Resonance effects in multiphoton ionization of atoms.

Multiphoton ionization of atoms emphasizes both atomic properties and laser characteristics, namely, frequency, coherence, and polarization. Resonance effects arise when the energy of an integral number of photons is close to the energy of an atomic level shifted under the influence of the laser field. The agreement between theory and experiment is excellent for resonance effects in a moderate laser intensity range (10(7)-10(9) W.cm(-2)). A simplified approach to resonance effects in a high intensity range (10(13) W.cm(-2)) is proposed in terms of the resonant multiphoton ionization time.