Inelastic collisions molecule-surface scattering

The site specificity of reaction can also be a state-dependent site specificity, that is, molecules incident in different quantum states react more readily at different sites. This has recently been demonstrated by Kroes and co-workers for the Fl2/Cu(100) system [66]. Additionally, we can find reactivity dominated by certain sites, while inelastic collisions leading to changes in the rotational or vibrational states of the scattering molecules occur primarily at other sites. This spatial separation of the active site according to the change of state occurring (dissociation, vibrational excitation etc) is a very surface specific phenomenon. 

Just as for gas-phase molecular collisions, gas-surface encounters can be elastic, inelastic, or reactive in nature. A wide range of scattering behavior is observed depending upon the gas molecules, the composition, structure, and temperature... 

Molecular projectiles offer the possibility of additional direct inelastic channels, namely, the excitation or de-excitation of the molecular internal modes, much as for gas-phase molecular inelastic scattering. Unlike a gas-phase collision of a molecule with a structureless projectile, here the energy balance of the internal modes of the molecule need not be met entirely by the translation. The participation of the surface degrees of freedom is possible and the low-energy modes of the surface, not only phonons but also electron-hole pairs, are particularly important in bridging the gap (remember the exponential gap principle) and thereby making such inelastic collisions quite efficient. 

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. 

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