IN-SITU SCANNING-TUNNELING-MICROSCOPY OF A REDOX MOLECULE AS A VIBRATIONALLY COHERENT ELECTRONIC 3-LEVEL PROCESS

We provide a theoretical frame for in situ scanning tunneling microsco py (STM) of adsorbate molecules with low-lying redox levels strongly c oupled to the environmental nuclear motion. The STM process is viewed as a coherent two-step electron transfer (ET) involving electron excha nge between the local redox level and the manifolds of electronic leve ls in the substrate and tip. The notion coherence is here taken to imp ly that the intermediate electron or hole state after the first ET ste p does not fully relax vibrationally before the second ET step. These views and the theoretical formalism are appropriate to in situ STM of large transition metal complexes and redox metalloproteins. The formal ism offers two kinds of spectroscopic features. One is the relation be tween the tunnel current and the bias voltage at fixed overvoltage of either the tip or the substrate relative to a reference electrode. The other one is the tunnel current dependence on the overvoltage, at fix ed bias voltage. Recent data on tunneling current patterns for adsorbe d or covalently tethered metalloporphyrins and the blue single-copper protein azurin are discussed in terms of the formalism.