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When exposed to intense light of -580 nm, the ground state of K shifts up in energy, passing through two-photon resonances with Rydberg states and finally crossing the two-photon ionization limit. Using laser pulses of 0.5 to 13 ps duration, we have shown experimentally that ionization occurs for short pulses, but for long pulses the population is diverted into the Rydberg states where some population survives the peak intensity of the pulse, in excellent agreement with a dynamic Floquet model.

Using the optical Ramsey method we have observed the configuration interaction in Ba drive atoms back and forth between the doubly excited 5d7d state and the degenerate 6snd Rydberg states. Our observations can be understood qualitatively as autoionization of the initially excited 5d7d state into the Rydberg states, followed by capture of the Rydberg electron by the Ba + ion a Kepler time later. After a time delay, the whole process repeats. A quantum defect theory description provides a quantitative picture and clarifies the connection to resonance scattering.

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