Visible and ultraviolet solar radiation is essential for virtually all life on earth as we know it, and photochemical reactions are some of the most important processes taking place in the human environment. Several examples of photochemical influences on our environment have already been given in Chapters 2 and 3. Direct utilization of solar energy as a possible alternative to fossil fuel combustion is discussed in Chapter 15, and its efficient usage is certain to be an increasingly important goal of research and development over the next few decades as we seek solutions for the worldwide energy problem. In Section 2.3.4 it is pointed out that atmospheric oxygen comes from photodissociation of water vapor in the upper atmosphere and from photosynthesis in the biosphere. Photosynthesis is a very important photochemical process that also leads to food production and storage of solar energy (Chapter 15). We discuss photochemical decomposition of petroleum, polymers, and PCBs and pesticides in Chapters 6, 7, and 8, respectively.

Photochemical processes occurring at high altitudes in the mesosphere and stratosphere are essential for the maintenance of the thermal and radiation balance at the surface of the earth (see Chapter 3), and they also provide phenomena such as night glow. The reactions following light absorption in the lower atmosphere that generate photochemical smog from atmospheric pollutants are less desirable! In association with living matter, light-induced reactions lead to such phenomena as vision, cyclic activities of plants and animals, and both cellular damage and repair. In a more prehistoric context, it has been suggested that the origin of life may have occurred via the photochemical synthesis of purines, essential building blocks of nucleic acids.

As we shall show later, absorption of light by a molecule results in excitation of the molecule to a higher electronic energy level. This is followed in many cases by reactions such as dissociation or interaction with other molecules, leading to the overall photochemical process. We shall see that the specific electronic state reached often determines the types of reaction that follow, so that the study of the absorption process is important to our understanding of photochemistry. In this chapter, we shall develop the basic principles of light absorption, electronic excitation, and subsequent photochemical and photophysical processes. As these principles are being developed, they will be applied to specific examples of photochemical processes occurring in our environment.

Photochemistry is the study of the interaction of a "photon" or "light quantum" of electromagnetic energy with an atom or molecule, and of the resulting chemical and related physical changes that occur. The energy necessary for the reaction (or at least for it to be initiated) is thus gained from the photon. This is in contrast to thermal reactions, in which the energy needed for the reaction is distributed among the molecules and among the internal vibra-tional and rotational motions of the molecule according to the Boltzmann distribution law (Figure 4-1).

As an example, consider the possible thermal dissociation reaction of molecular oxygen,

The fraction of molecules with thermal kinetic energy equal to or greater than a "threshold" energy Ec is approximately equal to the Boltzmann factor

where Nc is the number of molecules with energies equal to or greater than Ec, N is the total number of molecules, R is the gas constant, 8.314 J KT1 mol_1, and T is the temperature in kelvin. The energy required to break an oxygen bond is 5.1 eV, which is equivalent to 492kJ/mol.1 Equation 4-2 gives the fraction of 02

1The electron-volt (eV) is the energy required to move an electron through a potential difference of one volt; it is equal to 1.602 x 10~19 J, or 96.49 kJ/mol.

FIGURE 4-1 Distribution of kinetic energy for O2 at 1500 K, expressed as the fraction of O2 molecules having energies between E and (E + 1) kj/mol.

FIGURE 4-1 Distribution of kinetic energy for O2 at 1500 K, expressed as the fraction of O2 molecules having energies between E and (E + 1) kj/mol.

molecules with energies Ec equal to or greater than 5.1 eV at 1500 K to be approximately 7 x 10~18, which is too small to lead to even the most efficient thermal reactions involving oxygen atoms. Many reactions such as this one that are not feasible thermally can, however, be initiated by light, as will be shown.

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