Landau-Zener treatment of intensity-tuned multiphoton resonances of potassium

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. We have used laser pulses of varying duration to study the nature of the population transfer from the ground state to the excited state due to the intensity-tuned resonances encountered during the rising edge of the pulse. A dynamic Floquet approach in which the resonances are treated as avoided crossings of the Floquet energy levels is used to model the population transfer and gives excellent agreement with the data. The model is extended into the strong-coupling regime where the ground state interacts with many excited states simultaneously, and we show that this model can be used to describe multiphoton ionization as a series of avoided crossings with the continuum.