RESONANT DOUBLE MULTIPHOTON IONIZATION VIA PLANETARY STATES

We have studied double ionization of Ba 6snl Rydberg states through re sonant two-photon excitation of 8snl double-Rydberg states with n = 13 and 14 and different l. The laser intensity dependence of Ba2+ yield measured for / greater-than-equal-to 8, when the electrons suffer long -range dipole interaction, shows a decrease from four-photon for l = 8 to less than three for l = 11. The four-photon dependence is due to a utoionization decay populating the Ba+(7p) levels which gives an indir ect signature to the Ba2+ production. By increasing l this decay vanis hes and the data show evidence of a direct double-ionization process v ia planetary-electron states for l greater-than-or-equal-to 10. Additi onal intensity measurements performed for low l = 6 states strongly mi xed with 5fn'l' ones by short-range multipole interaction, reveal indi rect Ba2+ production and an individual case of partial stabilization d ue to destructive interference leading to direct double ionization.