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 (O-2, N-2, H-2, and CO2) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; 2 X 10(14) and 6 x 10(14) W cm(-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 O-2 and CO2 extends far beyond the cutoff of Xe and Kr, respectively, in contrast with N-2 and H-2 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.