Mapping the Excited States of Single Hexa-peri-benzocoronene Oligomers

Electronic states of a molecule are usually analyzed via their decomposition in linear superposition of multielectronic Slater determinants built up from monoelectronics molecular orbitals. It is generally believed that a scanning tunneling microscope (STM) is able to map those molecular orbitals. Using a low-temperature ultrahigh vacuum (LT-UHV) STM, the dl/dV conductance maps of large single hexabenzocoronene (HBC) monomer, dimer, trimer, and tetramer molecules were recorded. We demonstrate that the attribution of a tunnel electronic resonance to a peculiar pi molecular orbital of the molecule (or sigma intermonomer chemical bond) in the STM junction is inappropriate. With an STM weak-measurement-like procedure, a dl/dV resonance results from the conductance contribution of many molecular states whose superposition makes it difficult to reconstruct an apparent molecular orbital electron probability density map.