Charge transfer and electronic relaxation effects in the photoemission of EMIM-DCA ionic liquid vapor

Abstract

Ultraviolet photoelectron spectroscopy (UPS) measurements of EMIM-DCA vapors have been conducted. Density functional theory (DFT), Møller–Plesset perturbation theory (MP2) and many-body perturbation theory calculations (GW) have been used to interpret the experimentally measured spectra. The vapor of ionic liquids normally consists of isolated ion-pairs and the corresponding photoemission spectra should reflect the electronic binding energies of the molecular orbitals. The UPS spectra have revealed that many photolines are shifted when compared to ab initio calculation results. The hybrid DFT functionals, which performed very good in the description of the UPS spectra of many other ionic liquid vapors have shortcomings in the EMIM-DCA case. The MP2 calculation and the quasiparticle GW approach are shown to offer a better description of the experimental UPS spectra. The charge distribution analysis showed that there seems to be a favorable charge transfer from the cation to the anion upon ionization. The new data provides strong support to the argument that in some cases DFT still has difficulties in the modeling of IL ion-pairs.