Electronic transitions and excitations in solid C70 studied by EELS and XPS C 1s satellite structures.

Abstract

The electronic transition and excitation properties of highly ordered ${\mathrm{C}}_{70}$ films have been studied by reflection electron-energy-loss spectroscopy (EELS) and x-ray photoemission spectroscopy (XPS) C 1s satellite structures. The EELS study revealed a total of 11 features in the energy-loss range 1--40 eV. These include two tight-binding Frenkel excitonic levels in ${\mathrm{C}}_{70}$ fullerite, at 1.53 and 1.88 eV; the feature at 2.20 eV correlated with the onset of the electron transitions between the highest occupied molecular-orbital- and lowest occupied molecular-orbital-derived bands; a doublet at around 6 eV showing evidence of the \ensuremath{\pi}-plasmon splitting of fullerite ${\mathrm{C}}_{70}$. A broad hump at \ensuremath{\sim}28 eV corresponds to the excitation of the (\ensuremath{\sigma}+\ensuremath{\pi}) plasmon. The XPS C 1s satellite structures were found to be in overall agreement with the EELS results in the corresponding energy-loss range. By comparing the EELS data with the direct and inverse photoemission data, the on-site Coulomb correlation energy has been determined to be 1.2\ifmmode\pm\else\textpm\fi{}0.2 eV for ${\mathrm{C}}_{70}$ fullerite.