Adsorption. Energy Accommodation Coefficients

When an atom approaches the surface, it may become trapped and only come to rest after traveling some distance along the surface to a suitable site of high binding 

All of these processes have been observed in atomic-beam surface-scattering experiments, during which a beam of atoms of well-defined kinetic energy impinges on a single-crystal surface. The kinetic energy distribution of the back-scattered atoms can be detected by using a correlation chopper velocity selector [18]. 

For an incident atom, = 0 if the kinetic energies of the incident and scattered atoms are the same incident = scauered- Conversely, = 1 if the kinetic energy of the scattered species is equal to the kinetic energy expected for desorbing from a surface of thermal energy kgT urface since, in this circumstance, scattered = surface-For a scattered atom, the energy accommodation coefficient becomes the translational-energy accommodation coefficient. 

---

The previous section noted that a lower kinetic energy results in a longer collision time which decreases the energy transfer. The minimum in surface accommodation coefficient is found for energies Eq- Eg kTAt can be concluded that only for physical adsorption and high temperatures does thermal accommodation at the surface becomes a slow process, but that in other situations rapid thermal equilibration applies. 

---