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