Scattering by gas molecules

Observations up to this time have been for systems chosen at random. The chemical reactions observed have been very far from equilibrium, and usually represent one step only of a more complicated situation present when steady state catalysis is proceeding, i.e., when all reactants and products are continually arriving on the surface and desorbing. The much more primitive experiments described below must precede such steady-state studies using LEED as a monitor. It should be remembered that LEED observations are restricted to pressures below 10 Torr because the diffracted beams are severely scattered by gas molecules above this limit. 

The intensity of the coherent electron scattering at the angle by gas molecules is usually represented by the expression... 

It is widely believed that On a clear day you can see forever, as proclaimed in the 1965 Broadway musical of the same name. While an admittedly beautiful thought, we all know that this concept is only figurative. Aside from Earth s curvature and Rayleigh scattering by air molecules, aerosols—colloidal suspensions of solid or liquid particles in a gas—limit our vision. 

Scattering of photons by gas molecules and particles can also reduce the radiant energy transmitted through a layer. In contrast to absorption, the radiant energy scattered remains in the form of radiation, but its direction is altered from that of the incident radiation. If we return to Figure 3.7 and assume that the medium does not absorb but only scatters radiation, then we can write, similarly to (3.10),... 

Smoluchowski-Einstein theory of fluctuations gives an expression for the Rayleigh ratio identical to that previously established [equation (6.12)] (see Appendix). This means that the intensity scattered by a collection of Nt particles of gas is merely the sum of the intensity scattered by individual molecules, with the scattering phenomenon resulting from the random presence/movement of a gas molecule. 

All measured intensities can be put on absolute scale by proceeding as follows. At high angles the scattering pattern can be considered as arising from a collection of noninteracting gas molecules rather than from a liquid sample. 

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