3.4 Outgassing (Desorption)
Why it is difficult to create Vacuum
Surfaces facing a vacuum release gas. If they did not, practical vacuum technology would be much simpler. The inner wall of a vacuum container would serve simply as a clean and inert barrier to gaseous molecules, and by the selection of a suitable pump all the air and vapor initially contained could be rapidly removed and very high vacuum attained quickly. In practice this is not so. Gas is continuously released, albeit at relatively small rates, but they prove to be a limitation on the attainable vacuum. Even with careful preparatory cleaning of surfaces, the steady release of gas, principally water vapor, limits the degree of vacuum achievable in reasonable times to values ∼10-6 mbar or a little less, the conventional high vacuum. If vacua substantially lower than this are to be achieved, special procedures to reduce out-gassing are necessary. Thus, boundary surfaces play an active role in the attainment of vacuum.

Origins of Outgassing
It is molecules desorbing from bound states at vacuum-facing surfaces that constitute the outgassing flux. Although their point of departure into the vacuum space is at the surface, they have various origins as follows

1. A surface that has been exposed to normal air at atmospheric pressure for some time will be in equilibrium with it and covered by an adsorbed phase, dominantly of water molecules from atmospheric water vapor, which adsorb more strongly than other atmospheric species. If the pressure of the adjacent gas and vapor to which it has been exposed is now lowered by the action of pumping, this equilibrium is upset and molecules desorb into the vacuum space.
2. Exposure to atmosphere will also have caused diffusion of gases into the near-surface region of the bulk material, assisted perhaps by the presence of rough, microcracked surface structure.
3. Metals frequently have an oxidized surface layer, the passivation layer, typically a few nanometers thick, that absorbs gases by their inward diffusion following initial adsorption. From this near-surface region it will tend to diffuse back to the surface and desorb under low-pressure conditions.
4. The material of the boundary wall of the vacuum, typically stainless steel and several millimeters or more in thickness, will also contain gas that was trapped inside it at the time of its manufacture. Under certain conditions its slow diffusion to the vacuum interface and desorption can be significant. Dissolved hydrogen, occurring as interstitial H atoms rather than H2, is present in this way and is particularly important.
5. Although, at very small rate, gas adsorbed on the outer wall of the container may permeate the wall to arrive by similar diffusive processes at the inner wall, prior to desorption. Permeation through the relatively small areas areas associated with elastomer seals that join metallic components can be a significant contribution to outgassing even when, as is usually the case, permeation through bulk walls is not.
Thus, the desorption of gas, its diffusion out of near-surface regions and the bulk, and its permeation through bulk solid material are matters of concern.

Analytical Modeling of Outgassing Process

Outgassing due to desorption of gassing adsorbed on the surface
Measurements show that under the action of pumping, the outgassing of surfaces diminishes with time approximately according to the relationship
In this equation, qG is the rate of release of gas per cm2, referred to as the specific gassing rate, and q1 its value after 1 h of pumping. The quantity t is the dimensionless ratio of the time in hours to 1 h, and α is an exponent that is frequently about 1 for gassing from metallic surfaces, and nearer 0.5 for elastomers.This dependence is followed for tens of hours and then evolves exponentially into a sensibly constant value.
Fig. Typical outgassing rates of adsorbed species on the surface as a function of time.

Outgassing due to desorption of gassing diffused from outside of chamber walls
Evidently, gas that is adsorbed relatively weakly, with q less than 60 kJmol−1 ,is quickly pumped away consistent with the discussion above. Strongly absorbed gas, with adsorption energy 100 kJ mol-1 or greater, des-orbs at such a slow rate that its contribution to the pressure, though pro-longed, is negligibly small. But gas adsorbed with q between these values, and particularly at about 80 kJ mol-1 ,has a sizeable contribution and is long lasting. This is approximately the energy with which the water molecule binds to many surfaces.
Fig. Outgassing rates of diffused species from outside chamber walls as a function of time
This 1/t½ dependence changes at later times into a faster exponential fall exp(-αDt) as the quantity of dissolved gas becomes substantially depleted. Initially, the slope is − 1/2, changing eventually to a steeper linear portion. It describes the hydrogen outgassing of well-baked stainless steel at long times and after all near- surface adsorbed gases have been pumped away.
Where C0 is initial uniform concentration of diffused gas and D is diffusion coefficient.

Outgassing due to desorption of permeated gases from outside of chamber walls
Permeation is the diffusive flow of gas through a vacuum wall of thickness t driven by the pressure difference P across it. For monatomic gases and those that do not dissociate on diffusing into the solid, the outgassing flux is described by the equation (O‟Hanlon, 2003)
where K is the permeation constant with unit m2s-1, qperm expressed in Pa ms-1 ,P in Pa, and t in m.
When vacuum systems are vented back up to atmospheric pressure, direct adsorption from the gas phase leads to the adsorption and absorption into the near-surface of atmo-spheric gases, principally water vapor in relatively large amounts. It is pru-dent to try to minimize water vapor adsorption by venting to dry nitrogen.

How Outgassing contribute to vacuum during pumping process starting from atmospheric pressure

In pumping down from atmospheric pressure, most of the free gas in the pumped volume is soon removed, and in typical systems, sub-millibar pressures of 10− 2 mbar or better are achieved in times of order of minutes. Desorbing gas starts to contribute to the gas load below about 10− 1mbar,and as the pressure continues to fall into the region below 10− 4 mbar the gas load (assuming no leaks) becomes increasingly due to outgassing. Of the molecules that desorb, a number will find the entrance to the pump and be removed immediately. But others, a majority in vacuum chambers of typical proportions, will traverse the chamber to another part of the vacuum wall and further surface interactions before being pumped away. It is this traffic of molecules in transit to and fro across the chamber, fed by desorption and diminished by pumping, that constitutes the number density n of molecules in the vacuum and determines the pressure achieved.
Gas that is loosely bound on internal surfaces is pumped away quickly.Gas that is tightly bound desorbs at a very slow rate and does not contribute a significant load. But water vapor, which has an appreciable probability of desorption, has been stored in large amounts during exposure to the atmosphere. The result is that there is protracted gassing of water vapor from structural materials such as stainless steel and glass. The outgassing rate does decrease, but only slowly, and desorbing water vapor accounts for the dominant gas load for times of order tens of hours, unless bakeout procedures are used. In circumstances in which pump performance did not limit the vacuum achieved, outgassing would be observed to diminish as time progressed, with the contribution to it of the small amount of diffusion of hydrogen from the interior becoming more important, and eventually, after very long times, becoming the dominant contribution. Beyond that, if all the gas stored initially in the wall were removed by diffusion, the ultimate source of gas would be permeation.

Effect of temperature on Outgassing (Baking Process)
Because the desorption of gases is a thermally activated process whose rate increases dramatically with temperature, it is possible by raising surface temperatures to greatly accelerate the desorption process and, indeed, other thermally activated processes such as diffusion that occurs within the bulk of the vacuum wall. This process iscalled “baking” and consists of heating vacuum systems, walls, and contents,to temperatures of 150–250 °C for times of order 10 h, with continual pumping.
Surface adsorbed gas is removed at a much greater rate than at room temperature, and gas absorbed in the bulk diffuses at a much greater rate to the interior surface where it desorbs and is pumped away. Both the surface and its hinterland in the vacuum wall are therefore depleted of gas mole-cules, and on cooling back to room temperature the outgassing rate is dramatically reduced by factors ~103 or more, consistent with the attainment of ultrahigh vacuum.