Spectroscopic signatures of molecular orbitals on a honeycomb lattice

A tendency to form benzene-like molecular orbitals has been recently shown to be a common feature of the $4d$ and $5d$ transition metal oxides with a honeycomb lattice. This tendency competes with other interactions such as the spin-orbit coupling and Hubbard correlations, and can be partially or completely suppressed. In the calculations, SrRu$_{2}$O$_{6}$ presents the cleanest, so far, case of well-formed molecular orbitals, however, direct experimental evidence for or against this proposition has been missing. In this paper, we show that combined photoemission and optical studies can be used to identify molecular orbitals in SrRu$_{2}$O$_{6}$. Symmetry-driven election selection rules suppress optical transitions between certain molecular orbitals, while photoemission and inverse photoemission measurements are insensitive to them. Comparing the photoemission and optical conductivity spectra one should be able to observe clear signatures of molecular orbitals.