Tunneling ionization in the multiphoton regime

Data on the threshold ionization intensities of rare-gas atoms with 248-nm irradiation in the 10^13--10^16-W/cm^2 range of intensity are presented in the context of models involving tunneling ionization. Comparison is also made with the full Keldysh theory. It is shown that, although the experimental conditions fall well outside the tunneling regime as defined by the Keldysh theory, these relatively simple pictures accurately predict the threshold ionization intensities observed under the conditions studied. The main deviation of these results from the Keldysh theory occurs for the heavier elements at high charge states.