ELECTRONIC STRUCTURE AND SPECTRA IN DIATOMICS

It is customary to obtain fluorescence excitation spectra rather than absorption spectra in jets, where the total fluorescence intensity is monitored as a function of excitation laser wavelength. In cases where the fluorescence quantum efficiency (defined as fluorescence photons emitted/laser photons absorbed) is independent of excitation wavelength, the fluorescence excitation spectrum coincides with the absorption spectrum. Part of the fluorescence excitation spectrum of an Na2 jet operated with vibrational and rotational temperatures of 50 and 30 K, respectively, is shown in Fig. 4.3. The four intense bands arise from electronic transitions from v = 0 in the ground state to v = 25 

This chapter begins with a treatment of symmetry and electronic structure in diatomic molecules. The symmetry selection rules for electronic transitions are derived, and vibrational band intensities (cf. Fig. 4.1) are described in terms of 

Franck-Condon factors. The most common angular momentum coupling cases are discussed, and rotational fine structure in electronic transitions (cf. Fig. 4.3) is rationalized for heteronuclear and homonuclear diatomics using Herzberg diagrams. 

All diatomic molecules belong to either the or point group, and so much of their electronic structure and nomenclature is derived from the properties of these two groups. In what follows, the Cartesian z axis is always taken to be along the molecular axis (the line connecting the two nuclei). The x and y axes are both normal to the intemuclear axis, as shown in Fig. 4.4. 

The partial character table for the heteronuclear point group (which has an infinite number of classes and irreducible representations) is 

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