Diatomic molecules electronic band shapes

A diffuseness or weakening of discrete band lines, instead of being due to predissociation, may also arise from a different cause to which 01denberg(24) has first called attention. The energy curves V (p) which govern the nuclear vibration of a diatomic molecule in a given electronic state as ordinarily spoken of refer to the non-rotating molecule. For a stable molecular state they have the shape shown in fig. 66 (a). If the molecule rotates, i.e. if J 0, then, as pointed out already in section 55, we must add to this curve the term... 

In this chapter, we extend our treatment of rotation in diatomic molecules to nonlinear polyatomic molecules. A traditional motivation for treating polyatomic rotations quantum mechanically is that they form a basis for experimental determination for bond lengths and bond angles in gas-phase molecules. Microwave spectroscopy, a prolific area in chemical physics since 1946, has provided the most accurate available equilibrium geometries for many polar molecules. A background in polyatomic rotations is also a prerequisite for understanding rotational fine structure in polyatomic vibrational spectra (Chapter 6). The shapes of rotational contours (i.e., unresolved rotational fine structure) in polyatomic electronic band spectra are sensitive to the relative orientations of the principal rotational axes and the electronic transition moment (Chapter 7). Rotational contour analysis has thus provided an invaluable means of assigning symmetries to the electronic states involved in such spectra. 

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