ac Stark shifts in Rydberg NO levels induced by intense laser pulses

We report a quantitative assessment of the ac Stark effect induced in the NO [Formula Presented] Rydberg state by femtosecond laser pulses at intensities up to [Formula Presented] The [Formula Presented] vibrational level of [Formula Presented] NO is excited by two-photon absorption at frequencies above and below the weak-field [Formula Presented] resonance and its time-dependent population monitored by fluorescence detection at different incident laser intensities. Dispersed [Formula Presented] spectra recorded at [Formula Presented] exhibit prominent features due to fluorescence from the [Formula Presented] 1, and 0 levels of the [Formula Presented] state. These results can be quantitatively described by a kinetic treatment of multiphoton absorption and ionization that takes into account ac Stark shifting of the vibrational levels of the [Formula Presented] state at different coordinates in space time mapped out by the laser pulse. An analysis of the experimental data within the terms of this model leads to the deduction that the ac Stark shift is 0.4 times the ponderomotive energy of a free electron at the laser fields used, corresponding to a shift of 0.36 eV at an intensity of [Formula Presented] at [Formula Presented]