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19 Processes at solid surfaces
19A An introduction to solid surfaces
Answers to discussion questions
D19A.2 �eAuger e�ect is described in Section 19A.3(b) on page 828. InAuger electron
spectroscopy the electron beam is only able to a�ect species on the surface
and perhaps a few layers into the bulk, and therefore the emitted electrons are
characteristic of the surface and the species adsorbed onto it. �e energies of
the emitted electrons are characteristic of the material present and so provide
a �ngerprint of the sample.
In scanningAugermicroscopy (SAM) the electron beam is narrowly focused so
that electrons are excited only from a small region (of dimension about 50 nm).
By scanning the beam across the sample it is possible to build up a map of the
composition.
In scanning tunnelling microscopy (STM) a topological map of the surface is
built up, and under favourable circumstances individual atoms may be identi-
�ed; the resolution is thus much greater than that achieved in SAM. However,
STM gives no clues as to the identity of the atoms on the surface, in contrast to
SAM.
Solutions to exercises
E19A.1(b) �e collision �ux, Zw, is given by [19A.1–825], Zw = p/(2πMkT/NA)1/2 where
p is the pressure of gas, M is the molar mass of the molecule, k is Boltzmann’s
constant, T is the temperature and NA is Avogadro’s constant. From inside the
front cover, 760 Torr = 1 atm = 1.01325× 105 Pa, therefore 1 Torr is 133.32 Pa.
(i) For a nitrogen molecule, the molar mass M = 2 × (14.01 gmol−1) =
28.02 gmol−1, therefore for p = 10.0 Pa
Zw =
p
(2πMkT/NA)1/2
= (10.0 Pa)×(6.0221 × 1023mol−1)1/2
[2π×(28.02 × 10−3 kgmol−1)×(1.3806 × 10−23 JK−1)×(298.15 K)]
1/2
= 2.88... × 1023 m−2 s−1 = 2.88 × 1019 cm−2 s−1