The mechanism of gas release from the system in the process from ultrahigh vacuum to extreme-high vacuum was overviewed based on the basic evacuation model, quantitative outgassing measurement, and the examples using actual vacuum vessels. The main residual gas species after exposure to the atmosphere is water molecules adsorbed on the vessel wall. Accelerating thermal desorption of adsorbed molecules is the main role of baking. The gas species in the vacuum chamber that has been baked sufficiently is hydrogen molecules, and for extreme-high vacuum generation, a vacuum pumping system having a sufficient pumping speed for hydrogen is important. In the near future, it is highly expected that the extreme-high vacuum region will be a routine for researchers in surface science.
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