Settling a long-standing question on strong-field double ionization

Abstract
Through a theoretical analysis combined with new experimental data, obtained through a technique that allows observation of ionization originating from a constant, sharply limited ion extraction volume, we show that double multiphoton ionization of Xe and Kr in the wavelength range 527 to 531 nm does not involve an observable direct two-electron channel but does involve an inner subshell (5s for Xe and 4s for Kr) excited state which can leave the ion in an excited state. This explains the unusual laser power dependence that had been attributed to a direct two-electron channel.