Page 160 - Introduction to Colloid and Surface Chemistry

The solid-gas interface 149

the anode. Electrodes are wrenched from the metal tip, accelerated
along radial lines and hit the fluorescent screen where they display a
6
much magnified (c. 10 ) picture of the distribution of surface atoms in
the metal tip.
In field ionisation microscopy (FIM), helium at low pressure is
introduced into the above system and the polarity of the applied
potential difference is reversed. Helium atoms in the vicinity of the
now positively charged metal tip are stripped of an electron and the
resulting helium ions are accelerated radially to the negatively
charged fluorescent screen.

Real space Reciprocal space

•II

(b)
o ® o ®
o o o o o
o ® o ® o ®
o o o o o o o
® O ® O ®-0~®
O O O O O
i i
® O ® O

Figure 5.21 Real and reciprocal space. Substrate atoms are represented by dots and
adatoms by circles. The relationship, atb - b*/a*, holds, but the absolute dimensions
of the reciprocal space mesh, of course, depend on instrumental magnification