Electronic structures of ionic liquids and studied by ultraviolet photoemission, inverse photoemission, and near-edge X-ray absorption fine structure spectroscopies

Electronic structures of the ionic liquids formed by 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n = 4, 8, 10) with fluorine-containing anions (tetrafluoroboronate BF 4 - , hexafluorophosphate PF 6 - ) were studied by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Comparison of the calculated density of states with the observed spectra has revealed that the energies of the molecular orbitals of these ionic liquids are significantly affected by the electrostatic Madelung potential among the ions, by which the top of the occupied states and the bottom of the unoccupied states are both derived from the cation.