Quantum optimal control of electron ring currents in chiral aromatic molecules

We report the results of optimal control simulations of pi-electron rotation (ring current) in a six-membered chiral aromatic molecule, 2,5-dichloro[n](3,6)pyrazinophane (DCP), attached at a surface and excited by a linearly polarized UV laser. DCP has a pair of optically allowed, quasidegenerate pi-electronic excited states. The laser pulse to generate an approximate angular momentum eigenstate consisting of the quasidegenerate states was designed using the global optimal control theory. For both counterclockwise and clockwise pi-electron rotations, the calculated objective functional and target yield as a function of the angle of the photon polarization vector show two maxima and two minima. The origin of the two minima is coherent excitation to only one of the quasidegenerate states. The two maxima arise from creation of a superposition of the quasidegenerate states. The optimal control pulse at the maxima is a two-color laser field resonant with the quasidegenerate states. The electric field of the optimal control pulse consists of two parts: a slowly oscillating part with phase phi(env) and a rapidly oscillating one. The phase phi(env) is a crucial parameter for determination of the rotation direction of pi electrons at the end of control. The results of the optimal control simulations suggest that pi-electron rotation can be controlled by applying a two-color laser field with adjusted phases.