Perturbative study of multiphoton processes in the tunneling regime

A perturbative study of the Schrodinger equation in a strong electromagneti c field with dipole approximation is accomplished in the Kramers-Henneberge r frame. A proof that just odd harmonics appear in the spectrum for a linea r polarized laser field is given, assuming that the atomic radius is much l esser than the free-electron quiver motion amplitude. Within this approxima tion a perturbation series is obtained in the Keldysh parameter giving a de scription of multiphoton processes in the tunneling regime. The theory is a pplied to the case of hydrogen-like atoms: The spectrum of higher order har monics and the above-threshold ionization rate are derived. The ionization rate computed in this way determines the amplitudes of the harmonics. The w ave function of the atom proves to be rigid with respect to the perturbatio n so that the effect of the laser field on the Coulomb potential in the com putation of the probability amplitudes can be neglected as a first approxim ation: This approximation improves as the ratio between the amplitude of th e quiver motion of the electron and the atom radius becomes larger. The sem iclassical description currently adopted for harmonic generation is so rede rived by solving perturbatively the Schrodinger equation. (C) 1999 Elsevier Science B.V. All rights reserved.