Theory of High-Order Harmonic Generation from Molecules by Intense Laser Pulses

We show that high-order harmonics generated from molecules by intense laser pulses can be expressed as the product of a returning electron wave packet and the photo-recombination cross section where the electron wave packet can be obtained from the simple strong-field approximation (SFA) or from a companion atomic target. Using these wave packets but replacing the photo-recombination cross sections obtained from SFA or from the atomic target by the accurate cross sections from molecules, the resulting high-order harmonic spectra are shown to agree well with the benchmark results from direct numerical solution of the time-dependent Schrödinger equation, for the case of H⁺₂ in laser fields. The result illustrates that these powerful theoretical tools can be used for obtaining high-order harmonic spectra from molecules. More importantly, the results imply that the photo-recombination cross section extracted from laser-induced high-order harmonic spectra can be used for time-resolved dynamic chemical imaging of transient molecules with temporal resolutions down to a few femtoseconds