Exploring the spatial features of electronic transitions in molecular and biomolecular systems by swift electrons

Abstract

Electron energy loss spectroscopy is consolidating as a powerful tool to explore electronic (as well as vibrational) excitations of matter, including molecules. Performed in a scanning transmission electron microscope, this technique is based on inelastic scattering of fast electrons in a thin specimen. Very recently, new electron optics configuration have been introduced, opening the way to the analysis of the single components of orbital angular momentum of the outcoming electrons, that convey additional information on the spatial features of the investigated excitations: innovative double-dispersed spectroscopic experiments for metallic nanostructures have been therefore suggested. We propose here to extend this technology to probe molecular and supra-molecular systems, devising new kind of experiments: using state of the art quantum chemical methods to describe the molecular system in presence of an electron beam in a configuration that avoid molecular damage, we show that scattered electrons acquire the different azimuthal components of induced molecular transition potentials. Numerical simulations performed for systems of increasing size, point out that the conceived new technique can open up the possibility of probing the multipolar components and even the chirality of molecular transitions, superseding the usual optical spectroscopies for those cases that are problematic, such as dipole-forbidden transitions, at a very high spatial resolution.