High-order harmonic generation in helium: A comparison study

We report a detailed study of high-order harmonic generation (HHG) in helium. When comparing predictions from a single-active-electron model with those from all-electron simulations, such as the attomesa code and $R$-matrix-with-time-dependence method, which can include different numbers of states in the close-coupling expansion, it seems imperative to generate absolute numbers for the HHG spectrum in a well-defined framework. While qualitative agreement in the overall frequency dependence of the spectrum, including the cutoff frequency predicted by a semiclassical model, can be achieved by many models in arbitrary units, only absolute numbers can be used for benchmark comparisons between different approaches.