Hydrogen ionization by a coherent superposition of three harmonics

We consider the two-photon ionization of the ground state hydrogen atom due to the simultaneous interaction with three consecutive odd harmonics of th e same laser, namely the eleventh, the thirteenth and the fifteenth. Five A TI peaks are reached by the net absorption of the energies 22 (h) over bar omega, 24 (h) over bar omega 26 (h) over bar omega, 28 (h) over bar omega a nd 30 (h) over bar omega (omega) being the laser fundamental frequency). We give numerical results, based on analytical equations valid in the second order of the perturbation theory, for the two available IR lasers Nd: YAG ( (h) over bar omega = 1.17 eV) and Ti: Sapphire ((h) over bar omega = 1.55 e V). The results illustrate the influence of the harmonic intensities and th eir relative phases with respect to the fundamental on the total ionization rates.