  We present a theoretical analysis aimed at understanding electrical
conduction in molecular tunnel junctions. We focus on discussing the validity
of coherent versus incoherent theoretical formulations for single-level
tunneling to explain experimental results obtained under a wide range of
experimental conditions, including measurements in individual molecules
connecting the leads of electromigrated single-electron transistors and
junctions of self-assembled monolayers (SAM) of molecules sandwiched between
two macroscopic contacts. We show that the restriction of transport through a
single level in solid state junctions (no solvent) makes coherent and
incoherent tunneling formalisms indistinguishable when only one level
participates in transport. Similar to Marcus relaxation processes in wet
electrochemistry, the thermal broadening of the Fermi distribution describing
the electronic occupation energies in the electrodes accounts for the
exponential dependence of the tunneling current on temperature. We demonstrate
that a single-level tunnel model satisfactorily explains experimental results
obtained in three different molecular junctions (both single-molecule and
SAM-based) formed by ferrocene-based molecules. Among other things, we use the
model to map the electrostatic potential profile in EGaIn-based SAM junctions
in which the ferrocene unit is placed at different positions within the
molecule, and we find that electrical screening gives rise to a strongly
non-linear profile across the junction.
