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.

 

[astrokreng]

for your question. Quasi elastic scattering and non elastic scattering are two types of scattering phenomena that occur when light interacts with matter. The main difference between these two types of scattering lies in the energy transfer between the light and the particles it scatters off of.

Quasi elastic scattering, also known as Rayleigh scattering, occurs when the energy of the scattered light remains the same as the incident light. This means that there is no energy transfer between the light and the particles it scatters off of. Dynamic light scattering, also known as photon correlation spectroscopy, is a type of quasi elastic scattering where the motion of particles in a solution can be measured by analyzing the fluctuations in the scattered light. This type of scattering is often used to study the size and movement of particles in a solution.

On the other hand, non elastic scattering, also known as Raman scattering, occurs when the energy of the scattered light is changed due to interactions with the particles. This means that there is energy transfer between the light and the particles, resulting in a shift in the frequency of the scattered light. This type of scattering is often used to identify the chemical composition of a sample as different molecules will have unique Raman spectra.

In regards to your question about dynamic light scattering being classified as quasi elastic scattering, this is because although the scattered light may have fluctuations in intensity, the energy of the light itself remains unchanged. The frequency shift that you observed is due to the Brownian motion of particles in the solution, which causes slight changes in the path length of the scattered light, resulting in a Doppler shift in frequency. This does not indicate a loss of energy, but rather a change in the path of the light due to the movement of particles.

I hope this helps clarify the difference between quasi elastic and non elastic scattering. Both types of scattering are important tools in the study of matter and can provide valuable information about the properties and behavior of particles in a sample.