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This paper presents the first detailed theoretical calculation of numerous observables investigated in Sr using stepwise laser excitation techniques. Measurements concern the 5p12/ns and 5p32/ns J=1 autoionising levels excited through the 5sns 1S0 Rydberg states; isolated resonances as well as interacting autoionising 5pns levels have been investigated. Results on the excitation spectra, the positions and widths of the levels as well as on the branching ratios and the angular distributions of the photoelectrons have been reported. All the above-mentioned observables are calculated through a combination of the eigenchannel R matrix and the multichannel quantum defect (MQDT) methods. Short-range parameters used by MQDT to describe final-state channel interactions are determined by performing R-matrix calculations in LS coupling and including spin-orbit effects through a geometric (jj/LS) frame transformation. The calculation of the required dipole matrix elements is based on the isolated core excitation approximation. All the measured quantities are accurately reproduced except the asymmetry parameters characterising the angular distributions of electrons ejected to the Sr+ 5s level, these latter quantities being found to be the most sensitive to the interseries interaction. In addition, the calculated MQDT parameters are compared with those previously obtained empirically from experiment.

The approach based on the eigenchannel R-matrix method and multichannel quantum-defect theory, introduced by Robicheaux and Gao to calculate two-photon processes in light alkaline-earth atoms, has been implemented in jj -coupling introducing explicitly spin-orbit effects and employing both the length and velocity forms of the electric dipole transition operator. Emphasis is placed on the identification of the intermediate-and final-state resonances appearing in the cross sections. The efficiency and potential of the method are demonstrated in magnesium, by calculating two-photon ionization cross sections, branching ratios and photoelectron angular distributions for an extended photon energy range, including above-threshold ionization. The contributions of electronic correlation, core-polarization and spin-orbit effects are investigated by a systematic comparison with experimental results and earlier theoretical calculations.

6p3,,nl(n=1lto15, 1=5ton-I)and6d,,,nl(n=Il to 14,1=6ton-I)doubly excited Rydherg states of barium have been studied using a multistep laser excitation technique. The excitation is via the hound 6s,,,nl states which are populated using an electric-field switching method which allows us the selective population of high-l angular momentum states. The resulting spectra clearly demonstrate the presence of dielectronic correlation. We show that this correlation is well described in terms ofthe ( j -I)K coupling scheme, taking into account in a perturbalive way the dipole and quadrupole interadion terms.