EFFECTS OF COHERENT COUPLING OF AUTOIONIZING STATES ON MULTIPHOTON IONIZATION

We present the experimental and theoretical investigation of the effec ts of the strong electromagnetic coupling of two autoionizing states ( AIS's) on the photoionization properties of an atomic system. These ef fects are observed by probing the ladder system from the ground state of the atom through a weak two-photon transition to the region of the lower AIS. The experimental observation of the modification of the tot al ionization and partial photoelectron spectra at different wavelengt hs of the coupling and probe lasers are accurately reproduced by a the oretical analysis which uses a density-matrix approach. Both theory an d experiment indicate a strong inhibition of the ionization of the sys tem in the case when both lasers are resonant with the two atomic tran sitions, while displaying a rapidly varying asymmetry induced as the d etuning between the two states and the fields increases. In the extrem e case of a large detuning, two separate structures appear as distinct total ionization and photoelectron peaks. The above findings combined with the full theoretical treatment indicate that the phenomena obser ved are an experimental demonstration of ac Stark splitting between tw o AIS's.