Reversible isomerization of an azobenzene derivative adsorbed on Au(1 1 1): Analysis using vibrational spectroscopy

High resolution electron energy loss spectroscopy (HREELS) is employed to analyze reversible changes in the geometrical structure of the molecular switch di-methoxy-tetra-tert-butyl-azobenzene (diM-TBA) adsorbed on Au(1 1 1), which are induced by UV-light and thermal activation. While for 1 monolayer (ML) coverage no changes in the vibrational structure due to UV-light exposure at 3.5 eV are observed, illumination of 2 ML of diM-TBA leads to a pronounced modification of the vibrational spectra, which we assign to a trans→cistrans→cis isomerization. The reverse process, i.e., the cis→transcis→trans isomerization, can be stimulated by thermal activation. We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the adsorbed diM-TBA molecules in the second ML analogous to diM-TBA in the liquid phase.