As the incident electron energy is increased, the wavevector of the electrons increases. This means that the radius of the Ewald sphere becomes larger and more of the reciprocal lattice rods are cut. Thus the first effect is that as the electron energy is increased more spots appear on the LEED screen.

Secondly, because there is a linear relationship between sin q and l , as the electron energy increases and thus l decreases, sin q will fall. Therefore the angle between any given diffracted beam and the surface normal will decrease. This means that a diffraction spot will move towards the (00) (directly reflected) beam as the incident electron energy is increased.

(Someday I’ll get round to writing a simple Java applet to show this!).

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