Light absorption, directional nature

Figure 3.12 Directional nature of light absorption in H2 molecule.

Let the coordinate system be such as that given in Figure 4. IS. The electric vectors of a plane polarized radiation vibrate along OZ in the ZX plane and OX is the direction of propagation of the plane polarized wave. When a solution of anisotropic molecules is exposed to this plane polarized radiation, the electric vector will find the solute molecules in random orientation. Only those molecules absorb with maximum probability which have their transition moment oriented parallel to OZ (photoselection). Those molecules which are oriented by an angle 6 to this direction will have their absorption probability reduced by a factor cos 6, and the intensity of absorption by cos2 6. Finally, the molecules oriented perpendicular to the electric vector will not absorb at all. These statements are direct consequences of directional nature of light absorption... 

The extinction I is proportional to the concentration C of light absorbers in the medium and the directional nature of absorption in an anisotropic medium is denoted by the vector g, known as the molar extinction. Because Equation 6.8 was derived for a wave propagating in the vacuum, the contribution of the extinction to the light intensity is lost in Equation 6.7. For an optical wave propagating in an isotropic medium, Equation 6.7 can be rewritten in the form of Equation 6.10. 

Scheme 1. Conceptual model for direct and indirect photoreactions in heterogeneous systems. Symbols P represents the photoreactive halocarbon in solution, R-P represents halocarbon sorbed in reactive components of the sorbent, and U-P represents the halocarbon sorbed in unreactive components of the sorbent. Sorbents are sediments, natural organic matter, or biota. The types of reaction are direct with light absorption by P and indirect with light absorption...

Now it becomes increasingly clear that selectivity can be achieved in photochemical reactions. This is due to highly selective nature of light absorption which allows injection of energy into particular bonds of particular molecule. Under proper conditions, photochemistry may provide a short route for the synthesis of systems which are essentially unavailable by alternate synthetic methods. The development of spectroscopic techniques provides the way for direct study of transient intermediates. In this manner photochemistry gets today s face. 

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