Abstract
The study of the response of a two-active electron system exposed to an intense laser pulse represents a tremendous task from the theoretical point of view. To tackle this problem, we have analysed various coordinate systems and studied their appropriateness, having in mind a good description of the physics of the problem as well as the numerical resolution of the corresponding time-dependent Schrodinger equation.

We have singled out one particular coordinate system and presented in this thesis detailed results about the field free energies, oscillator strengths, Rayleigh and Raman scattering (from the ground state and the first excited state).

We present also an original method to treat One and Two colour photoionization of helium problems. This method consists in solving numerically the time-dependent Schrodinger equation for a few discrete complex eigenstates obtained by complex scaling of the atomic and interaction Hamiltonians.

The results, obtained for the study of the Two-colour photoionization with exitation of an intermediate autoionizing state, show a Autler-Townes splitting of this intermediate autoionizing state at low intensities of the second laser.