What is outgassing in vacuum science?

The generation of gas resulting from the desorption is known as outgassing. The outgassing constant is defined as the rate at which gas appears to emanate from unit area of surface, and is usually measured in units of Torr.Liter.Sec-1.Cm-2.     

If the temperature of the material is raised (baking), the outgasssing rate rises to a peak value.

Together with the acceleration of desorption, heating may also have the effect of causing activated chemisorption of physically adsorbed gas (in particular water vapour), which can then be desorbed only by prolonged heating at much high temperatures.

Chemi-adsorbed water vapor continues to be evolved at temperatures in excess of 300 degC. It should therefore appear that a degassing programme should begin with pumping at room temperature to remove physically adsorbed water vapor, before baking is commenced.

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