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.
