92168.pdf

The solid-gas interface 147

Fluorescent

screen

Observation

window

Crystal

UHV

chamber

Electron gun

(monoenergetic

collimated

source

Grids

Figure 5.18 Schematic illustration of a LEED apparatus

tional to the spacings between the scattering sources. The resulting
LEED pattern (as with any other diffraction pattern), therefore,
shows the surface net in 'reciprocal space' rather than directly, as
illustrated in Figure 5.21.
The C (2 x 2) surface layer shown in Figure 5.21b is, in effect, two
slightly displaced (2 x 2) surface layers, the first as shown and the

Incident

electrons

Scattered

electrons

Figure 5,19 Diffraction by a row of atoms for normal incidence