Coherent Control of Xe-136 within the Atomic Collision Window

Two-photon absorption of a circularly polarized UV beam of a femtosecond laser at 256 nm is used to pump ground state electrons of Xe-136 to the m = +2 or m = -2 sublevels of the 6p [5/2]2 excited state depending on the handedness of the circular polarization. A copropagating circularly polarized IR beam of a CW laser at 788.134 nm is used to further excite these electrons to the 8s [3/2]2 state through a single photon absorption process. The absorption of the IR beam is measured by a fast detector. In theory, the same and the opposite handedness of the circular polarization of the two beams respectively result in a forbidden and an allowed final transition, hence zero and finite absorption of IR photons, respectively. In practice, finite absorption is observed in both cases. However, the absorption signal in the forbidden case is less intense and delayed by about 70 ps relative to the absorption in the allowed case. This time delay is a measure of the depolarization rate at which exited electrons in m = +/-2 sublevels move to other sublevels, namely, m = +/-1 and m = 0 making the final transition possible, resulting in the delayed absorption of IR photons.