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.

What is desorption?

When a material is placed in Vacuum, the gas which was previously adsorbed begins to desorb i.e. to leave the material.

The desorption is influenced by

1.       Pressure

2.       Temperature

3.       Shape of material

4.       Kind of its surface

The pressure has a basic influence on the desorption phenomenon since according to its tendency of increasing over or decreasing below the equilibrium , the phenomenon of sorption or that of desorption appears.

The temperature has a clear influence on desorption phenomena. Desorption is endothermic, thus it is accelerated by increase of temperature.

The shape of the material influences desorption either if the gas is adsorbed or absorbed.

If the gas is adsorbed, then only the amount of surface is the influencing factor, but if the gas has to diffuse from the interior of the material to the surface, then the third dimension “thickness” is also influencing the rate of desorption.