Ellipticity effects in HHG from aligned molecules.

Two-centre interference in the recombination step of molecular high harmonic generation (HHG) has been probed in CO2 and O2. We report the order dependence of characteristic enhancements or suppressions of high harmonic production in aligned samples of both molecules. The ellipticity dependent studies in CO2 clearly show how the destructive interference can be “switched off” by increasing the degree of ellipticity and thus shifting the effective resonance condition. The presence of an elliptically polarised field leads to a tilting in the electron momentum which in turn breaks the cylindrical symmetry introducing a dependence on the azimuthal angle . The observed shift can be explained by assuming that molecules for which the tilting leads to a reduction of  (angle between the molecular axis and the recolliding electron momentum), give larger contribution to HHG. In a simple classical approach we have considered uniform contribution from half the molecules obtaining a result in agreement with the experimental findings. References.1) C. Vozzi et al., Phys. Rev. Lett. 95, 153902 (2005).2) C. Altucci et al., in preparation.