High-order harmonic generation from organic molecules in ultra-short pulses

We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes cyclopropane and cyclohexane (cyclic) and n-hex ane (linear). Harmonic intensities were measured both as a function of lase r intensity (in the range 5 x 10(13) -5 X 10(15) W cm(-2)) and as a functio n of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom. For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like b ehaviour for molecules when the laser pulse duration is shorter than the fr agmentation timescale of the molecule. T;Ve note significant differences be tween molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these differences in the context of the mole cular properties, electronic structure and behaviour of ionisation and frag mentation that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG shows that the har monic yield in molecules is less sensitive to the polarisation than for ato ms (xenon). This is consistent with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system co mpared to the atom. Our results suggest that study of HHG from molecules ex posed to ultra-short pulses is potentially a powerful tool for understandin g the electron dynamics of molecules exposed to an intense field.