Energy accommodation and trapping

Where only a physisorption potential energy well exists at a surface for an incident particle, such as helium, three processes can be distinguished  

Transfer of kinetic energy during an inelastic collision must take place to excitations of the combined system, solid plus adsorbate. These excitations are vibrational modes, of which phonons are the low lying ones, electronic modes, two-dimensional translation, and, for polyatomic incident molecules, rotational modes. Each of these energy-loss processes is considered in turn. 

There are no gas—metal systems for which the dominant loss mechanism has been determined. However, it can be anticipated that developments in angle-resolved inelastic atom beam scattering experiments, exemplified by the recent work of Feuerbacher and Allison [380] with scattering from LiF 100, will make good this deficiency. In cases where single surface phonons are responsible for the inelasticity in He scattering, time-of-flight measurements with the detector scanned away from the molecular beam enable the dispersion curves for surface phonons to be constructed. 

Two experimental parameters relating to energy transfer have been quite widely measured, the accommodation coefficient, ac, and the trapping probability a. In the former, all loss processes are integrated it is defined by 

Experimentally, ac may be determined using a metal filament by measuring the incremental electrical energy that has to be applied to the filament to maintain a constant temperature when gas is introduced to the experimental chamber [381]. 

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