Valence and core electronic excitations in LiC6

Abstract

Electron-energy-loss spectroscopy has been used to measure the electronic excitations in Li${\mathrm{C}}_6$ from 0.3 to 288 eV. Plasmon losses at 2.85 and 6.3 eV are in good agreement with optical results. In the region between 15 and 40 eV essentially no fine structure associated with backfolded interband transitions is observed contrary to similar measurements in K${\mathrm{C}}_8$. These differences are explained by the different bandfolding in $M{\mathrm{C}}_6$ and $M{\mathrm{C}}_8$ compounds. The carbon $1s$ absorption edge shows a sharp Fermi-edge discontinuity followed by a shoulder 0.5 eV to higher energy. This $1s$ absorption feature is similar to but weaker than what is seen in $M{\mathrm{C}}_8$ compounds; while the line shape may be a reflection of C $\pi$ final-state relaxation, a quantitative understanding of this phenomenon is still lacking. The Li $1s$ absorption edge shows two weak peaks at 57.1 and 58.7 eV, which are assigned to transitions to $E_F$ and to a three-dimensional conduction band, followed by two stronger peaks at 63.0 and 65.2 eV. The latter two exhibit relative intensity changes as a function of orientation leading to their identification as atomiclike transitions to crystal-field-split Li $2p$ levels.