The electronic transparency of a single CO molecule at contact

At the exact contacting distance of a CO molecule inserted in the tunnel junction between a Cu(3 1 1) surface and an STM tip, it is demonstrated that the calculated conductance of a single CO is smaller (G = 6 × 10−4 Go with Go = 2e2/h) than the corresponding vacuum junction conductance. This is not due to a destructive interference between tunnel channels through the junction but owing to the very large CO–Cu(3 1 1) electronic coupling which desequilibrates the electronic structure of the tunnel junction. A molecule chemisorbed on at least one of its contacting electrodes is not a guaranty for a large junction conductance.