Controllable surfaces of path interference in the multiphoton ionization of atoms by a weak trichromatic field

Multiphoton detachment rates for the H− 1S ground state irradiated by a weak trichromatic ac field consisting of the fundamental frequency ω = 0.272 eV and its second, third or fourth higher harmonics were computed from first principles. The weak intensities are in the range of 107–108 W cm−2. The calculations incorporated systematically electronic structure and electron correlation effects. They were done by implementing a time-independent, nonperturbative many-electron, many-photon theory (MEMPT) which obtains cycle-averaged complex eigenvalues, whose real part gives the field-induced energy shift, Δ, and the imaginary part is the multiphoton ionization rate, Γ. Through analysis, plausible arguments and computation, we show that when the intensities are weak the dependence of Γ on phase differences is simple. Specifically, Γs are depicted in the form of plane surfaces, with minor ripples due to higher order ionization paths, in terms of trigonometric functions of the phase differences. This dependence is likely to be applicable to other atomic systems as well, and to provide a definition of the weak field regime in the trichromatic case. When the field intensities are such that higher order ionization paths become important, these dependences break down and we reach the strong field regime.