Angular scattering measurements

Angular scattering measurements [336] for Ba + 02 indicate that the reaction proceeds via a long-lived collision complex and that the product recoil distributions are well described by statistical models using a loose transition state with 0.40. The BaO product vibrational and rotational distributions have been determined by laser-induced fluorescence methods, showing that about two-thirds of the available... 

The most crucial measurements would probably be improved angular scattering measurements using the crossed supersonic beam technique. This method will simultaneously deliver an improvement of the signal-to-noise ratio and velocity selection. This will result in an extension of this work to wider angles and simplify the theoretical analysis. The emergence of quantum undulations will almost certainly increase the decisive character of the measurements. 

An example of practical importance in atmospheric physics is the inference of effective optical constants for atmospheric aerosols composed of various kinds of particles and the subsequent use of these optical constants in other ways. One might infer effective n and k from measurements—made either in the laboratory or remotely by, for example, using bistatic lidar—of angular scattering fitting the experimental data with Mie theory would give effective optical constants. But how effectual would they be Would they have more than a limited applicability Would they be more than merely consistent with an experiment of limited scope It is by no means certain that they would lead to correct calculations of extinction or backscattering or absorption. We shall return to these questions in Section 14.2. 

An instrument for angular light scattering measurements is often called a nephelometer, from the Greek work nephele for cloud. To be more precise, the instrument shown schematically in Fig. 13.5 is a polar nephelometer, so named because of its angular detection capability. Its essential elements are a col-... 

Figure 13.5 Schematic diagram of a polar nephelometer for measuring angular scattering. FI and F2 are possible polarizing filters.

Angular light scattering measurements are sometimes classified as either absolute or relative. In an absolute measurement 7s//(, which is directly related to the differential scattering cross section (13.2), is determined in a relative measurement the irradiance is referred to some arbitrary scattering angle, say 10°, so that (assuming azimuthal symmetry)... 

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. 

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