92168.pdf

148 The solid-gas interface

Incident

beam

Line of

scatterers

Figure 5.20 Cones of diffracted radiation from a one-dimensional row of point

scatterers

second comprising the 'centre atoms'. The resulting diffraction

pattern is, therefore, that of a (2 x 2) surface layer in which the

spacing is half of that which would be given by the underlying

substrate atoms. Since the immediate substrate atoms will in practice

give some (albeit attenuated) signal, the LEED pattern will consist of

an array of bright (surface layer only) and very bright (surface layer

plus substrate) spots, as illustrated.

Suppose now that only a proportion of these surface layer sites are

occupied by adsorbate. The consequent combination of order and

randomness in the surface layer will give a LEED pattern which

shows the same spots (but at reduced intensity) together with some

diffuseness around the centre spot.

Field emission and field ionisation microscopy

In field emission spectroscopy (FEM), a refactory metal, such as
tungsten, is fabricated to give a very fine hemispherical tip of radius
of curvature about 10~^7 m. The tip is located at the centre of
curvature of a hemispherical fluorescent screen and a potential
difference of about 10 kV is applied, with the fluorescent screen as