THE ELECTRONIC TUNNEL FACTOR IN SIMPLE ELECTROCHEMICAL PROCESSES AND IN-SITU SCANNING TUNNEL MICROSCOPY

The tunnel factor in homogeneous long-range electron transfer (ET) has been in recent focus, stimulated by the appearance of many new chemic al and biological ET systems characterized in great detail, Well chara cterized electrochemical systems where the electron is transferred ove r considerable distances are also beginning to appear, In addition, sc anning tunnel microscopy (STM) can image large adsorbates including bi ological macromolecules. New attention to the tunnel factor in electro chemical ET and in sis STM is therefore warranted. We overview some el ements of ET theory appropriate to long-range ET in electrochemistry a nd STM. We introduce an electron density formalism that points to an i nteresting fluctuation effect where the metallic electron density expa nds and contracts as the electrode is charged negatively and positivel y, respectively. The effect can amount to several orders of magnitude for low-density metals over accessible overpotential ranges and approa ches here the overpotential effect on the nuclear activation factor.