Microwave Excitation and Ionization
Atoms may be ionized by static fields in excess of a threshold field E ≈ l/16n4 where n is the principal quantum number . On the other hand photoionization has a frequency threshold, requiring photons with energy in excess of the binding energy W. The rate of photoionization, though, is proportional to the intensity of the radiation, or E. Midway between these two processes is multiphoton ionization. Ionization by m photons requires that the photon energy be in excess of W/m, and in lowest order perturbation theory the ionization rate is proportional to Im or E2m. This scaling as Im has been observed for m = 21. For higher values of m, or even for m = 21, it is difficult to measure an Im dependence due to dynamic range considerations. Furthermore, it is not clear that the region in which the lowest order solution is valid is physically very interesting, for appreciable ionization will only occur where it is not valid. In these cases it is more useful to describe the multiphoton processes as having field or intensity thresholds. Thus multiphoton ionization qualitatively resembles field ionization.
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