Time-resolved spectroscopy of the dynamic Stark effect

Abstract

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 A2Σ+ ← X2Π1/2 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+ vA = 2 ← X2Π1/2 vX = 0 transition is found to be Δϵ(S)2←0(800 nm) = 0.34 ± 0.05Up, where Up is the ponderomotive energy. By varying the time delay between component fields, the time dependence of the population in the A2Σ+ vA = 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 X2Π1/2 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+ ← X2Π1/2 transition depends on the spatial orientation of the molecule with respect to the linear polarization sense of the applied laser field.