We demonstrate a simple method to improve the Lewenstein model for the description of high-order harmonic generation (HHG). It is shown that HHG spectra can be expressed as the product of a returning electron wave packet and the photo-recombination cross sections, where the former can be extracted from the Lewenstein model. By replacing plane waves with scattering waves in the calculation of recombination matrix elements, we showed that the resulting HHG spectra agree well with those from solving the time-dependent Schrodinger equation. The improved model can be used for quantitative calculations of high harmonics generated by molecules.
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