When optical frequency fields are used to couple a ground state to a Rydberg state, the resonant dipole coupling is to a low angular momentum state. Higher angular momentum states are typically thought not to play a role in the excitation. The extremely large dipole matrix elements coupling Rydberg states of the same n but differing l, however, allow optical fields of modest strengths to produce Rabi frequencies larger than optical frequencies. We demonstrate that these optical fields can therefore readily excite the higher angular momentum states, and we examine the consequences of this coupling.[S0031-9007(97)03635-1]

We describe a pulsed single-mode dye laser whose output characteristics satisfy the stringent requirements of coherent control experiments. This simple, low cost laser exhibits 1.6ϫ transform-limited frequency performance with a single-shot linewidth of 58 MHz, and a time averaged linewidth of 240 MHz. The spatial mode is nearly Gaussian and has a beam quality parameter M 2 ϭ1.2. We modified the pulse shape using saturable absorption in the dye amplifier. The resultant pulses are temporally smooth and have a duration of approximately 20 ns.

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Three-photon ionization is measured as the frequency of an optical pulse is tuned through two-photon resonance with the Rydberg series of potassium. When the intermediate Rydberg resonance is far below the ionization threshold, the standard resonance profile indicates the enhancement due to a single isolated level. However, as the frequency is tuned closer to threshold, continuous behavior is reached because of the coherent bandwidth of the short pulse and power broadening of the bound-bound transition line. Between these two limiting regions a transition region is observed where discrete and continuous features are manifested. The discrete nature is seen through resonance peaks that blend together as the threshold is approached. The continuous nature manifests itself through the creation of a coherent superposition state that produces localized radial motion. The laser pulse effectively probes the radial orbital motion of this wave packet. Since the ionization rate is drastically reduced while the wave packet is far from the core, drops in the ion signal are seen when the number of times the wavepacket can return to the core is reduced by one. These drops are abrupt because the electron travels much faster near the core in these highly elliptical orbits.

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