Origin of the extended fine structure in the secondary-electron-emission spectrum of graphite.

Abstract

An extended fine structure (EFS) has been observed in the electron-excited secondary-electron-emission (SEE) spectrum of graphite. Measurements were performed in the 0--150-eV electron kinetic range as a function of primary-electron energy above and below the threshold for carbon core-level 1s ionization. For primary electron energies of 200, 500, and 1000 eV the spectra display a well-defined fine structure that does not depend strongly on the primary electron energy. However, a dependence of the spectrum on orientation of the sample with respect to the primary electron beam and the electron analyzer was found. The effect of structural disorder on the EFS-SEE spectrum of graphite was investigated by performing in situ measurements as a function of the 2-keV Ar-ion irradiation dose. It was found that the EFS-SEE is very sensitive to imperfections created by the ion beam for very low dose levels of ion irradiation. The sensitivity of the spectrum to short-range crystalline order was assessed by comparing the spectrum obtained for graphite to that of glassy carbon. It is concluded that the EFS-SEE of graphite originates from interband electronic excitation processes involving excitations of valence electrons by primary electrons and secondary electrons of sufficient energy into unoccupied electronic states of high density in the graphite crystal field. This conclusion is supported by comparison with other electron spectroscopic measurements of graphite.