Population transfer via adiabatic passage in the rubidium quantum ladder system

Generally, high optical intensity is required for effective multiphoton exc itation of quantum systems to highly excited states. Ln certain situations, however, lower-intensity, chirped pulses can provide more efficient transf er of population to the upper states by the process of adiabatic passage. W e have studied the relative importance of these two mechanisms in the anhar monic 5s-5p-5d quantum ladder system of rubidium using frequency chirped la ser pulses from an amplified Ti:sapphire laser (lambda=780 and Delta lambda =10 nm). We measure simultaneously the three-photon ionization signal due t o the Ti:sapphire and the population that remains in the 5d state with a po stionizing 532-nm Nd:YAG pulse (where YAG denotes yttrium aluminum garnet). At low infrared fluences (80 mu J/cm(2)), the transfer to the 5d state is significantly enhanced when the pulse frequency is swept from the red to th e blue, such that it follows the frequency spacing of the rubidium ladder. Counterintuitively, population is also transferred efficiently for the blue -to-red chirp at high fluences (>5 mJ/cm(2)). We attribute both of these ef fects to adiabatic passage from the 5s state to the 5d state. Even at the h ighest fluences, more efficient transfer occurs for tither direction of chi rp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magn itude and functional form. These results have important implications for th e understanding of population transfer in complex ladder systems, such as m olecular anharmonic vibrational ladders. [S1050-2947(99)01502-4].