Elastic scattering, molecule surface

Being formed in the gaseous phase medium, the electronically excited particles (EEPs) reach the solid surface by diffusion. The diffusion coefficients of EEPs are, as a rule, smaller than the self-diffusion coefficients of parent gas, a factor that is associated with increasing of the EEP elastic scattering cross-section at parent molecules due to the redis-... 

Insoluble monolayers on an aqueous substrate have been investigated by means of the capillary wave method for many years. Lucassen and Hansen (1966) in their pioneering work neglected the surface viscosity and considered only pure elastic films. Subsequent studies showed that the surface elasticity of real surface films is a complex quantity, and both the equilibrium surface properties and the kinetic coefficients of relaxation processes in the films influence the characteristics of surface waves. However, it has been discovered recently that the real situation is even more complicated and the macroscopic structure of surface films influences the dependency of the damping coefficient of capillary waves on the area per molecule (Miyano and Tamada 1992, 1993, Noskov and Zubkova 1995, Noskov et al. 1997, Chou and Nelson 1994, Chou et al. 1995, Noskov 1991, 1998, Huhnerfuss et al. this issue). Some peculiarities of the experimental data can be explained, if one takes into account the capillary wave scattering by two-dimensional particles (Noskov et al. 1997). 

Fig. 5. A detailed comparison between STM experimental (a) and calculated constant current (b) images of HBDC molecules. The calculated image was obtained using the elastic scattering quantum chemistry (ESQC) technique with additional molecular mechanics optimization of the conformation on the surface to converge with the experimental image. Different heights in the images of the ferf-butyl lobes are observed experimentally, corresponding to different propeller-like conformations of the HBDC molecules. The intramolecular conformation of each experimentally observed molecule can be accurately extracted using ESQC plus molecular mechanics calculations...

The lifetime of the beam is influenced by many factors. The dominant beam loss mechanism results from collisions of the electrons with residual gas molecules in the machine vacuum. Both inelastic and elastic scattering can take place off the nuclei and orbital electrons of the gas molecules. The beam lifetime is inversely proportional to the vacuum pressure that can be achieved. After the start-up of a new storage ring, or one for which the vacuum has been let up to atmosphere and then the ring pumped down and baked, the lifetime will be poor. However, it will improve with operation. This is because the gas molecules adsorbed to the vacuum vessel surfaces are desorbed by the SR itself. 

Elastic and inelastic colUsions of atoms and molecules at surfaces are also of importance. The scattering of hydrogen and helium from surfaces leads to diffraction patterns in the same manner as with LEED, but since the atoms penetrate the surface far less deeply than even low-energy electrons, the structures obtained reflect the very surface of the sample. The inelastic surface scattering of molecules can be examined in detail using laser and mass spectromet-ric detection for the scattered molecules. Such measurements can be used to model the form of the gas-surface interaction potential, knowledge of which is a prerequisite for any detailed picture of gas-surface reaction dynamics. 

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