Dependence upon the molecular and atomic ground state of higher-order harmonic generation in the few-optical-cycle regime

High-order harmonic generation from molecules (O₂, N₂, H₂, and CO₂) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; 2×10¹⁴ and 6×10¹⁴0.3em0exW0.2em0excm^-2, were compared. At the lower intensity spectra were approximately the same for molecules and atoms with the same ionization potential, at higher laser intensity the cutoff of O₂ and CO₂ extends far beyond the cutoff of Xe and Kr, respectively, in contrast with N₂ and H₂ which exhibit cutoffs very close to that of Ar. This behavior is well explained by adopting an atomlike approximation for the molecule response in the high-field regime and employing the Lewenstein's model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble.