AES Theory: Electron Beam Effects

When the electron beam strikes a sample surface, it produces a plethora of different interactions. Elastic scattering occurs when a high energy electron (1 to 30 keV) strikes a sample atom and recoils with essentially all of its energy. (The RBS kinematic factor equation applies to this situation and indicates an energy loss of 1 eV for a 25 keV electrons striking a surface iron atom and scattering back at 180 degrees.) The electron beam loses energy as it passes through material, thereby broadening the energy distribution of backscattered electrons. Inelastic scattering occurs by several mechanisms as the primary electron gives up larger amounts of its energy.

• Plasmon excitation, occurs with high probablility as the free electron gas between ionic cores absorbs energy. Typical plasmon excitations involve transfer of around 15 eV to the solid.
• Conduction band excitation ejects loosely bound conduction electrons as secondary electrons. The majority leave with 0 to 50 eV kinetic energies.
• Bremsstrahlung (from the German for "braking radiation") occurs when a primary electron undergoes deceleration in the Coulombic field of an atom. The bremsstralung consists of X-ray photons with energies spread between zero and the primary beam energy.
• Excitation of lattice oscillations (phonons) transfers a substantial portion of beam energy to the sample as heat.
• Inner shell ionization leaves the atom in an a highly energetic state while absorbing a large amount of primary electron energy. Decay of this excited state produces characteristic Auger electrons and X-rays.

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