Translational energy accommodation

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 translational-energy accommodation and transfer between a monatomic gas and a metal surface depends on both the translational energy, Ej, of the incident atom and the temperature of the solid, surface- Describe the nature of energy transfer in the two extremes when (a) Ej ... 

Fig. 5. Rotational temperatures ofNO desorbing from Pt(l 11). The data are representative of data published for (x) neat thermal desorption , ( +) thermal desorption in the presence of coadsorbed C0 ° (solid squares) and (solid triangles) trapping/desorption in molecular beam scattering, (open triangle) reaction limited desorption from NO-NHj complexes, (open circle) and (open square) NHj oxidation reactions. The solid line is for full accommodation. The dashed curve represents results for translational energy measurements in direct inelastic scattering ...

Figure 4.8. Translational, vibrational, and rotational energy accommodation coefficients for NO scattered from Pt(l 11) as a function of crystal temperature [93J.

In case the molecular partner in the dimer complex has more than one internal degree of freedom, additional dissociation channels (product internal energies) must be taken into account. These can be accommodated by using different final vibrational state wavefunctions, 4>,/-( ) and correspondingly different translational energies for the departing fragments. 

Scattering results from the well-ordered surfaces have certain features in common. In all these cases, direct-inelastic scattering of NO(v=l) —> NO(v=l) was observed, as characterized by angular distributions peaked near the specular direction and translational energies much higher than expected from accommodation to the surface temperature. 

A molecule adsorbing on a metal surface also has to lose its energy. In vacuum this can only occur by a loss of translational energy to the lattice vibrations of the solid. This process is called thermal accommodation. 

In the case of polyatomic molecules, one may consider separately the accommodation coefficients for translational and for vibrational energy. Values of the latter, civ, are discussed by Nilsson and Rabinovitch [7]. 

Excess energy in Reaction (75) would not be readily accommodated in the translational degrees of freedom available. 

The NO LID results on Pt(lll), Pd(lll) and Pt(foil) are strongly marked both by their dramatic similarities and their subtle differences. All exhibit a thermal LID component with high degrees of translational and rotational accommodation. For the Pt substrates the non-Boltzmann component exhibits (1) anomolously high, state-dependent kinetic energies (2) spin-orbit... 

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