This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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For many materials the plasmon energy, based on free-electron theory (Drude model [1]), E_plasmon, is proportional to the square root of the density of valence electrons (n):

                  ------------------------- [3368]

where,
          ħ -- The reduced Planck constant;
          e -- The charge of the electron
          m_e -- The effective mass of the electron
          ε₀ -- The permittivity of vacuum           

In Equation 3368 the only parameter that depends on material properties is the electron density, determined by the mean valence of the atoms and the lattice parameter. If the mean valence of the atoms in specific crystals is constant, the changes in plasmon energy are only due to variations in lattice parameter.

[1] P. Drude, Phys. Z. 14, 161 (1900).