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 vibrati onal levels of a molecule that are Condon inaccessible or are otherwise dif ficult to reach by direct optical excitation. Dynamic Stark shifting of the C(2)Pi (r), n(C) = 0 Rydberg vibrational level relative to vibrational lev els of the B(2)Pi (r), valence state of NO serves as the vehicle for this s tudy. Quantum dynamics calculations of two-photon C(2)Pi (r) n(C) = 0 <-- X (2)Pi (r) n(X) = 0 intense-laser excitation, Stark shifting of the C(2)Pi ( r), state and Rydberg-valence state mixing provide a conceptual basis for t he proposed test of intense-field optical control, in which the C(2)Pi (r), state acts as a 'molecular elevator', depositing population in B(2)Pi (r), n(B) = 7-10 vibrational levels. The viability of this approach is assessed through a combination of spectrally and temporally resolved measurements o f B(2)Pi (r), NO production. Spectrally resolved B(2)Pi (r) n(B) --> X(2)Pi (r) n(X) fluorescence induced by a 100 fs laser field at an intensity of 6 .0 x 10(13) W cm(-2) and wavelength of 382 nm shows evidence of formation o f B(2)Pi (r) n(B) = 9, 10 levels via Stark shifting of the optically pumped C(2)Pi (r) Rydberg state. In bichromatic pump-probe experiments, an intens e, off-resonant Stark field is applied to NO at different times to bring ab out formation of B(2)Pi (r), n(B) = 9, 10 from C(2)Pi (r), n(C) = 0 prepare d by a spatially overlapping excitation field. These experiments were unabl e unambiguously to confirm the feasibility for optical control of the B(2)P i (r), nB = 9. 10 <-- C(2)Pi (r) n(C) = 0 <-- X(2)Pi (r) n(X) = 0 pathway s uggested by the spectral measurements of the B(2)Pi (r) n(B) --> X(2)Pi (r) n(X) band system, and reasons for this are discussed. (C) 2001 Elsevier Sc ience B.V. All rights reserved.