Stark effect in diatomic, linear and symmetric rotor molecules

3 Stark effect in diatomic, linear and symmetric rotor molecules 

As we saw in Equation (1.61), space quantization of rolational angular momenlum of a diatomic or linear polyatomic molecule is expressed by 

In this rather formidable equation there are two points to notice  

Because of point 2, rotational microwave and millimetre wave spectroscopy are powerllil techniques for determining dipole moments. However, the direction of the dipole moment cannot be determined. In the case of 0=C=S, for which /r = 0.715 21 0.000 20 D [(2.3857 0.0007) x 10 ° C m], a simple electronegativity argument leads to the correct conclusion - that the oxygen end of the molecule is the negative end of the dipole. However, in CO, the value of 0.112 D (3.74 x 10 C m) is so small that only accurate electronic stmcture calculations can be relied upon to conclude correctly that the carbon end is the negative one. 

For a symmetric rotor the modification Eg to the rotational energy levels in an electric field S is larger than in a linear molecule and is given, approximately, by 

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