Photon density and the correspondence principle of electromagnetic interaction

We report here the remarkable result that a correspondence principle for electromagnetic radiation can exactly reproduce the apparently anomalous behavior that has been observed by a number of experimenters in the transition regime between multiphoton ionization and tunneling ionization for tenuous gases irradiated with high intensity lasers. Further, when the photon density of a laser beam of frequency ω is equated to the electron density of a plasma at its critical value (plasma frequency ω_p≡ω), it turns out that the corresponding laser intensity is just the critical laser intensity derived from this correspondence principle which separates classical and quantum ranges and which has been previously identified by various experiments. The relationship between this purely classical plasma property and basic quantum and relativistic effects are discussed. It may also follow from this discussion that higher harmonic generation with fsec lasers can be explained as a photon collective effect.