Desorption is the physical process where adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding energy that keep it attached to the surface. 
Desorption is the reverse of the process of adsorption, which differs from absorption in that adsorption it refers to substances bound to the surface, rather than being absorbed into the bulk.  
Desorption can occur from any of several processes, or a combination of them: it may result from heat (thermal energy); incident light such as infrared, visible, or ultraviolet photons; or a incident beam of energetic particles such as electrons. It may also occur following chemical reactions such as oxidation or reduction in an electrochemical cell or after a chemical reaction of a adsorbed compounds in which the surface may act as a catalyst.