Multiphoton adiabatic rapid passage

A recent experiment showed that significant population transfer for a ∼10 photon resonance could be obtained with a relatively small chirp in the light field. The system investigated was the microwave transition between Rydberg states in Li from n = 72, = 1 to n ∼ 82. This is a multiphoton resonance in a one-electron system. We present quantum calculations using a model one-dimensional system and a fully three-dimensional system that agree with the measured results and the original interpretation of the transition as an adiabatic passage through a multiphoton resonance. Although the measured results strongly suggest a simple multiphoton transition, the quantum calculations clearly indicate that more than 50 photons are absorbed/emitted in this process. We also performed classical calculations for this system and found that the classical transitions also give a small energy band near the multiphoton resonance. However, the interpretation of the results is quite different and is an example of transitions induced by separatrix crossing. The separatrix arises from the classical resonance between the microwave frequency and the classical Rydberg frequency at an energy between the initial and final states.