Spectroscopic Terms and Selection Rules

In chemical equations for the reactimis in the atmosphere, atoms and molecules are sometimes denoted with symbols in a parenthesis, e.g. 0( P), CX D), O2 ( 2g ), O2 ( IIu), etc. These notation are called spectroscopic terms differentiating electronic states by the symmetry of angular momentum wave functions. Here, we do not enter into the theory of quantum chemistiy, the meaning and usefulness of the symbols and selection rules which is important for the discussion of photoabsorption and photolysis reactions are remarked. 

giving three different electronic states, Pq, Pi, 2. When these spin-orbit states are not differentiated, it is denoted simply by P omitting the J values. In atmospheric chemistiy, ground and excited states of oxygen atoms are usually denoted as 0( P), 

For general non-linear polyatomic molecules, symbols A, A , Ai, A2, Bi, B2 etc. are used depending on whether the sign of wave function is changed or not for the symmetric operation at the symmetric axis or plane. However, they seldom appear in the discussion of atmospheric chemistry. 

Selection Rules The intensity of photo-absorption for transition from a lower to higher energy state can be determined by the transition probability IRp, where R expresses 

Transition probability is determined only by electronic eigenfunctions for atoms, since there is no vibrational or rotational motion. 

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