Appendix A: Principles of Operation
© Copyright 2010 RBD Insruments, Inc.
Appendix A: Principles of Operation
Energy, imparted to the sorbed water molecules, will raise their internal energy to a high enough level to
exceed these weak bonds and allow the molecules to desorb. The two most common energy sources are
heat and UV. Heat, the traditional energy source, will result in rapid desorption but it has the disadvantages
of heat-up and cool-down time along with thermal pyrolitic degradation problems with some vacuum
materials such as O-rings.
UV, though, is essentially a non-thermal effect where the UV energy is imparted directly from the UV
source to the sorbed water molecules and thusly requires no heat-up or cool-down time penalty. For
systems that will need to operate in the ultrahigh vacuum hydrogen zone below the drydown zone, heat is
more effective since it can drive both adsorbed gases from surfaces and absorbed gases from the material’s
bulk. UV will only be effective on surface-sorbed gas or gas already released from its original source. UV,
then, should be viewed as a pumpdown enhancement tool instead of a replacement for a 150
°
to 200
°
C
bakeout with metal seals.
The UV Source
The UV band encompasses a wide range of wavelengths, and not all UV sources will provide effective
energy for desorption. The best results are obtained with a hot cathode mercury (Hg) discharge tube of the
type used for ozone (O
3
) formation. These are emitters fabricated from ultra-pure quartz and filled with
inert gas and a trace of Hg.
When electrically energized, the Hg discharge emits UV light in two major wavelength peaks: 254 nm
(about 90%) and 185 nm (about 10%). Only the highly energetic 185nm wavelength UV is effective in
increasing net water vapor desorption, and it is the wavelength that converts oxygen (O
2
) to ozone (O
3
).
Since the 185 nm radiation will be adsorbed by the O
2
in air, the emitters must be operated in vacuum to
allow the UV to reach the internal surfaces. This means operating the emitter within the chamber. Since
the emitter will be exposed to the vacuum, it’s important that they are constructed entirely of vacuum-
compatible materials.
The Process
Although it would seem to be intuitive that the emitter(s) should be mounted to provide direct line-of-
sight to all surfaces, this is really not required. The UV energy will reflect from internal surfaces to
spread through the chamber, but direct desorption is only part of the process. With 185 nm wavelength
light flooding the chamber, water molecules that are desorbing or already desorbed will be continually
energized with the UV, and as they impact other surfaces prior to being pumped away, they will transfer
some of that energy to the molecules on the surfaces. Emitters can even be operated within an appendage
nipple as a floodlight with only slightly reduced results.
Thusly, the UV serves, directly and indirectly, as an energy pump to maintain a total high energy level of
all the water molecules within the system. In essence, this keeps them in motion until they enter the pump
and reduces their chance of resorbing on a surface.
Additionally, the 185 nm radiation converts much of the water molecules to the energetic free radical OH
8
.
The free radicals also serve to maintain a high energy level. Their effects can be seen in a large increase
in carbon dioxide (CO
2
) in the residual gases while the emitter is operating. This is caused by oxidation
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