and the electron energy E in eV above the Fermi level (conductors) or above the bottom of the conduction band (non-conductors). m e is the electron mass, c the vacuum velocity of light, N ν ν is the number of valence electrons per atom or molecule, ρ ρ describes the density (in g c m 3 ), M is the atomic or molecular weight and β β , γ γ , C and D are parameters determined in the following. Equation 2 calculates the IMFP and its dependence on the electron energy in condensed matter.