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Ultra High Vacuum is a a regime of vacuum characterised by pressures lower than typically about 10-9 torr.
Ultra high vacuum is necessary for many surface analytic techniques such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, Secondary Ion Mass Spectrometry etc, as well as thin film preparation techniques that have stringent requirements for purity.
There is typically no pump that can take you all the way from atmospheric pressure to the regime of Ultra high vacuum. Thus we typically use a combination of pumps and switch over from one regime to the other.
At extremely low pressures, outgassing of the vacuum vessel occurs over time. Even if a high vacuum is generated in a hermetically sealed container, there is no guarantee that an adequately low pressure will continue unless outgassing has been accounted for. Outgassing is generally worse at higher temperatures. Even materials which are not naively considered absorbent will outgas. Water vapor is a primary outgas component, even in hard metal vessels (such as stainless steel or titanium). Outgassing can be reduced by desiccation prior to vacuum pumping. Baking the UHV chamber is a common procedure to get rid of the adsorbed gases on the surface of the chamber. It involves heating the chamber to roughly 250 to 400 deg. C. Vessels lined with a highly gas-permeable material such as palladiumThis article describes the chemical element palladium. For other meanings, see palladium (disambiguation Palladium is a chemical element with symbol Pd and atomic number 46. A rare silver-white transition metal of the platinum group, palladium resembles p (which is a high-capacity hydrogenhydrogen helium H Li Full table General Name, Symbol, NumberHydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1 , s Density, Hardness 0. 0899 kg/m3, NA Appearance colorless Atomic properties Atomic weight 1. 00794 amu Atomic radius (ca sponge) create special outgassing problems.