PHASE DEPENDENCE OF (N-COLOR (N-GREATER-THAN-1) IR-UV PHOTOIONIZATIONOF ATOMS WITH HIGHER HARMONICS(1))

We discuss, via a numerical calculation, the main properties of atomic photoelectron spectra, as they would be obtained by using a radiation pulse containing N+1 frequencies associated with a ''Dirac comb'' of N higher harmonics together with the laser which has been used to gene rate them. We address more precisely the physically relevant situation in which the harmonics have much weaker intensities than the one of t he laser. In such (N+1)-color photoionization processes, the atom can simultaneously absorb harmonic uv photons and exchange, i.e., absorb a nd/or emit, (via stimulated emission) laser ir photons. We have simula ted the photoelectron spectra by numerically solving the time-dependen t Schrodinger equation for a three-dimensional hydrogen atom in the pr esence of the radiation pulse. Our results show that, everything else being kept fixed, the magnitudes of the photoelectron peaks are strong ly dependent on the difference of phase between successive harmonics. This strong dependence results from interference effects taking place between competing quantum paths leading to a given final state. An int eresting feature is that these interferences involve transitions in th e continuum states of the atom and do not depend on resonances in the discrete spectrum. Another interesting outcome of our study is to show that such effects should be observable with currently developed harmo nic sources.