Electronic structure of ionic liquids at the surface studied by UV photoemission

Electronic structures of room-temperature ionic liquids (RILs) were studied by ultraviolet photoemission spectroscopy (UPS) with synchrotron radiation. The samples contain 1-buthyl-3-methylimidazolium ion [bmim]+ as the common organic cation, combined with fluorine-containing inorganic anions. We also performed molecular orbital (MO) calculations for the isolated ions. Comparing the calculated density of the states with the observed spectra, we found that the top of the valence states in the liquid salts is derived from the organic cation, although the highest occupied molecular orbitals (HOMOs) of the isolated anions are higher than that of the isolated cation. The Madelung energies at the surface region, which are probably responsible for this reversal, are estimated by comparison between UPS and MO calculation. We also simulated the observed spectra based on the independent-atomic-center (IAC) approximation.