SETTLING A LONG-STANDING QUESTION ON STRONG-FIELD DOUBLE-IONIZATION

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