Non-relativistic quantum electrodynamics for strong laser-atom interaction

A formulation of quantum electrodynamics is given that applies to atoms in a strong laser field by perturbation theory in a non-relativistic regime. Dipole approximation is assumed. With the current wavelengths, squeezing can be proved to be negligible and then, just the linear term in the Hamiltonian can be retained. The dual Dyson series, here discussed by referring it to the Birkhoff theorem for singularly perturbed linear differential equations, can be applied and a perturbation series obtained transforming the Hamiltonian by a Pauli-Fierz transformation. But, if just few photons are present highorder harmonics cannot be generated. So, it is proven that odd high-order harmonics only appear when the laser field is intense and one can substitute the creation and annihilation operators by the square root of the mean number of photons taken to be huge, the field retaining its coherency property as observed experimentally for harmonics. In this case, the Hamiltonian for perturbation theory comes to the Kramers-Henneberger form. From this Hamiltonian it is shown that just odd harmonics of the laser frequency contribute to the spectrum for a spherically symmetric potential. This contribution is dipolar when the free-electron quiver motion amplitude is larger than the atomic radius. For a Coulomb potential one has that the outer electron is periodically kicked, and so a prove is given that the same should happen to Rydberg atoms in intense microwave fields. The distribution representing the kicking has a Fourier series with just odd terms. Using a modified RayleighSchrödinger perturbation theory, it is shown that under the same condition of validity of the quiver motion amplitude to atomic radius ratio, the atomic wave function is only slightly modified by the laser field due to the way the energy levels rearrange themselves. This gives a prove of stabilization in the limit of laser frequency going to infinity. Then, perturbation theory can be applied when the product of the laser frequency and the square root of the ratio between the ionization energy and the ponderomotive energy, that is the Keldysh parameter, becomes smaller with respect to the shifted distance between the energy levels of the atom. PACS: 32.80.-t, 42.50.Ct, 42.65.Ky, 05.45.+a Typeset using REVTEX 1