Page 147 - Introduction to Colloid and Surface Chemistry

The solid-gas interface 137

quotes over twenty (not counting minor variations)), but only a few
of the more important techniques have been singled out for
discussion here.
In addition to their particular instrumental requirements, the
viability of most of these techniques is due to advances in high
vacuum technology. In order that clean surfaces may be examined,
they must be presented in a suitably uncontaminated state (see page
119) and recontamination during the course of the experiment must
be avoided. Conventional high vacuum equipment (diffusion pumps)
permits evacuation down to about 10~ 6 torr and is used to remove
most of the bulk gas. However, even at this pressure, of the order of
ten of the remaining gas molecules will collide with each surface atom
every second, so, with only 1 per cent of these molecules actually
sticking, the solid surface would be well contaminated within ten
seconds. It is, therefore, often necessary to achieve ultra-high
10
vacuum (UHV) conditions down to about 10" torr. There are
several ways of doing this, but the pumped gas remains in the system
rather than being removed. Ion pumps use electrons to ionise the gas
molecules, which are collected and remain adsorbed at a metallic
cathode. Cryopumps use very low temperature to promote gas
removal by adsorption. lonisation gauges are used to measure the
UHV pressure thus achieved. Stainless steel and aluininosilicate glass
are the materials favoured for apparatus construction and a malleable
metal, such as gold, is used to obtain leak-free valves.
Having considered the preservation of a clean solid surface during
the course of its study, some further comment on the preparation of
the solid sample in the first place is appropriate. Some cleaning and
polishing of the solid prior to its introduction into the UHV system
may be necessary. Various subsequent treatments are then possible.
Heating will help to remove chernisorbed gas, but introduces the risk
of sintering. Ion bombardment (Ar + or electrons) or the use of
pulsed laser light is also effective in removing chemisorbed gas, but
can cause structural damage and leave the solid surface pitted. The
researcher is, to some extent, faced with the dilemma that no
treatment means a contaminated surface, but treatment means a
modified surface. The solid surface to be studied is often generated in
situ. Metal films can be produced by evaporating metal from an
electrically heated filament. By means of remote control manipula-
tion, a solid sample can be crushed under UHV to expose a large area
of fresh surface. In order to obtain far more detailed information,