GW calculations and ultraviolet photoelectron spectroscopy of gas phase ion pairs - a window into the electronic structures of ionic liquids

Room temperature ionic liquids have extremely low equilibrium vapor pressures, but in ultrahigh vacuum vapors consisting of neutral ion pairs can be detected. Spectroscopic measurements of these ion pairs, the fundamental building blocks of ionic liquids, can yield insights into the electronic structures of these unusual and technologically important materials. From a theoretical perspective, the description of these ion pairs is challenging due to the presence of long range charge transfer. For example, in density functional theory different exchange correlation functionals can produce qualitatively different ground state electronic structures. In this study, it is shown that the GW method yields a consistent description of the gas phase ion pairs that is only weakly dependent on the mean field starting point. The effect of different levels of self-consistency in the GW calculations is analyzed. Theoretical valence level photoelectron spectra are calculated, and it is found that G0W0 at a hybrid DFT starting point yields excellent quantitative agreement with experiment. In one instance, GW calculations highlighted the presence of a contaminant in the experimental spectrum that had not been previously recognized, and corroborated its assignment to a decomposition product.