Here "d" is the penetration depth, and formula_2 denotes the inelastic mean free path, defined as the distance an electron can travel before its intensity has decreased by the factor 1/"e". While the inelastic scattering processes and consequently the electronic mean free path depend on the energy, it is relatively independent of the material. The mean free path turns out to be minimal (5–10 Å) in the energy range of low-energy electrons (20–200 eV). This effective attenuation means that only a few atomic layers are sampled by the electron beam, and, as a consequence, the contribution of deeper atoms to the diffraction progressively decreases.