Light, polarisation scattering

The electron clouds of a molecule irradiated with visible light will orient themselves to some extent in line with the electric vector of the light. If the light is plane polarised, scattered light is visible only in one plane, and if the dimensions of the molecule are small compared with the wavelength of the light, the Rayleigh equation, eqn. (4.21), holds ... 

In this work uniaxially oriented specimens of polyethylene terephthalate were studied, most of which were from the first series used in the work of Nobbs et al. described in Section 5.3.1. The Raman intensity measurements were made using a Coderg PHO spectrometer and a CRL 52A argon ion laser tuned to 488 nm. The tape samples were mounted parallel to the spectrometer slit. The partially focused laser beam was incident normally on them and the scattered light was collected in directions making approximately 180° with the incident light direction. The incident and scattered light polarisation vectors could be chosen parallel or... 

V vertically polarised incident light, vertically polarised scattered light). 

In this technique, normally incident plane-polarised light is scattered by thin films ( 20 pm thickness) of nematic aligned with the director either parallel or perpendicular to the incident light direction. The scattered light is detected as a function of angle, and for different polarisations of the incident (i) and scattered (f) beams. The differential scattering cross-section per unit volume is given by [12] ... 

When the electric field strength of the incident light is ED, the induced dipole will be m, = aE0 where a is the optical polarisability. The electric field strength of radiation scattered by the induced dipole Es, depends on second derivative of m1 with respect to time. The useful experimental quantities are intensities of scattered light (Is) and incident important light (is). These are respectively proportional to Es2 and E02, averaged over a vibrational period, i.e., from time t = 0 to 10/C, where 1 is wavelength of... 

The expressions for scattered light intensity (and Rayleigh ratio) must be corrected by dividing by the appropriate Cabannes factor. Effectively this is equivalent to replacing the optical constant K as defined in Eq. (24) by Kf and by 2 Kfj for unpolarised and vertically polarised incident light respectively. 

Note 3 Chiral nematic mesophases exhibit Bragg scattering of circularly polarised light at a wavelength proportional to the pitch P (Xr = P, where is the mean refractive index). 

As we shall see, the intensity, polarisation and angular distribution of the light scattered from a colloidal system depend on the size and shape of the scattering particles, the interactions between them, and the difference between the refractive indices of the particles and the dispersion medium. Light-scattering measurements are, therefore, of great value for estimating particle size, shape and interactions, and have found wide application in the study of colloidal dispersions, association colloids, and solutions of natural and synthetic macro-molecules. 

Figure 3.6 Radiation envelope for light scattered from small particles. Distances from the origin of the dotted, dashed and smooth lines represent the relative intensities of the horizontally polarised component, vertically polarised component and total scattered light, respectively...

The quantity Re (1+cos2 0) is called the Rayleigh ratio. The unity term in (1+cos20) refers to the vertically polarised component of the scattered light, and the cos2 0 term to the horizontally polarised component. 

The interaction between light and matter can be viewed as the creation of a coherent quantum superposition of initial and final electron states that has an associated polarization [3], as shown in Figure 1. The coherence between states with different wave vector requires an intermediate virtual state and the presence of a coherent phonon. A transition between the initial and final states may occur when the coherence of the system is broken either due to the finite width of an optical wave packet or by scattering from the environment. The transition results in the absorption of a photon and the creation of a hot electron-hole pair. Otherwise, the photon is re-radiated with a different phase and, perhaps, polarisation. 

Since the light scattering method yields the mass-average molar mass, hence a may be written as cNAyJ Mni. Combining Eqs. 5.47 and 5.50 and assuming that the polarisability of the solvent, a0 is negligible, we obtain... 

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