Here on the earth vacuum is achieved by pumping on a vessel, the
degree of vacuum increasing as the pressure exerted by the residual
gas decreases below atmospheric. Measuring a system's absolute
pressure is the traditional way to classify the degree of vacuum. Thus,
we speak of low, medium, high and ultra-high vacuum corresponding to
regions of lower and lower pressures.
At first approach the limits of these various ranges may look as
arbitrary, since for each range there are specific kinds of pumps and
measuring instruments. In fact, each of these various vacuum ranges
corresponds to a different physical situation. In order to describe
these situations it is useful to utilize the concepts of molecular
density, mean free path, and the lime constant to form a monolayer,
concepts which are related to the pressure, as well as to the kind of
gas and its temperature.
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Molecular density is the average number of molecules per unit volume.
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Mean free path is the average distance that a molecule travels in a
gas between two successive collisions with other molecules of that gas.
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Time to form a monolayer is the time required for a freshly cleaved
surface to be covered by a layer of the gas of one molecule thickness.
This time is given by the ratio between the number of molecules
required to form a compact monolayer (about 8 x 1011 molec/cm2) and
the molecular incidence rate (at which molecules strike a surface).
Low (and medium) vacuum - the number of molecules of the gas phase is
large compared to that covering the surfaces, thus in this range the
pumping is directed toward rarefying the existing gas phase. The range
extends from atmospheric pressure to about 10-2 Torr.
High vacuum - the gas molecules in the system are located principally
on surfaces, and the mean free path equals or is greater than the
pertinent dimensions of the enclosure. Therefore the pumping consists
in evacuating or capturing the molecules leaving the surfaces and
individually reaching (molecular flow) the pump. This is the range
where particles can travel in the vacuum enclosure without colliding
with other particles. The range extends from about 10-3 to 10-7 Torr.
Ultra-high vacuum - the time to form a monolayer is equal or longer
than the usual time for laboratory measurements, thus "clean" surfaces
can be prepared and their properties can be determined before the
adsorbed gas layer is formed. This vacuum range extends from about
10-7 to 10~K TOIT (lower limit decreasing with the progress of the
technology).
Composition of the gas - While the total pressure in a vacuum chamber
decreases, the composition of the gas phase changes as well. In the
low vacuum range the composition of the gas mainly resembles that of
the atmosphere. In the high vacuum range the composition changes
continuously, toward one which contains 70-90 percent water vapour.
The water molecules come from the surfaces. As pumping is continued
and heating is applied, the carbon monoxide content increases. In the
ultra-high vacuum range hydrogen is the dominant component, coming
mostly from the bulk of the materials (perineation).. |
