The energy of the electrons in LEED:

Figure 7.05: Energy of electron and diffraction pattern.

The short free path length of electrons with 20 - 200 eV for LEED gives very high surface sensitivity. However, the electrons still penetrate several atomic layers. The scattering from the several layers modulates the 2D Ewald's construction toward 3D case as shown in Figure 7.05. This gives the variation in intensity of each diffracted spot with changing electron energy.