RESONANT 2-PHOTON IONIZATION OF ATOMS VIA 2 COUPLED RYDBERG SERIES

We present non-perturbative ab initio calculations of resonant two-pho ton ionization in which the ground state of a complex atom is resonant ly coupled by a laser field to two Rydberg states, each of which belon gs to a different Rydberg series. As an example, we have studied the r esonant two-photon ionization of neon via the 5s and 4d Rydberg states using the R-matrix-Floquet theory. We find that for weak fields, wher e the Rydberg states are not overlapping, interference effects are alr eady present which lead to a suppression of ionization for a particula r laser frequency. For strong fields, the field induced coupling betwe en the two Rydberg states can strongly modify the weak-field behaviour . We give a qualitative explanation of these phenomena by considering the dressed states of a simple three-level model atom coupled to two c ontinua. Finally we show, by varying the intensity, how the Floquet ei genstates corresponding to the ground state and the two Rydberg states of neon are adiabatically connected to the field-free states.