Symmetric rotor degeneracy

There is, in addition, the (27+ l)-fold M degeneracy. A symmetric rotor is classified as/ ro/are if /a /c andoWareif /a>/b, /c-A linear molecule is a degenerate case of a prolate-symmetric rotor, with Ia and Ja = 0. Dipole-allowed transitions of a symmetric rotor have the additional selection rule AA" = 0, since the electric dipole moment lies in the principal axis and cannot be accelerated to a different orientation within the molecule. 

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