The relation between intensity and dipole moment for bending modes in linear molecules

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Abstract
By careful summation over the rotational fine structure, we examine the relation between dipole moment derivative and intensity of bending modes in linear molecules. The relation is derived without the usual assumption of the harmonic oscillator approximation and it is emphasized that the intensity of the degenerate mode cannot be viewed simply as a sum of two orthogonal one-dimensional oscillators, although the intensity is found to be close to two times the intensity of a nondegenerate mode with the same dipole moment derivative. The importance of using the GAMMA intensity rather than the S intensity for the total band intensity is demonstrated, and finally the effect of the Coriolis coupling is summarized. It is pointed out that most recent quantum mechanical computations predict band-intensities of the bending modes in linear molecules that are approximately a factor of 2 bigger than the observed intensities, and it is shown that the problem relates to the ab initio treatment.