Probing two-centre interference in molecular high harmonic generation

Two-centre interference in the recombination step of molecular high harmonic generation (HHG) has been probed in CO2 and O-2. We report the order dependence of characteristic enhancements or suppressions of high harmonic production in aligned samples of both molecules. In CO2, a robust destructive interference was seen consistent with the known separation of the oxygen atoms that are active in HHG. In O-2, a harmonic enhancement was found indicating constructive interference. A good agreement was found with a simple two-centre interference model that includes the angular distribution function of the sample. The effective momentum of the electron wave was determined from the spectral position of these interferences. 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.