Tunneling versus sequential long-range electron transfer: Analogy with pump–probe spectroscopy

Abstract

The interplay between the sequential and the superexchange (tunneling) mechanisms for electron transfer in condensed phases is studied by formulating the problem using the density matrix. The sequential mechanism proceeds via populations of intermediate electronic states (diagonal density matrix elements) whereas the superexchange proceeds through coherences (off diagonal density matrix elements). The present formulation establishes a complete formal analogy between these mechanisms and the incoherent and the coherent pathways in nonlinear optical measurements, in particular, pump–probe spectroscopy.