Enhancement of the autoionization rate of two-photon excited states of Ba (6d_{5/2}nd_{5/2(3/2)})_{J=4} near the Ba^{+} 6d_{3/2} limit

We report the excitation of highly excited $6d_{\frac{5}{2}\left(\frac{3}{2}\right)}\mathrm{nd}\left(\mathrm{ns}\right)\mathrm{}$ states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound $6snd\left(\mathrm{ns}\right)\mathrm{}$ Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the ${\left(6\mathrm{}{d}_{\frac{5}{2}}n{d}_j\right)}_{J=4\mathrm{}}$ states exhibit a sharp enhancement in the autoionization rate beyond $n=25\mathrm{}$ which is accompanied by the onset of ejection of very-low-energy electrons (${6d_{\frac{5}{2}}n{d}_j)}_{J=4\mathrm{}}\mathrm{}(n>\mathrm{}25\mathrm{})\mathrm{}$ states predominantly autoionize to the nearly degenerate $6d_{\left(\frac{3}{2}\right)}$ state of ${\mathrm{Ba}}^{+}$. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.