Autoionization Inhibited by Internal Interferences

We have measured the linewidths of ${\left[5\mathrm{}{d}_{\frac{3}{2}}{n}_2{d}_{\frac{3}{2}}\right]}_{J=0\mathrm{}}$ autoionizing Rydberg states of barium with principal quantum numbers ranging between $n_2=14\mathrm{}$ and $n_2=100\mathrm{}$. Dramatic deviations of the autoionization rates from the normal $n_2^{*-3\mathrm{}}$ scaling law have been observed. Whereas the ${\left[5\mathrm{}{d}_{\frac{3}{2}}60d_{\frac{3}{2}}\right]}_{J=0\mathrm{}}$ level exhibits a linewidth $\Gamma$ (FWMH) as low as 6 MHz, higher ($n_2\approx 80$) members of this series are broadened up to 5 GHz. Autoionization is inhibited for principal quantum numbers $n_2\approx 60$ because of configuration interaction between the ${\left[5\mathrm{}{d}_{\frac{3}{2}}{n}_2{d}_{\frac{3}{2}}\right]}_{J=0\mathrm{}}$ and ${\left[5\mathrm{}{d}_{\frac{5}{2}}{n}_3{d}_{\frac{5}{2}}\right]}_{J=0\mathrm{}}$ Rydberg series. This causes the state at $n_2=60\mathrm{}$ to be metastable.