Nonperturbative theory and computation of the nonlinear response of He to dc and ac fields - art. no. 013407

We have nonperturbatively computed de-field tunneling rates, linear and non linear dynamic polarizabilities, and two-, three-, four-, and five-photon i onization rates for He, for frequencies omega in the range (h) over bar ome ga=4.9-26.4 eV and for intensities; in the range 3.5 X 10(12)-1.715 X 10(14 ) W/cm(2). The calculations systematically incorporated electronic structur e and electron correlation effects in the discrete and continuous spectra, while the two-electron state symmetries were S-1, P-1, D-1, F-1, (1)G, and H-1 of even and odd parities. The calculations were made by implementing a time-independent many-electron, many-photon theory which obtains cycle-aver aged energy shifts Delta(omega,F) and energy widths Gamma(omega,F) nonpertu rbatively, where F is the field strength. These quantities are the real and imaginary parts of a complex eigenvalue z(0) (the result of the overall ca lculation), after the subtraction of the unperturbed real energy E-0.