Above-threshold ionization of Cesium under femtosecond laser pulses: New substructure due to strongly coupled bound states

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Abstract
First above-threshold-ionization (ATI) spectra on a cesium atoms obtained with laser pulses as short as 70 fs are presented. Up to 7 ATI peaks exhibiting new substructure related to the extremely large Rabi coupling between the ground and first excited states have been detected. The angular and intensity dependence of the ATI signal has been studied. It is shown that the ATI peaks shift to higher electron energies with rising laser intensity. The obtained results are compared with a perturbative as well as a nonperturbative theory which predicts the structure.