Loss mechanisms scatterers

It is important to understand the sources and loss mechanisms of stratospheric sulfate aerosols. These aerosols are linked to the decrease in ozone at mid-latitudes because they hydrolyse N2O5, reducing the amount of NOx that would otherwise limit the efficiency of chlorine-catalysed ozone depletion. In addition these aerosols scatter light, cooling the planet [127]. Their concentration increases dramatically following major volcanic eruptions however they are always present at background levels. The source of these background aerosols is a matter of debate. In 1976 Paul Crutzen presented the idea that sulfate aerosols result from the photolysis of carbonyl sulphide [128] ... 

A large amount of the studies on this field are devoted to the analysis of the energy loss of atomic particles scattered off surfaces under grazing angle of incidence. The aim of this work is to review some recent developments on this problem. We focus on the theoretical approaches that have been used to characterize and understand the energy loss mechanisms under these conditions. 

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. 

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. 

Only the first four loss mechanisms (viscous, thermal, scattering, and intrinsic) make a significant contribution to the overall attenuation spectra in most cases. Structural losses are significant only in structured systems that require a quite different theoretical framework. These four meeha-nisms form the basis for acoustic spectroscopy. 

Cooled atoms enable fundamental experiments in collision physics [9.452]. When the particles move very slowly, scattering can be described accurately by theory. The study of collisions is also important since they constitute a loss mechanism in laser cooling. On the other hand, collisions are necessary to achieve the evaporative cooling needed for Bose-Einstein condensation (see below). 

---