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A systematic development is presented of a strong-field method for photoionization of atoms and molecules. The strong-field requirement demands a relativistic treatment. A Dirac theory is developed. The exact S matrix is expanded on the premise that the plane-wave field is stronger than the binding of the atom or molecule. The leading term of this expansion is the strong-field approximation. This general method is applied to photoionization from the ground state of a Dirac hydrogenic atom by circularly polarized monochromatic radiation. Numerical examples show that relativistic effects should be discernible with present-day pulsed lasers. The nonrelativistic limit is exactly one of the standard forms of the Keldysh or Keldysh–Faisal–Reiss approximation.
Howard Reiss (Sun,) studied this question.