We report a quantitative assessment of the ac Stark effect induced in the NO $3s\sigma A{}^2{\Sigma }^{+}$ Rydberg state by femtosecond laser pulses at intensities up to $I<~6\times {10}^{13}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ The $n^{\prime }=2\mathrm{}$ vibrational level of $A{}^2{\Sigma }^{+}$ NO is excited by two-photon absorption at frequencies above and below the weak-field $A{}^2{\Sigma }^{+}\leftarrow X{}^2{\Pi }_r+2\mathrm{}ħ\omega$ resonance and its time-dependent population monitored by fluorescence detection at different incident laser intensities. Dispersed $A{}^2{\Sigma }^{+}\to X{}^2{\Pi }_r$ spectra recorded at $I>~{10}^{13}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}$ exhibit prominent features due to fluorescence from the $n^{\prime }=2,$ 1, and 0 levels of the $A{}^2{\Sigma }^{+}$ 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 $A{}^2{\Sigma }^{+}$ 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 $I=6\mathrm{}\times {10}^{13}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}$ at $\lambda =400\mathrm{}\mathrm{nm}.$

• Received 3 January 2000

DOI:https://doi.org/10.1103/PhysRevA.62.013414