We present results from a time-dependent two-level-plus-continuum model of multiphoton ionization in order to better understand the short-pulse photoelectron spectrum resulting from multiphoton ionization and the role of intermediate states. Both resonant and nonresonant population transfer to the upper state are found to play an important role in the multiphonon ionization process. The dynamic modeling reveals the potential for broadening and shifting peaks in the photoelectron spectrum. However, we conclude that despite the complex dynamics of the excitation, short-pulse photoelectron spectra can be used to measure ac Stark shifts of both ponderomotively and nonponderomotively shifted states in strong laser fields.

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Journal Phys. Rev. A
Gibson, G. N, Freeman, R. R, McIlrath, T. J, & Muller, H. G. (1994). Excitation and ionization dynamics in short-pulse multiphoton ionization. Phys. Rev. A, 49, 3870–3874.