Electronic spectra of diatomic molecules

The electronic spectra of molecules, even the smallest diatomic molecules, are more complicated than those of atoms because more than one nucleus is present. However, now we can take advantage of molecular symmetry. Just as with vibrational spectroscopy, electronic spectroscopy of molecules uses group-theoretical ideas for simplification. Because all diatomic molecules have either Coov or symmetry, for the present those two point groups will be important. There are also some similarities in electronic spectra and rotational spectra for diatomic molecules, so a review of rotational motions might be useful. 

As for atoms, term symbols can be defined for diatomic molecules. The term symbols for diatomic molecules are determined similarly to the K quantum number 

Unless otheiwise noted, all art on this page is Cengage Learning 2014. 

The term symbols look very similar to those for atoms, but the quantum numbers involved are given different labels. Instead of the term symbol for 

FIGURE 15.8 Electronic spectra of diatomic molecules are described on the basis of the component of electron angular momentum about the molecular axis, as shown. 

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The major changes in the new edition are as follows There are three new chapters. Chapter 1 is a review and summary of aspects of quantum mechanics and electronic structure relevant to molecular spectroscopy. This chapter replaces the chapter on electronic structure of polyatomic molecules that was repeated from Volume I of Quantum Chemistry. Chapter 2 is a substantially expanded presentation of matrices. Previously, matrices were covered in the last chapter. The placement of matrices early in the book allows their use throughout the book in particular, the very tedious and involved treatment of normal vibrations has been replaced by a simpler and clearer treatment using matrices. Chapter 7 covers molecular electronic spectroscopy, and contains two new sections, one on electronic spectra of polyatomic molecules, and one on photoelectron spectroscopy, together with the section on electronic spectra of diatomic molecules from the previous edition. In addition to the new material on matrices, electronic spectra of polyatomic molecules, and photoelectron... 

The theory of the electronic spectra of diatomic molecules has been treated comprehensively by Herzberg3 The electronic properties of diatomic molecules belonging either to the point group C v or (Si2) are commonly represented by MO schemes (Fig. 2). In order to obtain a consistent notation,... 

Rotation-Vibration-Electronic Spectra of Diatomic Molecules... 

The effect of isotopic substitution appears also in the electronic spectra of diatomic molecules that led to the discovery of 0, 0, C, and by the methods of optical molecular spectroscopy in 1929 (see, e.g., Nash 1975). 

FIGURE 15.9 Electronic spectra of diatomic molecules are more specifically defined in terms of A, fi, and S. A is defined in terms of the orbital angular momentum of the electrons, L. Cl is defined in terms of the total angular momentum, /. The vector difference between L and / is, of course, S. 

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