Signatures of symmetry and electronic structure in high-order harmonic generation in polyatomic molecules

We report detailed measurements of high-order harmonic generation in chloromethane molecules (CCl4, CHCl3, and CH2Cl2) to show that fingerprints of symmetry and electronic structure can be decoded from high-order harmonic generation even in complex randomly oriented molecules. In our measurements, orbital symmetries of these molecules are manifested as both extended harmonic cutoffs and a local minimum in the ellipticity dependence of the cut-off harmonics, suggesting the occurrence of quantum interferences during ionization. The harmonic spectra exhibit distinct interference minima at similar to 42 and similar to 60 eV. We attribute the former to the Cooper minimum in the photoionization cross section and the latter to intramolecular interference during the recombination process.