Measuring the inverted region of an electron transfer reaction with a scanning tunneling microscope

A procedure to measure the inverted region of an electron-transfer reaction with a tunneling microscope is presented. A Fe(III)-protoporphyrin film adsorbed on a highly oriented pyrolytic electrode was chosen as a particularly suitable system. Experimental results indicate a strong decrease of the rate constant in the inverted region, and an approximately Gaussian density of states of the redox couple. The concomitant energy of reorganization is relatively small, of the order of 0.2 eV. Theoretical considerations show that inelastic tunneling transition would give rise to additional structure in the apparent density of states, but there is no evidence for this in the experimental data.