Effect of electron irradiation on the spectra of elementary excitations in C60 fullerite

Abstract

Electron irradiation produces changes in the spectra of elementary excitations of C60 fullerite, which are manifested by decrease in the π-plasmon energy, bandgap width, and energies of the HOMO-LUMO and other molecular transitions, smoothening of the corresponding spectral features, and significant growth in the quasicontinuous low-energy background intensity, the latter being indicative of an increase in the conductivity. The observed “red shifts” are related to collectivization of a part of the π electrons, the formation of chemical bonds between adjacent molecules (polymerization), and the corresponding increase in the proportion of sp3-hybridized electrons. Characteristic electron energy loss (EEL) spectra of an intact fullerite sample non-perturbed by the polymerization process were measured. The EEL spectra of fullerite exhibit a multipole structure due to the (σ + π)-plasmon and reveal an exciton feature which is highly sensitive with respect to electron irradiation and can be used to characterize the initial fullerite structure and to indicate the polymerization onset.