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

Abstract

We report detailed measurements of high-order harmonic generation in chloromethane molecules (${\mathrm{CCl}}_{4}$, ${\mathrm{CHCl}}_{3}$, and ${\mathrm{CH}}_{2}$${\mathrm{Cl}}_{2}$) 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 $~42$ and $~60$ eV. We attribute the former to the Cooper minimum in the photoionization cross section and the latter to intramolecular interference during the recombination process.