The linear-Stark-effect approximation is employed to investigate the effects of electric fields on autoionizing doubly excited atomic states corresponding to relatively large values of the outer-electron principal quantum number
n. By assuming that a statistical distribution of the inner-electron magnetic sublevels has been established, simple expressions are derived which relate the autoionization and radiative transition rates in the presence of an electric field to the respective field-free rates. It is pointed out that the multiphoton laser excitation of the doubly excited states can give rise to a nonstatistical distribution of the magnetic sublevels and to complex interference terms involving transition amplitudes associated with different values of the outer-electron angular momentum quantum number
l. The results of calculations are presented for the autoionization and radiative-transition rates which play the most important role in the dielectronic recombination of the Li-like
Fe+23 ion through the $2p\ensuremath{\rightarrow}2s$ inner-electron transition, where the contributions from large-
n values are particularly important. In these calculations it is assumed that the magnetic sublevels are statistically populated and only the simple expressions for the transition rates are employed. The effects of the electric field on the dominant (inner-electron) radiative decay rate can be neglected, but the electric field mixing of the outer-electron
l substates substantially increases the autoionization rates from the higher-
l sublevels.
We present a detailed theoretical study of two-photon above-threshold excitation to the lowest dipole forbidden
1Se and
1De autoionizing states of helium with free-electron-laser pulses. We consider the case where the formation of these transient states is detected by monitoring the yield of atoms in the metastable
1s2s states, which are reached in the radiative cascade decays of the autoionizing states. We describe the two-photon near-resonant driving with an effective Hamiltonian operator within the density matrix formalism, and we report the relevant parameters of the model, such as the field-induced shifts, the effective Rabi frequencies, and the cascade branching ratios of both the autoionizing states and the $1sn\ensuremath{\ell}$ states, to which the autoionizing states decay. We compare the calculated metastable atom yield to the vacuum ultraviolet florescence yield and to the electron yield. We discuss the effects of the intensity of the source, admixture of higher harmonics in the incident beam, polarization of the incident light, and the pulse chirp on the metastable atom yield.