Temporal interference picture for subpeak structures between Autler-Townes doublets in photoelectron energy spectra

The existence of subpeak structures between the Autler-Townes (AT) doublet of the photoelectron spectrum during resonant ionization of an atom has been predicted in a large number of theoretical calculations, but a convincing physical explanation is still lacking. In this study we establish a temporal interference picture to elucidate the physical origin of these subpeaks. We derive an analytical estimation for the photoelectron spectra based on a three-level model, which agrees well with the numerical solution of the time-dependent Schr"odinger equation at moderately strong pulse intensities. Our interpretation suggests that the interference of the photoelectron wave packet emitted during each Rabi cycle results in the subpeak structure between the AT doublet. By analyzing the phase difference between two temporally adjacent photoelectron wave packets, we successfully predict the interference minima in the photoelectron energy spectra.