92168.pdf

62 Optical properties

Mixing this broadened, scattered signal with incident light produces a

pattern of beat frequencies, the measurement of which allows the

diffusion coefficient of the particles to be calculated.

Stationary

COQ

Frequency

Figure 3.9 Doppler broadening. For a typical colloidal dispersion, Ao>1/2 = 103 with

<o 0 « 10 i4 Hz

The Doppler shift will vary from zero for zero scattering angle to
maximum for back-scattering (0 = 180°). The width of the Doppler-
broadened peak at half-height (Figure 3.9) is related to the diffusion
coefficient by

= D\n sin (3.10)

where 0 is the scattering angle.
The technique is alternatively called photon correlation spectro-
scopy (PCS) or quasi-elastic light scattering (QELS).
An interesting modification of this technique is the fibre-optic
dynamic anemometer (FODA)^143. A length of fibre-optic cable
carries the laser beam to the interior of the dispersion. Back-
scattered light, with its Doppler frequency shift, is returned to the
detector along with reflected light and, again, the resulting beat
frequency pattern is analysed. Since only a very small volume around