Adsorption And Desorption Of Nitrogen Molecules On The Copper Single Crystal Surface Of Cu(Ⅲ)

In this thesis, a model was described for calculation of the kinetic energy and angular distributions of molecules recombinatively desorbing from a single crystal surface. Using the model, we have investigated the kinetic energy and angular distributions of nitrogen molecules desorbing from the copper single crystal surface Cu(111). Calculation results demonstrated that the energy released into the desorbing nitrogen molecules has a distribution, ranging from 2.0eV to 6.0eV. The average value of the released energy is about 4.0eV. The value of adsorption barrier of nitrogen molecules on a clean Cu(111) surface is calculated from the Density function Theory to be 6.0eV. According to the principle of detailed balance, the energy released during desorption should be equal to the adsorption barrier. There should be two possible explanations for the difference between the calculated adsorption barrier and the released energy during desorption. The first explanation is that thermal desorption should occur preferentially at the sites of surface defects and steps of a single crystal surface, at which there are lower heights of adsorption barriers. There would be small possibilities that desorption takes place at terraces of the surface. Therefore we suggest that the low end of the distribution of released energy, which has barriers around 2.0 eV, should be related to adsorption at surface defects and steps of a single crystal surface. The high end of the distribution of released energy, which has barriers around 6.0 eV, should be attributed to adsorption occurring at terraces of the single crystal surface. The other explanation is that the calculated adsorption barrier is the adsorption barrier at a clean surface but desorption does not take place at a clean surface. The surface at which desorption occurred had some adsorbed nitrogen atoms. These adsorbed nitrogen atoms have a significant effect on the adsorption barrier. If one nitrogen molecule desorbs from a clean Cu(111) surface, the energy released during desorption should have been close to the calculated adsorption barrier.The kinetic energy distribution of nitrogen molecules desorbing into the ground state in vibration is almost the same as that of nitrogen molecules desorbing into the vibrationally excited state, and so the excitation in vibration has little effect on adsorption barrier for the N₂/Cu(111) system. Moreover, the energy released during desorption is not only along the direction normal to the surface but also along the direction parallel to the surface.