Role of orbital symmetry in high-order harmonic generation from aligned molecules

High-order harmonic generation has been explored theoretically and experimentally in CO₂, a linear molecule characterized by doubly antisymmetric highest-occupied molecular orbitals. For the first time to our knowledge, the high-harmonic yield is studied as a function of the angle between the molecular axis and the polarization direction. A minimum yield is found at 0^∘ for all harmonics, which constitutes evidence for destructively interfering terms in the ionization probability amplitude playing an important role in strong-field phenomena in molecules. The maximum yield is always found at intermediate angles, but the detailed behavior is harmonic specific. Numerical simulations reproduce the main features observed in the experiment.