DETECTION OF ENERGY-SELECTED SECONDARY ELECTRONS IN COINCIDENCE WITH ENERGY-LOSS EVENTS IN THIN CARBON FOILS

The ultimate resolution in secondary-electron images obtained in a scanning transmission electron microscope depends strongly on secondary-electron-production mechanisms. In such a microscope, equipped with an electron-energy-loss spectrometer as well as a secondary-electron-energy analyzer, coincidences have been achieved between secondary electrons and energy-loss electrons. This has resulted in additional information on the origin of secondary electrons in thin carbon films. The average probability for the production of secondary electrons of a given kinetic energy rises linearly with the energy lost by the primary electrons minus this kinetic energy and a few eV. These secondary electrons seem to appear from a cascade which was started by the original energy-loss event. Since most of these events are volume-plasmon excitations, volume-plasmon decay followed by a cascade contributes strongly to the secondary-electron emission. The linear relation seems to break down at energy losses of more than almost-equal-to 125 eV. Energy losses associated with surface plasmons give a high probability for emission and detection of secondary electrons with a kinetic energy of 2-3 eV. For losses which are about 11 eV higher than the kinetic energy of the secondary electrons, an enhanced probability of emission exists. This enhancement is attributed to direct ionization from the valence band. The results are obtained in non-ultrahigh-vacuum conditions but show one of the many possibilities of coincidence spectroscopy in a scanning transmission electron microscope.