Double ionisation of Ba from 550 to 670 nm

The authors have observed the spectra for the production of Ba + and Ba 2+ between 550 and 670 nm. They observe sharp resonances for the production of Ba 2+ which generally do not coincide with peaks in the Ba + production. While they can easily assign most of the resonances in the Ba + spectra to two-photon resonances with bound states it is not possible to assign most of the Ba 2+ peaks to two- or three-photon Ba + transitions. This suggests that the production of Ba 2+ proceeds both via Ba autoionising states and low-lying Ba + states.