Chapter3 - Surface Processes & Outgassing

3.1 Introduction

Although an obvious and important role of surfaces in vacuum practice is that of the inner wall of the vessel in providing the boundary of the vacuum within, there are a number of processes occurring between gases and surfaces, both inside the vacuum and at its boundary wall, that make the role of surfaces more important than their mere mechanical presence.

There are few circumstances in which surfaces play a strictly neutral role. For example, as we shall see shortly, the rebound or scattering of gas molecules from surfaces is not as simple as it is modeled to be in the kinetic theory, though the results of that theory are correct.

In addition to scattering molecules in non-simple ways, surfaces may, depending on a number of factors, trap molecules in bound states, a process called  adsorption.  The reverse process, the release of molecules from trapped states, is called  desorption. Adsorption  finds applications in pumping, while, less benignly, desorption is the principal feature of the process of outgassing, a property of fundamental importance and characteristic of all surfaces in vacuum. It describes the prolonged desorption of gas molecules from a surface, some of which reach it from subsurface regions in which they have been earlier stored by exposure to the atmosphere and other mechanisms.

Thus, after the air in a vacuum vessel has been pumped away, pumping is still needed to remove the continual outgassing load from surfaces exposed to the vacuum. These matters, together with sputtering, which is the removal of surface atoms due to energetic particle bombardment, are to be discussed in this chapter.

3.2 The Scattering of Molecules by a Surface

There are three possible outcomes to collision of molecule with the surface.

1. An  elastic  rebound without energy loss, in which the angle of incidence made with the normal to the surface at the point of impact equals that of reflection, with the directions of incidence, reflection, and the normal all being in the same plane. In this “specular” reflection, the molecule’s linear momentum parallel to the surface is unchanged. Perpendicular to it, the momentum is unchanged inmagnitude but reversed in direction, as shown in figure (a) below.

2.  Although the rebound of the molecule to the gas phase is more or less immediate, some energy is either lost to the surface or gained from it (for example, by exchange of energy with vibrations of the surface atoms), in which case the collision is described as inelastic as shown in fig. (b) below.

3.  The molecule may lose sufficient kinetic energy that it becomes bonded to the surface in an adsorbed state. Its stay in this trapped state will not be permanent. A molecule that has lost sufficient energy to bring it into equilibrium with the host surface at the prevailing temperature will nevertheless have kinetic energy of vibrational motion under the restraints imposed by its bonding to the surface and will eventually, by statistical chance, acquire enough energy from favorable collisions with its neighbors to break free.

Depending on the identities of the molecule and the surface atoms, its stay may be brief or long. When the molecule does leave the surface, its direction of departure will be random and unrelated to its direction of arrival, because its initial momentum parallel to the surface, like the perpendicular component, will have been dissipated in the collisions that led to its capture. Thus, whereas elastic and inelastic rebounds off the surface will occur with either complete or partial preservation of the parallel component of momentum in the plane of incidence, the direction of release from a trapped state can be anywhere into the three-dimensional solid angle of 2 π  above the surface, as suggested in fig. (c) below.

Fig. Rebound of a molecule from a surface. (a) Specular, (b) inelastic, (c) via a trapped state.

In the context of molecules losing energy to a surface, one may define an energy accommodation coefficient αE by the equatio                                                              

in which E1 and  E2 are the energy of the incident molecule before and after scattering, respectively, and  ES the energy associated with equilibrium at the surface.

Thus, for elastic reflection  E1 = E2 and  αE = 0, and No adsorption

While if E2 = ES, the other extreme,  αE  = 1 and accommodation is complete.

In non equilibrium situations for example, if a gas is hotter than a surface on which it impinges the energies  E1and  E2 can represent the mean kinetic energies of translation associated with the  fluxes of arriving and scattered molecules.

Because the mean kinetic energy of translation is 3kT/2, above equation may then also be expressed as                                                             

For diatomic and polyatomic molecules like N2, CO, and H2O, for example,there are vibrational and rotational kinetic energies in addition to that of translation, although these may not be excited at 300 K. Potentially, however,the complexity of the scattering process is greater than for monatomic species such as argon, and accommodation coefficients appropriate to different aspects of the scattering process need to be defined.

3.3 Adsorption and Desorption

Adsorption

Adsorption is the process of adhesion of molecules of liquid or gases onto the surface of a solid particle. While absorption is a bulk phenomenon where molecules of absorbate enter into the absorbent.

