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

Multiphoton ionization rates for He irradiated by a dichromatic AC-field co nsisting of the fundamental wavelength lambda = 248 nm and its second, thir d and fourth higher harmonics have been computed. The calculations incorpor ated systematically electronic structure and electron correlation effects i n the discrete and in the continuous spectrum. They were done by implementi ng a time-independent. nonperturbative many-electron, many-photon theory (M EMPT) which obtains cycle-averaged complex eigenvalues, whose imaginary par t is the multiphoton ionization rate, Gamma. It is shown that, provided the intensities are weak, the dependence of Gamma on phase difference phi (2), is simple. Specifically, for odd higher harmonics, Gamma varies linearly w ith cos(phi (2)) whilst for even higher harmonics it varies linearly with c os(acp,). These relations may turn out to be applicable to other atomic sys tems as well. and provide a definition of the weak-field regime in the dich romatic case. When the intensities are such that higher powers of cos(phi ( 2)) and cos(2 phi (2)) become important, these rules break down and we reac h the strong-field regime. (C) 2001 Elsevier Science B.V. All rights reserv ed.