Helicity dependence of two-photon ionization by a tricolor field

We study the two-photon ionization of the ground state hydrogen atom by an electromagnetic field consisting of a coherent superposition of the fundame ntal frequency of the Ti:sapphire laser and two of its harmonics of orders 5 and 9, with constant relative phases. We consider the ionization process resulting from the interference of the amplitudes of two different routes: (i) the absorption of two photons from the fifth harmonic, (ii) the simulta neous absorption of one laser photon and of one from the ninth harmonic. Ou r results obtained via a perturbative calculation reveal dichroic effects b oth in the angular distribution and in the total rate. The influence of the helicity, of the phase difference and of the field intensities on the azim uthal angular distribution of photoelectrons and on the dichroic signal are analyzed. (C) 2001 Elsevier Science B.V. All rights reserved.