The UHV Barrier
Below about 10⁻⁴ Pa, the gas load from chamber walls exceeds the gas entering through leaks or from the volume. Water molecules, adsorbed in multiple layers on every surface exposed to air, desorb slowly over hours and days, maintaining a persistent gas load that limits base pressure. This outgassing is the central challenge of ultra-high vacuum technology.
Desorption Kinetics
The outgassing rate from metals follows an approximate 1/t decay during the first hours of pumping, reflecting the diffusion of water from bulk oxide layers. The characteristic energy for water desorption from stainless steel is about 0.9 eV, meaning that thermal activation at modest temperatures dramatically accelerates the process — the basis for bakeout procedures.
Bakeout Physics
Heating a vacuum chamber provides the thermal energy needed to overcome the desorption activation barrier. At 200°C, the desorption rate is thousands of times faster than at room temperature, so molecules that would take months to desorb at 25°C leave in hours. The key is that after cooldown, the cleaned surface has far fewer adsorbed molecules, giving a much lower room-temperature outgassing rate.
Material Selection
This simulation models outgassing for stainless steel, aluminum, copper, and glass — the four most common vacuum materials. Each has different initial outgassing rates and activation energies. Stainless steel dominates due to its strength and weldability, but OFHC copper achieves the lowest outgassing rates after bakeout, making it the material of choice for the most demanding UHV and XHV applications.