Abstract

Within the framework of perturbation theory and of dipole approximation, the angular distribution for third order processes can be expressed for any state of quantum numbers (n, l) of the hydrogen atom as ; whatever the polarisation state (linear or circular). The explicit expressions of the corresponding angular coefficients are given for ns and np initial states. It is shown for the particular case of linear polarization that, the isotropic term a occurs only for initial states with orbital quantum number l greater than zero, and contains exactly twenty one contributing terms for np states; in the case of circularly polarized light, the distribution follows a sin6q behaviour for ns states, while for np states, it is expressed as a combination of sin4q and sin6q. Using the implicit summation technique, nine radial transitions matrix elements are determined and a quantitative analysis of these coefficients is made for 1s, 2s, 3s, 2p and 3p initial states at various wavelengths, including the three photons ionization threshold; for linear polarization, the shapes are strongly dependent on both quantum numbers (n, l); while for circular polarization the shapes remain the same. Furthermore, the corresponding total cross sections obtained indirectly from these coefficients, are globally in agreement with that obtained directly from other methods.

Key words: Multiphoton, ionization, angular distribution, total cross section, polarization.