Understanding the electronic structure of matter on the atomic scale is one of the central goals of nanoscience. In particular, many chemical processes are governed by the exchange of electrons between atoms and molecules, a mechanism known as electron transfer. Probing such phenomena requires the capability to study atom and molecules at their own length scales, while at the same time being able to control their charge states.While Scanning Tunneling Microscopy (STM) allows imaging electronic states with Angstrom resolution, the requirement of a conductive substrate hinders the investigation of completely electronically isolated adsorbate structures in given charge states. Recently, combining elements of STM and AFM, we successfully developed a novel imaging and spectroscopy method, referred to as single-electron Alternate-Charging Scanning Tunneling Microscopy (AC-STM), in the framework of a running project. Here, in our renewal proposal, we aim at applying this novel method to various physical questions that, without the AC-STM technique, have been elusive. In fact, by enabling electronic state imaging on insulators, with this method one can also study higher lying excited states, in particular multiply-ionized molecules. We make use of these possibilities in five different contexts. First, we aim to study the electron-electron repulsion within single oligomeric molecules, as model systems for conducting polymers. Second, we would like to use AC-STM to identify reaction products, formed by charge injection. The need to access multiple charge states in these single-molecule reactions inhibits conventional STM characterization, highlighting the possibilities brought about by AC-STM. Third, we wish to expand the technique to its full spectroscopic capabilities, such that it can provide not only the energy of different charge state transitions, but also level broadening, polaronic shifts and maybe even vibrational excitations. Fourth, we would like to explore the possibilities to also access non-equilibrium states within each charge state. For example, by a suitable sequential injection of different charge carriers, it should be possible to prepare a molecule with a HOMO-LUMO excitation (exciton), providing access to the energetics of charge conserving optical transitions with electron-transfer reactions. Finally, we propose to combine AC-STM with luminescence experiments on thick insulators. This project will open up a new arena of single-molecule experiments with access to the entire class of phenomena involving out-of-equilibrium charge states.
