Ultrafast switching of persistent electron and hole ring currents in a molecule

A circularly polarized laser pulse can induce persistent intramolecular currents by either exciting or ionizing molecules. These two cases are identified as electron currents and hole currents, respectively, and up to now they have been studied only separately. We report ab initio time-dependent density-functional theory (TDDFT) simulations of currents in a molecule, which reveal for the first time that both electron and hole currents can be present simultaneously. By adjusting the intensity of the laser pulse, the balance between the two types of current can be controlled and the overall sign of the current can be switched. We provide a physical explanation for the effect in terms of molecular orbitals, which is consistent with the TDDFT simulations.