Specimens that charge under electron beam irradiation in the scanning
electron microscope (SEM) can be stabilized by choosing the beam energy to
be such a value that the sum of the secondary and backscatter electron
yields is unity, as this establishes a dynamic charge balance. We show
here that for pure elements, the energies El and E2, for which charge
balance occurs, are related directly to the atomic number of the material.
Although generally there is no comparable relation for compounds, we also
show that for polymers, the E2 energy is related both to the ratio of the
number of valence electrons to molecular weight and to the
electro-negativity of the monomer units that form the polymer.