Optical control of electronic state populations via the dynamic Stark effect

Through a synergic combination of theoretical calculations and experimental measurements, we explore the possibility of taking advantage of different AC Stark shifts in different electronic states to populate selected vibrational levels of a molecule that are Condon inaccessible or are otherwise difficult to reach by direct optical excitation. Dynamic Stark shifting of the C²Π_r n_C = 0 Rydberg vibrational level relative to vibrational levels of the B²Π_r valence state of NO serves as the vehicle for this study. Quantum dynamics calculations of two-photon C²Π_r n_C = 0 ← X²Π_r n_X = 0 intense-laser excitation, Stark shifting of the C²Π_r state and Rydberg-valence state mixing provide a conceptual basis for the proposed test of intense-field optical control, in which the C²Π_r, state acts as a 'molecular elevator', depositing population in B²Π_r n_B = 7-10 vibrational levels. The viability of this approach is assessed through a combination of spectrally and temporally resolved measurements of B²Π_r NO production. Spectrally resolved B²Π_r n_B → X²Π_r n_X fluorescence induced by a 100 fs laser field at an intensity of 6.0 × 10¹³ W cm^-2 and wavelength of 382 nm shows evidence of formation of B²Π_r n_B = 9, 10 levels via Stark shifting of the optically pumped C²Π_r Rydberg state. In bichromatic pump-probe experiments, an intense, off-resonant Stark field is applied to NO at different times to bring about formation of B²Π_r n_B = 9, 10 from C²Π_r n_C = 0 prepared by a spatially overlapping excitation field. These experiments were unable unambiguously to confirm the feasibility for optical control of the B²Π_r n_B =9, 10 ← C²Π_r n_C = 0 ← X²Π_r n_X = 0 pathway suggested by the spectral measurements of the B²Π_r n_B → X²Π_r n_X band system, and reasons for this are discussed.