Th e solid-gas interface 141
Suppose that a primary Is electron is ejected, following which a 2s
electron falls into the vacant orbital and a 2p Auger electron is
ejected. Labelling these electrons K, LI and L2, respectively, this is a
KLL-type emission and the energy of the resulting Auger electron is
given by£Auger='(K)-/(Ll)-/(L2) (5,15)Since this is a function of ionisation energies only, it is not necessary
to use a monochromatic energy source,, which is an attractive
experimental feature of the technique. Indeed, Auger electrons can
be identified as such by their independence of the incident energy.
With many experimental features in common, it is possible to
construct a single instrument with facilities for XPS, AES, LEED and
certain other studies. The energy source for AES is usually an
electron beam, rather than X-rays. The disadvantage of an electron
beam is that it may cause structural damage. The advantage is that it
can be focussed (as in the electron microscope) and scanned over the
surface - scanning Auger electron spectroscopy (SAES). It is possible
to focus down t610"^14 m^2 and systematically scan an area of 10~^8 m^2.
A facility for sputter-ion etching by Ar+ ion bombardment can be
included. This enables the surface layers to be removed progressively
prior to studying the underlying surface features. AES is similar, in
principle, to SEM (see page 49) except that the latter employs an
incident electron beam of higher energy.
AES can be sensitive to as little as 1 per cent of a monolayer of
adsorbed material and is often used as a preliminary investigation of
the cleanliness of a surface prior to its study by LEED.
Figure 5.14 shows SAES analysis of surface iron, aluminium,
calcium and potassium on reduced ammonia synthesis catalyst. As
can be seen, these surface distributions are patchy rather than
uniform. The calcium and, to a lesser extent, the aluminium tend to
be located in areas of their own where iron is absent, but the
potassium tends to be located alongside the iron. This supports the
view that calcium and aluminium oxides are structural promoters,
with the role of keeping the iron crystallites apart and minimising loss
of surface area through sintering. Potassium is probably a chemical
promoter, i.e. the positively charged potassium ions influence the
electronic structure in the nearby iron such as to give enhanced
catalytic activity.