Extended fine structure in the secondary electron emission spectra of graphite and glassy carbon

The electron excited secondary electron emission (SEE) spectra of highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC) were measured in the 0-150 eV electron kinetic energy range. For HOPG, the SEE spectrum was measured as a function of primary electron beam energy above and below the threshold for carbon core level 1s ionization. The SEE spectrum in the 0-70 eV range reflects high energy conduction band states, in agreement with band structure calculations and other spectroscopic techniques sensitive to the empty density of states. For electron kinetic energies above approximately 70 eV the spectrum of HOPG displays a well defined structure which has not been studied previously. We found that the SEE fine structure in the low energy range up to approximately 40 eV shows some dependence on primary electron energy, whereas for larger energies a much weaker dependence was found. It has been established that the SEE spectrum is strongly affected by low-energy Ar ion irradiation of the HOPG surface. This result suggests that the SEE spectrum is very sensitive to crystal defects induced by the irradiation process. The lower-energy range of the spectrum, up to approximately 40 eV was found to be more sensitive to damage induced by the ion irradiation process than the higher range of the spectrum. For GC, the SEE spectrum in the 0-50 eV range is smeared out compared to that of HOPG. However, in the higher electron kinetic range some of the peaks measured for HOPG were also observed for GC. These results suggest that the lower-energy range of the SEE spectrum is more sensitive to long-range order, whereas the higher-energy range of the spectrum reflects a shorter-range order. It is suggested that SEE spectroscopy can be used as a very sensitive tool for the characterization of different carbon structures.