92168.pdf

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

much magnified (c. 106 ) 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)

i i

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 ® 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