Abstract

It is shown that absorption of the minimum number of photons needed to ionize an atom in a light beam of moderate intensity is no longer energetically possible when the external field is strong. However, subsequent absorption of a large number of photons by the "emerging" electron in the field of its parent ion can occur via multiphoton inverse "half-Bremsstrahlung" and ionization is possible again. This new theory leads to a simple interpretation of the appearance of a large number of peaks in the electron spectrum together with the disappearance of the low-energy peaks. The roles played by the laser intensity and wavelength are singled out, and quantitative agreement is found between the conclusions of the present theory and existing experimental data. Furthermore, using an argument developped in a recent paper (Phys. Rev. Lett., 54, 1385 (1985)), it is demonstrated that the ponderomotive force does not contribute to any modification of the energy of the electron when it leaves the laser beam. An intuitive physical explanation of this fact is given.

© 1986 Optical Society of America