Publication

Journal	Physical Chemistry Chemical Physics
Volume (Issue)	17
Page(s)	15771
Title of proceedings	Physical Chemistry Chemical Physics
DOI	10.1039/c5cp01417d

URL	http://homeweb.unifr.ch/allanm/pub/ma/dir_allan/Regeta_ionic_liquids_PCCP_2015.pdf
Type of Open Access	Website

Free electrons and ionic liquids: study of excited states by means of electron-energy loss spectroscopy and the density functional theory multireference configuration interaction method The technique of low energy (0–30 eV) electron impact spectroscopy, originally developed for gas phase molecules, is applied to room temperature ionic liquids (IL). Electron energy loss (EEL) spectra recorded near threshold, by collecting 0–2 eV electrons, are largely continuous, assigned to excitation of a quasi-continuum of high overtones and combination vibrations of low-frequency modes. EEL spectra recorded by collecting 10 eV electrons show predominantly discrete vibrational and electronic bands. The vibrational energy-loss spectra correspond well to IR spectra except for a broadening (0.04 eV) caused by the liquid surroundings, and enhanced overtone activity indicating a contribution from resonant excitation mechanism. The spectra of four representative ILs were recorded in the energy range of electronic excitations and compared to density functional theory multireference configuration interaction (DFT/MRCI) calculations, with good agreement. The spectra up to about 8 eV are dominated by pi–pi* transitions of the aromatic cations. The lowest bands were identified as triplet states. The spectral region 2–8 eV was empty in the case of a cation without pi orbitals. The EEL spectrum of a saturated solution of methylene green in an IL band showed the methylene green EEL band at 2 eV, indicating that ILs may be used as a host to study nonvolatile compounds by this technique in the future.