Multistep excitation of autoionizing Rydberg states

We describe the photoexcitation spectra from bound Ba Rydberg states $6snl$ to the analogous autoionizing $6pnl$ states. The spectra are obtained with high laser power bringing out weak spectral features corresponding to changes in $n$ of the outer $\mathrm{nl}$ electron. These spectra are explained using a quantum-defect-theory approach which shows that the optical cross section is a product of the spectral density of the autoionizing states and the overlap integral from the initial bound state. When a saturation effect is taken into account, synthetic spectra calculated in this way accurately reproduce the observed spectra. Experimental spectra of unperturbed autoionizing series are presented together with the two-channel quantum-defect theory relevant to this case. In addition, the new features which arise in the case of interacting series are shown as well as a model three-channel quantum-defect-theory treatment.