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+ 6s1/2, 5d3/2 and 5d5/2 states. The wavelengths of the observed Ba2+ 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 Ba2+ photoionisation threshold and the angular distributions of the electrons ejected in the production of Ba2+.
