Time-resolved spectroscopy of the dynamic Stark effect

This study reports on the ac Stark effect of nitric oxide electron-vibration (vibronic) levels dressed by an intense, bichromatic laser field. The aims of the work are to make quantitative measurements of the magnitude of the ac Stark shift of vibrational levels connected by the A ²Σ⁺ ← X²Π_1/2 transition and to map out the cycle-averaged level separation during the Stark field. In pump-probe experiments at intensities up to 30 TW cm^-2, the ac Stark shift of the A²Σ⁺ v_A = 2 ← X²Π_1/2 v_X = 0 transition is found to be Δε_2←0^(S)(800nm) = 0.34 ± 0.05U_p, where U_p is the ponderomotive energy. By varying the time delay between component fields, the time dependence of the population in the A²Σ⁺ v_A = 2 state is mapped out under different intensity conditions. Fluorescence monitoring of the upper level serves to identify the Stark-shifted transition and reveals that real population remains in the upper level after interacting with the short-pulse laser fields. Semiclassical calculations of NO irradiated by a bichromatic field indicate the direction of the A²Σ⁺ and X ²Π_1/2 level shifts and show that the population promoted to and retained in the A²Σ⁺ state is determined predominantly by intra-Rydberg state interactions rather than by multiphoton loss processes. The calculations also indicate that the magnitude of the ac Stark effect of the A²Σ⁺ ← X ²Π_1/2 transition depends on the spatial orientation of the molecule with respect to the linear polarization sense of the applied laser field.