Effect of electron correlation on high harmonic generation of helium atoms in intense laser fields

We present a time-dependent generalized pseudospectral (TDGPS) approach in {\em hyperspherical coordinates} for fully {\em ab initio} nonperturbative and high-precision treatment of multiphoton electron-correlated dynamics of atomic systems in intense laser fields [1]. The procedure is applied to the investigation of high-order-harmonic generation (HHG) of helium atoms in ultrashort laser pulses at a KrF wavelength of 248.6 nm. The 6D coupled hyperspherical-adiabatic-channel equations are discretized and solved efficiently and accurately by means of the TDGPS method. The effects of electron correlation and doubly excited states on HHG are explored in detail. A HHG peak with Fano line profile is identified which can be attributed to a broad resonance of doubly excited states. Comparison of the HHG spectra of the {\em ab initio} two-electron and the single- active-electron model calculations is also presented. \vspace{1mm} \noindent [1] X. Guan, X.-M. Tong, and S. I. Chu, Phys. Rev. A (in press).