We present experimental and theoretical data on the electron energy spectra from resonance-enhanced multiphoton ionisation of atoms. In experiments performed with 594-nm wavelength, linearly polarized laser pulses of 300-fs and 600-fs durations, it is found that excitation of the intermediate resonances during the rising edge of the laser pulse can dominate the electron spectrum. Good agreement is found between the experimental results and a model based on the Landau-Zener energy level crossing theory. Theoretical results obtained with direct integration of the Schrödinger equation show that intermediate resonances can be excited appreciably off resonance if the coupling between the ground state and the excited state is highly nonlinear. The excitation of the intermediate state is not localized at the intensity which makes it six-photon resonant with the ground state. A multilevel model used to simulate the experimental spectra, based on direct integration of the Schrödinger equation, shows excellent agreement with the data.

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Journal Mod. Phys. Lett. B
Vrijen, R. B, Hoogenraad, J. H, & Noordam, L. D. (1994). The role of resonances in multiphoton ionization of atoms with short intense laser pulses. Mod. Phys. Lett. B, 8, 205–232.