Correlation enhancement of high-order harmonics generation in Xe

We consider the process of high-order harmonics generation (HHG) in the xenon atom enhanced by the intershell correlation between the valence $5p$ and inner $4d$ shells. We derive the HHG spectrum from a numerical solution of the one-electron time-dependent Schr\"odinger equation multiplied by the enhancement factor taken as the photoionization cross-sections ratio calculated with and without the intershell correlation. Such a simplified approach is adequate to describe the experimental HHG spectrum reported by Shiner et al. [Nat. Phys. 7, 464 (2011)] generated by a single color IR laser. Similarly, we find good agreement when applied to the two-color driven HHG spectra reported by Faccial\'a et al. [Phys. Rev. Lett. 117, 093902 (2016)].