Multichannel quantum-defect approach for two-photon processes

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Abstract
We present a method which allows the efficient calculation of the amplitudes for two-photon processes based on variations of the eigenchannel R-matrix method and of the multichannel quantum-defect theory (MQDT). This method contains the flexibility to describe structure due to intermediate- as well as final-state Rydberg series, autoionizing series, and continua. Because the method is based on MQDT, we would be able to describe easily two-photon processes in negative ions as well as in any of the situations previously described by the generalization of MQDT in one-photon processes. We present calculations for two-photon absorption from the ground state of Mg and Ca.