Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temperature ionic liquids composed of bis(trifluoromethylsulfonyl)imide anions ([Tf2N]−) and either 1-ethyl-3-methyl-imidazolium ([EMIm]+) or 1-octyl-3-methyl-imidazolium cations ([OMIm]+). [T. Ikari, A. Keppler, M. Reinmoller, W. J. D. Beenken, S. Krischok, M. Marschewski, W. Maus-Friedrichs, O. Hofft and F. Endres, e-J. Surf. Sci. Nanotechnol., 2010, 8, 241.] In the present work we analyze these spectra by means of partial density of states (pDOS) as calculated from a single ion pair of the respective ionic liquid using density functional theory (DFT). Subsequently we reconstruct the XPS and UPS spectra by considering photoemission cross sections and analyze the MIES spectra by pDOS, which provides us decisive hints to the ionic liquid surface structure.