Tunneling time for electron transfer reactions

The tunneling time for nonadiabatic electron transfer reactions described within the superexchange model is estimated using a Biittiker type internal clock: the electron is taken to possess two internal spin states that are weakly coupled on the bridge. By studying the transition probability between these channels during the tunneling process the traversal time through the bridge can be estimated. Like the Büttiker-Landauer result it is linear in the bridge length, but its dependence on the barrier energy U_B approaches the Büttiker-Landauer form only in the limit of strong interstate coupling (broad band). In the "normal" superexchange (weak coupling) limit it is inversely proportional to the barrier energy.