Macroscopic properties of high-order harmonic generation from molecular ions.

High harmonic spectroscopy utilizes the extremely nonlinear optical process of high-order harmonic generation (HHG) to measure complex attosecond-scale dynamics within the emitting atom or molecule subject to a strong laser field. However, it can be difficult to compare theory and experiment, since the dynamics under investigation are often very sensitive to the laser intensity, which inevitably varies over the Gaussian profile of a typical laser beam. This discrepancy would usually be resolved by so-called macroscopic HHG simulations, but such methods almost always use a simplified model of the internal dynamics of the molecule, which is not necessarily applicable for high harmonic spectroscopy. In this Letter, we extend the existing framework of macroscopic HHG so that high-accuracy ab initio calculations can be used as the microscopic input. This new (to the best of our knowledge) approach is applied to a recent theoretical prediction involving the HHG spectra of open-shell molecules undergoing nonadiabatic dynamics. We demonstrate that the predicted features in the HHG spectrum unambiguously survive macroscopic response calculations, and furthermore they exhibit a nontrivial angular pattern in the far field.