the process in a scanning electron microscope (SEM) where a sample is exposed to a focused beam of high-energy electrons. These electrons collide with the atoms on the surface of the sample, and as a result of these collisions, the sample emits various signals, including secondary electrons, backscattered electrons, and X-rays. These emitted signals provide information about the sample's composition, topography, and other properties. So, "when it's bombarded with electrons" means that the sample is subjected to this electron beam to gather information about it.