z k’ kθqq

Let us assume that the incident beam has an axis of symmetry. The average

incident wave vector, k0, is given by

k0 = k0 ( sin χ0 cos δ0 ex + sinχ0 sin δ0 ey + cosχ0 ez

) . (7.28)

In this expression, the incident beam direction (the k0 direction) with respect

to the main rotation axis of the probed atom site (chosen as z axis), is defined by the

colatitude angle χ0 and the azimuth angle δ0 (see Fig. 7.3). Generally, the direction

of an incident electron wave vector is slightly different from the average incident

wave vector. Let us introduce a new basis set (Ex, Ey, Ez), in which Ez is oriented

along k0.