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

Abstract

We report the excitation of highly excited $6{d}_{\frac{5}{2}(\frac{3}{2})}\mathrm{nd}(\mathrm{ns})$ states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound $6snd(\mathrm{ns})$ Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the ${(6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}$ 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 ${6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}(n25)$ states predominantly autoionize to the nearly degenerate $6{d}_{(\frac{3}{2})}$ state of ${\mathrm{Ba}}^{+}$. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.