The character of the adsorbed state in which a molecule may be trapped on a surface will depend, of course, on the identity of the partners. For example, one would expect that the interaction between an inert gas such as argon and a glass surface would differ from that of a chemically active gas, such as oxygen, and a metal.

The bonds established between a molecule and a surface are not intrinsically different in nature from those existing within molecules and in bulk phases.

Physical Adsorption:

If molecule adsorbs on a surface via van der Waals forces, which arise from the attraction between fluctuating dipole moments such adsorption is called physical adsorption. Van der Waals forces are responsible for the condensation of gas to liquid, and are the source of the attractive force between a molecule and a surface.

It is the weakest form of adsorption and this type of bonding leaves the structure of the molecule essentially unchanged and is purely physical in origin. Physical adsorption, sometimes shortened to  “Physisorption”.

Heats of adsorption in physical adsorption, referred to one mole, are in the range of 6–40 kJ per mole. For nitrogen on various surfaces, it is about15 kJ mol-1

Chemical Adsorption:

In contrast, in Chemical adsorption bonding involves electron transfer or sharing between the molecule and atoms of the surface.

Bonding in chemisorption is generally much stronger. It can be thought of more as a chemical reaction between the two. Chemical adsorption sometimes shortened to “Chemisorption” 

In many cases it alters a molecule’s structure and occurs with dissociation of the molecule into its constituent atoms. In others, the molecular identity is retained. For example, when adsorbing at room temperature on nickel, hydrogen dissociates, whereas carbon monoxide on nickel does not.

Heats of adsorption in chemical adsorption, referred to one mole, are in the range of, from about 40 up to 1000 kJ per mole.

Molecular Stay or Adsorption Time

For a molecule that becomes adsorbed at t = 0, the average time it stays adsorbed τa, may be evaluated as follows.                                         

Evidently, this equation makes the sensible predictions that the stronger the binding to a surface and the lower the temperature, the greater is the average time of stay.

Variously, τa is called the mean stay time, or the adsorption time. Because of the exponential factor, the variation of τa with binding energy is quite dramatic, as Table below shows for species adsorbed at room temperature. The ratio of binding energy to the thermal energy is critical. For molar rather than molecular specifications of  q,  kT is,of course, replaced by R0T.

Notice the enormous range of times, from small fractions of a nanosecond in physisorption for the first two entries, through micro and millisecond times in weak chemisorption to seconds, days, and years, with progressively stronger chemisorption. But, as well as that of q, the effect of temperature is profound.

Consider physisorption at 15 kJ mol-1 at room temperature ~300K. Lowering the surface temperature to that of liquid nitrogen 77 K will give a roughly fourfold increase in  q/T and raise the stay time from 5  × 10-11s to that of the fourth entry in the table, 5.5 × 10-3s, i.e., increase it by a factor~108. Such is the behavior of thermally activated processes.

The strong effect of temperature is also evident in table below showing effect of temperature on stay time, which shows the stay time at 77, 295, and 600 K for a q value of 80 kJ mol-1 (~0.8 eV per molecule) which is approximately the binding energy of water on many surfaces.                                      

Fractional Coverage (θ)

It is the fraction or part of atoms adsorbed on a unit surface area of total number of atoms that can be adsorbed on that surface area forming a single monolayer of atoms. It is given by

                                                      θ = na/N0

Where na is number of molecules adsorbed per unit surface area and N0 total number of molecules that can adsorbed on unit surface area forming a monolayer.

Sticking Coefficient

The  sticking coefficient s = s(θ ) is defined as the probability that, on striking the surface already having coverage θ , a molecule becomes adsorbed.

Whatever its initial value, one would expect it to change with coverage, becoming less as the number of available sites is reduced. Measurements show that for chemisorption at room temperature, initial sticking coefficients  s(0) on fresh surfaces are usually between 0.1 and 1 and that they start to decrease at  θ ∼ 0.5, falling to very low values as  θ  approaches complete coverage.

Analytical Model for Population of Adsorbed Molecules on a Surface

Provided we restrict ourselves to dilute adsorbed layers, θ << 1, consistently with our earlier assumption for the desorption process, it is possible to create a simple model for the equilibrium state as follows. The rate of adsorption at coverage  θ  will be                                                      

The rate of desorption will be                                            

At equilibrium the rates balance and so                                                                

Where na is required population of adsorbed molecules at equilibrium state.