Two major factors control which effects can be detected from the interaction volume. Beam electrons lose energy as they traverse the sample due to interactions with it and if too much energy is required to produce an effect, it will not be possible to produce it from deeper portions of the volume. Secondly, the degree to which an effect, once produced, can be observed is controlled by how strongly it is diminished by absorption and scattering in the sample. For example, although secondary and Auger electrons are produced throughout the interaction volume, they have very low energies and can only escape from a thin layer near the sample's surface.
Absorption is an important phenomenon and is discussed in more detail below.
Figure I-1 . Effects produced by electron bombardment of a material. Figure I-2 . Generalized illustration of interaction volumes for various electron-specimen interactions. The excitation volume is a hemispherical to jug-shaped region with the neck of jug at the specimen surface.