Ultra-High Vacuum
Definition and meaning of Ultra-High Vacuum in chemistry.
Ultra-High Vacuum (UHV) is a gas pressure below 10^-7 Pascal, achieved through specialized multi-stage pumping systems that remove nearly all gas molecules from an enclosed chamber.
In more detail
At UHV conditions, the mean free path of gas molecules extends to tens of kilometers, meaning individual molecules travel extremely long distances before colliding with each other or with chamber surfaces. This extreme condition is essential for surface science, electron microscopy, and particle accelerators, where trace atmospheric gases would contaminate surfaces or distort measurements. Achieving UHV requires sophisticated turbomolecular and ion pumps, careful chamber design, and thermal baking to drive adsorbed water vapor and other gases from the chamber walls. The considerable effort and expense are justified by the unique ability to study atomic-scale surface phenomena, chemical reactions, and material properties without gaseous interference.
Key facts
| Pressure threshold | Below 10^-7 Pa (approximately 10^-9 torr) |
|---|---|
| Mean free path | On the order of tens of kilometers (about 66 km at 10^-7 Pa) |
| Equipment required | Turbomolecular and ion pumps, cryogenic traps, baking systems |
| Field | Physical Chemistry |
A scanning tunneling microscope operating in UHV can image individual atoms on a surface and study surface chemical reactions without oxidation from atmospheric oxygen.
Frequently asked questions
How does UHV differ from ordinary vacuum?
Regular vacuum maintains pressures of 10^-3 to 10^-6 Pa with significant gas molecules present. UHV achieves such extreme pressures that surface chemistry dominates all other effects, making atomic-scale surface study possible without contamination.
Why are turbomolecular pumps necessary for UHV?
Rotary vane pumps can only achieve medium vacuum. Turbomolecular pumps use rapidly rotating blades to propel gas molecules toward the exit, enabling the compression ratios needed for ultra-high vacuum conditions.