Probing ring currents in Mg-porphyrins by pump-probe spectroscopy

Abstract

Theoretical studies of Manz et al. have shown that upon excitation of a Mg-porphyrin molecule with a circularly polarized laser pulse, a ring current can be generated that is much stronger than what can be induced by means of an external magnetic field with present technology. We show that the circular dichroism signal of a Mg-porphyrin molecule that has been excited to a state with an inner ring current is proportional to the magnitude of the probability of this ring current and can be used for the detection of this current. In analogy to magnetic circular dichroism, it probes the symmetry of degenerate excited states.

Figure 1

Two pump–probe measurements. The pump is left circularly polarized and the probe is either (a) left circularly polarized or (b) right circularly polarized. The difference between the induced transmittance of the two measurements is related to the magnetic circular dichroism signal and is proportional to the magnitude of the frequency dependent ring current in the Mg-porphyrin. (c) Ring current in the Mg-porphyrin.

Figure 2

Two pathways for a transition from the state ∣e₁〉 to the state ∣e₂〉. The parameters are defined in section 2.

Figure 3

Feynman diagrams for the pump–probe experiment in a Mg-porphyrin molecule. The n and n′ may refer to states 1 or 2, associated with the Q_x and Q_y transitions.

Figure 4

Double-sided Feynman diagram for the coherences between degenerate excited states of the Q-band. ∣e_n〉 ≠ ∣e_n′〉 for n = 1 or 2 and n′ = 1 or 2 (the model is described in section 2). ∣g〉 is the ground state of the molecule.