Structure of autoionizing Rydberg series in strong laser fields: A multichannel-quantum-defect-theory approach

We study one-photon resonant two-photon ionization into an autoionizing Rydberg series for the case of a weak excitation from the ground state to a resonant intermediate state, followed by a strong-field transition to a structured continuum. The dipole strength for the transition into the autoionizing series is thereby modeled by a multichannel-quantum-defect-theory expression taking into account the infinite number of autoionizing Rydberg states. Analytical expressions are derived for the total ionization rate in the limit of extremely overlapping resonances.