AUTOIONIZING STATES OF NEGATIVE-IONS IN STRONG RESONANT LASER FIELDS - THE NEGATIVE RUBIDIUM ION

We have performed an experimental study of the atomic system Rb-, for which the ground state 5s2 is coupled to the autoionizing state 5p(1/2 )6s by means of a strong resonant laser field. The investigations are based on irradiation of Rb- with intense (I less-than-or-equal-to 1.5 x 10(10) W/cm2) laser pulses of duration approximately 1 ns. Single-ph oton detachment of Rb- is suppressed due to the Fano interference betw een transitions directly to the continuum and transitions via the auto ionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoi onizing states in strong laser fields. Furthermore, we discuss the rol e of autoionizing states in multiphoton double-ionization processes. E lectron spectroscopy has been used to show that double ionization of R b- primarily is the result of a sequential removal of the two electron s via an excited Rb state. The perspectives for direct two-electron io nization in negative ions are briefly discussed.