Multiphoton spectroscopy of doubly excited, bound, and autoionizing states of strontium

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Abstract
We present a theory attempting an interpretation of existing data on three- and four-photon ionization of Sr in the wavelength range 557–575 nm and at 532 nm. Two-photon resonances with bound excited states and three-photon resonances with doubly excited autoionizing states are studied. ac Stark shifts and broadening of resonances, as well as singlet-triplet transitions, are shown to yield interesting new information on atomic structure. The absorption of a fourth photon above the threshold leads to excited final ionic states, with autoionizing states having served as intermediate three-photon resonances. The calculation is based on a configuration-interaction scheme, with singlet-triplet mixing taken into account either through jl coupling or phenomenologically by adjustment to some aspects of the data. The apparatus of resonant multiphoton theory is combined with the above scheme. Depending on the quantity in question, the agreement between theory and experiment ranges from good to semiquantitative.