Sensitivity of high-order-harmonic generation to aromaticity

The influence of cyclic electron delocalization associated with aromaticity on the high-order-harmonic generation (HHG) process is investigated in organic molecules. We show that the aromatic molecules benzene $({\mathrm{C}}_{6}{\mathrm{H}}_{6})$ and furan $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O})$ produce high-order harmonics more efficiently than nonaromatic systems having the same ring structure. We also demonstrate that the relative strength of plateau harmonics is sensitive to the aromaticity in five-membered-ring molecules using furan, pyrrole $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{NH})$, and thiophene $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S})$. Numerical time-dependent Schr\"odinger equation simulations of total orientation-averaged strong-field ionization yields show that the HHG from aromatic molecules comes predominantly from the two highest $\ensuremath{\pi}$ molecular orbitals, which contribute to the aromatic character of the systems.