Molecular orbital dependence of high-order harmonic generation

We examine the role of the electronic orbital of a molecule in the process of high-order harmonic generation. To do this, measurements of high-order harmonic generation in molecules (O-2, N-2, H-2, CO2 and small alkanes) and atoms (Xe, Ar and Kr) have been carried out with intense few-optical-cycle pulses. Comparison between molecules and their 'companion' atoms (i.e. those with nearly equal ionization potential) was made. The molecule response in the high-field regime is treated by adopting an 'atom-like' model for the forms of the molecular orbitals employing the Lewenstein et al. model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble. The calculation procedure is described here in detail. There is good agreement between the spectra calculated using this model and the harmonic cut-off position and the shape of the measured spectra. The cut-offs of O-2 and CO2 extend far beyond the cut-offs of Xe and Kr respectively, in contrast with N-2 and H-2 which exhibit cutoffs very close to that of Ar. This behaviour is well explained by adopting the atom-like approximation. A series of alkanes has also been investigated and again the spectra are well matched by this simple model.