Orientation dependence of high-order harmonic generation in molecules

We present two- and three-dimensional model calculations of high-order harmonic generation in ${\mathrm{H}}_2^{+}.$ The harmonic spectra exhibit clear signatures of intramolecular interference. An interference minimum appears at a harmonic order that depends on the molecular orientation. Harmonic generation in three-center molecules is studied on the basis of two-dimensional calculations for a ${\mathrm{H}}_3^{2+}$ model system. From analytical considerations, the orientation dependence of the harmonic intensities in three-center molecules exhibits a double minimum due to intramolecular interference. In the numerical results, the double minimum is broadened into a single wide minimum. The effect of nonzero laser ellipticity on harmonic generation is investigated by means of two-dimensional simulations for ${\mathrm{H}}_2^{+}.$ We find that harmonic generation with elliptical polarization is governed by interference effects similar to linear polarization.