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.
© 1990 Optical Society of America
Original Manuscript: September 9, 1989
Manuscript Accepted: November 29, 1989
Published: April 1, 1990
H. R. Reiss, "Relativistic strong-field photoionization," J. Opt. Soc. Am. B 7, 574-586 (1990)