Multiphoton double ionisation of Ba via Ba+

In the spectral region 560-610 nm the multiphoton double ionisation of Ba is dominated by sharp resonances of incompletely determined origin. By selectively populating specific low lying states of Ba + with a secondary laser system the authors show unambiguously that the multiphoton double ionisation of Ba proceeds via real Ba + intermediate states. Specifically, by comparing multiphoton double ionisation spectra with and without the population of selected Ba + states the authors can identify the initial Ba + states for multiphoton double ionisation as the Ba + 6s 1/2 , 5d 3/2 and 5d 5/2 states. The wavelengths of the observed Ba 2+ features can be assigned with confidence as multiphoton resonances to Ba + intermediate states, and these assignments are confirmed in some cases by the wavelength of the Ba 2+ photoionisation threshold and the angular distributions of the electrons ejected in the production of Ba 2+ .