High Harmonic Generation in Sulfur Dioxide

We show results of high harmonic generation (HHG) on the triatomic molecule sulfur dioxide. This molecule is one of the simplest systems having a conical intersection (CI), a region of electron degeneracy in which the Born-Oppenheimer approximation breaks down. A CI of particular interest exists between the excited electronic $^{1}$B$_{1}$ and $^{1}$A$_{2}$ states in the sulfur dioxide molecule. This CI can be reached via a direct Frank-Condon transition from the electronic ground state. The ionization and recombination steps of HHG render HHG a sensitive probe of electronic structure. HHG thus allows us to monitor electronic symmetry changes within the CI region, which has been shown to contain contributions from multiple orbitals [McFarland et al. \textit{Science} 2008]. We spectrally resolve the high harmonics of the ground and excited states of sulfur dioxide to characterize the CI, paving the way for a more complete understanding of non-Born-Oppenheimer dynamics.