Generally, high optical intensity is required for effective multiphoton excitation of quantum systems to highly excited states. In certain situations, however, lower-intensity, chirped pulses can provide more efficient transfer of population to the upper states by the process of adiabatic passage. We have studied the relative importance of these two mechanisms in the anharmonic $5s-5p-5d$ quantum ladder system of rubidium using frequency chirped laser pulses from an amplified Ti:sapphire laser (λ=780 and Δλ=10 nm). We measure simultaneously the three-photon ionization signal due to the Ti:sapphire and the population that remains in the $5d$ state with a postionizing 532-nm Nd:YAG pulse (where YAG denotes yttrium aluminum garnet). At low infrared fluences (80 μ${\mathrm{J}/\mathrm{c}\mathrm{m}}^2$), the transfer to the $5d$ state is significantly enhanced when the pulse frequency is swept from the red to the 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 ${\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^2$). We attribute both of these effects to adiabatic passage from the $5s$ state to the $5d$ state. Even at the highest fluences, more efficient transfer occurs for either direction of chirp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magnitude and functional form. These results have important implications for the understanding of population transfer in complex ladder systems, such as molecular anharmonic vibrational ladders.

• Received 13 July 1998

DOI:https://doi.org/10.1103/PhysRevA.59.1374