Difference in quasi elastic scattering and non elastic scattering

[watertreader]

hi,

How do we differentiate between quasi elastic scattering and non elastic scattering?

Was told that dynamic light scattering falls under the quasi elastic scattering... but it seems to me that light undergoing such scattering have its frequency shifted... this seems to me to be that energy is being lost...shouldn't it be classified as non elastic scattering?

Thanks

 

[Andy Resnick]

Quasi-elastic light scattering (QELS) is simply the approximation that the frequency shift is much smaller than the frequency. Conceptually, it arises from particles that are moving rather than intrinsic processes (for example, Raman scattering).

The information gained is the velocity distribution of scatterers. This is why DLS has been so useful as a probe of soft matter (hard sphere colloids, for example) phases. The angular dependence of the scattered light is related to the velocity distribution, so by moving the detector, different velocities are measured. Forward scattered light corresponds to v = minimum, while backscattered light corresponds to maximal velocities.