We report the observation of the wavelength spectrum of the double ionization of Ba from - 280 nm to nearly 700 run, using laser pulses 5ns long of peak intensity - 1010 W/cm2. All the strong resonances can be assigned to Ba+ transitions, and many of the assignments have been verified by pump-probe techniques. Thus, the Be+ observed is due to sequential ionization. The shortest wavelengths used are not as effective as λ ~ 500 nm.
T F Gallagher,
R R Jones,
"Multiphoton Double Ionization Of Barium", Proc. SPIE 0875, Short and Ultrashort Wavelength Lasers, (27 April 1988); doi: 10.1117/12.943880; https://doi.org/10.1117/12.943880