In the 1880s a curious phenomena was observed: when a ray of light, no matter how weak, hit certain metals; electrons were emitted from the surface. Called the “photoelectric effect”, this puzzle was never explained until much later. In 1905, Albert Einstein put forth one possible explanation, which is currently accepted as correct. Einstein proposed that light propagated in discrete energy packets rather than as a continuous wave. While most scientists disbelieved Einstein theory, it was later proved in detail by Robert Milikan.1 Rays of light traveling in discrete packets hit metal surfaces, depositing energy. If the energy is high enough, electrons will be emitted. Since emission is only dependant on incident energy, electrons and even ions may also be used to deposit energy. Electrons emitted as a result of this energy deposition are called secondary electrons (SE).
Corbridge, Jodie, "Secondary Electron Emission Study of Annealed Graphitic Amorphous Carbon" (2003). Senior Theses and Projects. Paper 9.