The avoided crossing between Isa and 2sa

A perhaps overfanciful pictorial representation of this avoided crossing is portrayed in Fig. 3.8. If Iscr and 2sct were of different symmetry species, they 

Chapter 3. Diatomic Molecules and their Molecular Orbitals 

The state symmetries of heteronuclear diatomic molecules are simpler than those of homonuclear diatomics the g and u states of Doo/i simply coalesce in Coot . Table 3.2 summarizes the correlations of the irreps of Doo/i with those of its subgroup Coov and between the latter and those of C2v  

An X2 molecule has been set up in Fig. 3.9, where - as has been our wont -the X and y directions are fixed with a hypothetical quadrupolar field, so D2/1 labels can be used. 

As the molecule rotates, the displacements are assumed to remain perpendicular to the internuclear axis. Inertia does in fact tend to increase the XX distance, leading to centrifugal distortion and thus to interaction between rotation and vibration this effect can be ignored in the present context. 

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