He in two-color AC-fields of λ1=248 nm and λ2=(1/ m) 248 nm, m=2, 3, 4. The rate of multiphoton ionization, for weak fields, is a simple function of the phase

Abstract

Multiphoton ionization rates for He irradiated by a dichromatic AC-field consisting of the fundamental wavelength λ=248 nm and its second, third and fourth higher harmonics have been computed. The calculations incorporated systematically electronic structure and electron correlation effects in the discrete and in the continuous spectrum. They were done by implementing a time-independent, nonperturbative many-electron, many-photon theory (MEMPT) which obtains cycle-averaged complex eigenvalues, whose imaginary part is the multiphoton ionization rate, Γ. It is shown that, provided the intensities are weak, the dependence of Γ on phase difference ϕ 2, is simple. Specifically, for odd higher harmonics, Γ varies linearly with cos( ϕ 2) whilst for even higher harmonics it varies linearly with cos(2 ϕ 2). These relations may turn out to be applicable to other atomic systems as well, and provide a definition of the weak-field regime in the dichromatic case. When the intensities are such that higher powers of cos( ϕ 2) and cos(2 ϕ 2) become important, these rules break down and we reach the strong-field regime.