We have already noted some of the important photochemical reactions occurring in the atmosphere that are important to the nature of our environment. In Section 2.3.4 we saw that photodissociation of water vapor in the upper atmosphere may be an important source of atmospheric oxygen. Molecular oxygen is photodissociated into oxygen atoms in the thermosphere, mesosphere, and upper stratosphere, and in so doing it absorbs most of the high-energy radiation below 180 nm. Ozone is produced in the stratosphere by the combination of an oxygen atom and an oxygen molecule. We saw in Section 2.5 that O3 extends the earth's shield against lethal ultraviolet radiation from 180 nm to about 290 nm. The troposphere connects the biosphere (i.e., the earth's surface) to the stratosphere and therefore is closely involved in the chemistry of both. For example, anthropogenic emissions of oxides of nitrogen and hydrocarbons, from internal combustion automobile engines and stationary furnaces, are major contributors to photochemical reactions in the troposphere that drastically affect the quality of our environment. Strong vertical mixing occurs in the troposphere, allowing species like methane and water that are produced naturally, as well as anthropogenic substances such as oxides of nitrogen and chlorofluorocarbons, to be transported upward to contribute to photochemical processes occurring in the stratosphere.

It is pointed out in Chapter 4 that while direct photochemistry is primarily limited to initiation by electronic excitation of the absorbing molecule, different electronic states can result from absorption in different spectral regions, which in turn may lead to different intermediate electronic species and chemical products. A summary of the methods for generating the designations (or term symbols) for atoms and simple molecules is given in Appendix A. The following is a brief summary of these state designations.

The term symbol for an atom can be written in terms of atomic quantum numbers S, L, and J as

(25+1) Lj where S represents the net (i.e., the resultant or vector sum) spin angular momentum (or simply the spin) of all the electrons of the atom; it can be zero or an integral number for an even number of electrons in the atom, or a half-integral number for an odd number of electrons. The quantity (2S + 1) is called the multiplicity. The quantum number L describes the net electronic orbital angular momentum, and is given the symbols S, P, D, F, ..., for the allowed values of L = 0,1,2,3, ..., respectively. J is a quantum number designating the total angular momentum of the atom—that is, the vector sum of the spin (S) and orbital (L) angular momenta. For example, for an oxygen atom (eight electrons) in its lowest (ground) electronic state, S = 1, L = 1, and J = 2, so the term symbol for the ground-state O atom is 3P2.

For a diatomic molecule, the total term symbol is often given as

where S and (2S + 1) are the same as for atoms, A represents the total orbital angular momentum along the internuclear axis (analogous to L, A is given the symbols X,n, A, ..., for A = 0,1,2,3, ..., respectively), and (g or u) and (+ or —) are associated with two types of symmetry operation that the diatomic molecule can undergo. States in which S = 0, 1/2, and 1 (multiplicity = 1, 2, and 3) are called singlet, doublet, and triplet states, respectively.

For polyatomic molecules, in many cases it is sufficient to designate an excited state in terms of the type of molecular orbital from which an electron is excited (the initial state) and the molecular orbital to which it is excited (the final state). In addition, the multiplicity (2S + 1) of the molecule is included in the designation. Specific examples are given in Appendix A.

Radiative absorption leading to transitions between electronic states (in contrast to collisional energy transfer) are limited by selection rules. The following rules give the allowed transitions:1

(a) AS = 0 [A(multiplicity) = 0]; transitions where AS = 0 are called spin-forbidden

2. For diatomic and light-nuclei polyatomic molecules:

Sometimes we will use the complete designations to provide specific information about the states. In other situations, where a specific state is not key to our understanding of the system or the process being given, involvement of an electronically excited state generally is represented simply by an asterisk (*). If no designation is used, the species is in its ground electronic state.

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