Multiphoton population transfer in a kicked Rydberg atom: adiabatic rapid passage by separatrix crossing

Following an experimental observation, a recent simulation has shown that efficient population transfer can be achieved through adiabatic chirping of a microwave pulse through a 10-photon resonance connecting two Rydberg states with n = 72, l = 1 and n similar to 82. These simulations have revealed that this population transfer is essentially a classical transition caused by separatrix crossing in the classical phase space. Here, we present the results of our fully three-dimensional quantum and classical simulations of coherent multiphoton population transfer in a kicked Li atom in a Rydberg state. We were able to achieve similar to 76% population transfer from the 40p to 46p state in Li through a 6-photon resonance and contrast our results with those when the transition is driven by microwaves. We further discuss the case when the atom starts out from a Stark state in conjunction with the l-distribution of the transferred population. We use a one-dimensional classical model to investigate the classical processes taking place in the phase space and find that the same separatrix crossing mechanism observed in microwave transitions is also responsible for the transition when the atom is kicked.