Strongly perturbed Stark states and electron correlation in Ba

We present the results of experimental and theoretical investigations into the photoionization spectra of Ba Stark states at energies resonant with the 5d7d D2 perturber level starting from the 5d6p D1 o state. The electric field is strong enough to allow the electrons to classically escape the atom, thus transforming the perturber state into a resonance. Much of the dynamics of this remarkably complex system may be understood qualitatively. In particular, we stress the role played by electron escape time in qualitatively changing the correlation by reducing the singlet-triplet mixing and triplet excitation. A theoretical formulation of multichannel systems in static electric fields is developed which allows us to efficiently obtain both total and partial cross sections; this formulation can be used to describe the Rydberg states of atoms and molecules in static electric fields. @S1050-2947~99!11308-8#