He in dichromatic weak or strong ac fields of lambda(1)=248 nm and lambda(2) = (1/m) 248 nm (m=2,3,4) - art. no. 013411

We have computed multiphoton ionization rates for He irradiated by a dichro matic ac held consisting of the fundamental wavelength lambda = 248 nm and its second-, third-, and fourth-higher harmonics. The intensities are in th e range 1.0 x 10(12)-3.5 x 10(14) W/cm(2), with the intensity of the harmon ics being 1-2 orders of magnitude smaller. The calculations incorporated sy stematically electronic structure and electron correlation effects in the d iscrete and in the continuous spectrum, for S-1, P-1, D-1, F-1, (1)G, and H -1 two-electron states of even and odd parity. They were done by implementi ng a time-independent, nonperturbative many-electron, many-photon theory wh ich obtains cycle-averaged complex eigenvalues, whose real part gives the f ield-induced energy shift, Delta (w(1).F-1;omega (2),F-2,phi (2)), and the imaginary part is the multiphoton ionization rate, Gamma(omega (1),F-1;omeg a (2),F-2,phi (2)), where omega is the frequency. F is the field strength, and phi (2) is the phase difference. Through analysis and computation we sh ow that, provided the intensities are weak, the dependence of Gamma(omega ( 1),F-1;omega (2),F-2,phi (2)) on phi (2) is simple. Specifically, for odd h igher harmonics, Gamma varies linearly with cos(phi (2)) whilst for even hi gher harmonics it varies linearly with cos(2 phi (2)). These relations may turn out to be applicable to other atomic systems as well, and to provide a definition of the weak-field regime in the dichromatic case. When the comb ination of (omega (1),F-1) and (omega (2),F-2) is such that higher powers o f cos(omega (2)) and cos(2 omega (2)) become important, these rules break d own and we reach the strong-field regime.