Fine structure of spectra for secondary electron excited by electron impact – Novel spectrum data analysis and application to nearly free electron metals

In order to analyze the fine structure of the energy distribution of secondary electrons generated through the cascade process, we measured the electron spectra of NFE metals (Li, Mg, Al, Ca, Ga, In), their compounds (Al₂O₃ and GaAs) and Fe as transition metal with a CMA-type analyzer with absolute gain. We propose that the spectrum obtained by differentiating the energy spectrum in a logarithmic representation (defined as DLS) can be used to evaluate the excitation function that generates the fine structure of the secondary electron energy distribution. We found that the fine structure of the energy distribution of secondary electrons in NFE metals is mainly due to emission through a cascade process generated by the electrons from plasmon decay, which was obtained from analyses of the DLS, the excitation function, the energy distribution of secondary electrons N(E), and the EELS spectra. In addition, slow Auger electrons also generate secondary electrons through the cascade process. In Al₂O₃, a fine structure of secondary electrons was generated by electrons excited to unoccupied states by inter-band transitions and inner-shell excitation. In GaAs, both contributions from plasmon decay and electrons excited to unoccupied states by inter-band transitions and inner-shell excitation were observed. Based on these findings, it is concluded that the fine structure of the energy distribution of the secondary electrons emitted in the cascade process is due to the following electrons: 1) electrons from plasmon decay, 2) slow Auger electrons, and 3) electrons excited to unoccupied states by inter-band transitions or inner-shell excitation. These fine structures are superimposed on the structure of the individual excitation by primary electrons.