Interference in climbing a quantum ladder system with frequency-chirped laser pulses

Excitation of a quantum ladder system by frequency-swept (-chirped) laser pulses has been investigated by studying an atomic model system, the 5s-5p-5d ladder of rubidium. The population transfer is significantly enhanced when the frequency is swept such that it follows the spacing of the ladder. At intermediate intensities we observe oscillations in the population transfer to the upper level (5d), which can be attributed to interference between two routes of excitation: sequential transfer and direct two-photon excitation via a virtual intermediate state. We determine simultaneously the population transfer to the upper level (5d) and the direct three-photon ionization, the latter corresponding to a three-step ladder with the final state being in the continuum: 5s-5p-5d-ɛp/ɛf. Experiments were performed, using 100 fs pulses at ∼780 nm from a self-mode-locked Ti:${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ oscillator amplified at 10 Hz.