Ultra high vacuum
Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about 10−7pascal or 100 nanopascals (10−9mbar, ~10−9torr). UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is approximately 40 km, so gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber.
UHV conditions are integral to scientific research. Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates. Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV.
Maintaining UHV conditions requires the use of unusual materials for equipment, and heating of the entire system above 100 °C for many hours ("baking") to remove water and other trace gases which adsorb on the surfaces of the chamber. Materials which are not allowed due to high vapor pressure:
Technical limitations:
Ultra-high vacuum is necessary for many surface analytic techniques such as:
UHV is necessary for these applications to reduce surface contamination, by reducing the number of molecules reaching the sample over a given time period. At 0.1 mPa (10−6 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.
UHV is also required for:
and, while not compulsory, can prove beneficial in applications such as:
Typically, UHV requires:
Outgassing is a problem for UHV systems. Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by selection of materials with low vapor pressures (such as glass, stainless steel, and ceramics) for everything inside the system. Materials which are not generally considered absorbent can outgas, including most plastics and some metals. For example, vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.
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