Interaction between two Rydberg series of autoionizing levels in barium

Abstract

Using a three-step laser excitation approach, we have studied the autoionizing ${(6{p}_{j}nd)}_{J=3}$ states of neutral barium. The $6{p}_{\frac{3}{2}}\mathrm{nd}$ series is essentially unperturbed and exhibits a constant quantum defect and autoionization widths which scale as ${\ensuremath{\nu}}^{\ensuremath{-}3}$, where $\ensuremath{\nu}$ is the effective quantum number. The $6{p}_{\frac{1}{2}}\mathrm{nd}$ states are narrower than the $6{p}_{\frac{3}{2}}\mathrm{nd}$ states and are perturbed by their interaction with the $6{p}_{\frac{3}{2}}\mathrm{nd}$ states. The perturbation leads to variations in the quantum defects of the $6{p}_{\frac{1}{2}}\mathrm{nd}$ states and departures from the ${\ensuremath{\nu}}^{\ensuremath{-}3}$ scaling of the widths. The interaction is most apparent in the excitation of the $6{p}_{\frac{3}{2}}10d$ state which is degenerate with the $6{p}_{\frac{1}{2}}\mathrm{nd}$ states for $n\ensuremath{\sim}20$. The observed excitation spectrum clearly shows the influence of the degenerate $6{p}_{\frac{1}{2}}\mathrm{nd}$ states and is shown to represent the spectral density of the $6{p}_{\frac{3}{2}}10d$ state. This spectrum as well as all the above-mentioned observations may be connected by a quantum-defect treatment.