Systematic local spectroscopy of the affinity levels, by means of a scanning tunneling microscope, in highly ordered molecular semiconductor films of tetracene reveals strong energy level shifts by up to $\sim 1.0\mathrm{eV}$ from molecule to molecule. This final state effect can be traced back to the site specificity of the polarization energy in organic materials with complex unit cells, caused by a combination of different molecular environments, the intrinsically anisotropic molecular polarizability, and the influence of the substrate.

• Received 19 November 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.177405