Enhancement of the autoionization rate of two-photon excited states of Ba (6d5/2nd5/2(3/2))J=4 near the Ba⁺ 6d3/2 limit

We report the excitation of highly excited 6d5/2(3/2)nd(ns) states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound 6snd(ns) Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the (6d5/2ndⱼ)J=4 states exhibit a sharp enhancement in the autoionization rate beyond $n=25$ which is accompanied by the onset of ejection of very-low-energy electrons ( 0.02 eV). We observe that the 6d5/2ndⱼ)J=4(n25) states predominantly autoionize to the nearly degenerate 6d(3/2) state of Ba⁺. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.