  The electron acceptor molecule TCNQ is found in either of two distinct
integer charge states when embedded into a monolayer of a charge
transfer-complex on a gold surface. Scanning tun- neling spectroscopy
measurements identify these states through the presence/absence of a zero-bias
Kondo resonance. Increasing the (tip-induced) electric field allows us to
reversibly induce the ox- idation/reduction of TCNQ species from their anionic
or neutral ground state, respectively. We show that the different ground states
arise from slight variations in the underlying surface potential, pictured here
as the gate of a three-terminal device.
