Quasi-Elastic Light Scattering QELS Method

In the inelastic or quasi-elastic light scattering (QELS) method, the autocorrelation function of the scattered light is measured. Again for monodisperse spherical objects. 

In recent years, the quasi-elastic light scattering (QELS) method has become increasingly routine in experimental studies of polymer dynamics in both dilute and concentrated solutions. It allows us to obtain information about the possible modes of Brownian motion of a polymer molecule either isolated or entangled with others. Leaving its technical details and various practical applications to reference books [38], we here give a brief account of its important aspects which are closely related to the current theory of dilute polymer solutions. 

Photon correlation spectroscopy — also dynamic light scattering (DLS) or quasi-elastic light scattering (QELS) — is a robust and rapid method for size determinations in the colloidal range (from about 5nm to 1 m ). In PCS, intensity fluctuations of the scattered light due to particle motion are measured in dependence on time. By the diffusion coefficient of the particles in the measurement fluid, the particle diameter can be calculated according the Stokes-Einstein Eq. (9.1) under the assumption of a spherical particle shape ... 

Dynamic light scattering involves the study of time-dependent fiuctuations in the intensity of scattered light which are the result of the Brownian motion of the particles. The random movement of the particles causes the distances between the particles to fiuctuate, causing constantly varying constructive and destructive interference patterns. This time-dependent fiuctuation in the intensity can be correlated with itself to obtain the diffusion coefficient of the particles. This method is also called photon correlation spectroscopy (PCS) and quasi-elastic light scattering (QELS) [21, 22]. 

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