge of Rydberg states if alkaline-earth atoms Ca through Ba. The major motives for the interest in the two-electron systems is the ability to study interacting Rydberg series. The perturbations of principal Rydberg series by doubly-excited states are reflected not only on the level structure but also in various observable quantities such as oscillator strenghts, lifetimes, hyperfine structure, isotope shifts, Landé factors. The MQDT is a powerful tool for studying the mutual interaction of perturbing series but sometimes energy values represent a too limited data set to derive reliable wavefunctions. Experimental data on observables other than energies are then of prime importance to check or even extend MQDT models deduced from energies. A large part of the paper is devoted to the new possibilities offered by high-resolution spectroscopy to analyze state-mixing and thus to probe MQDT wavefunctions. Numerous illustrative examples as well as several prospects for future experimental and theoretical investigations are presented throughout this paper.