Analysis of the Ammonia Spectrum Symmetric Rotor

The spectrum of the molecule NH3 is of interest because the fine-line structure of some of the absorption bands can be seen even with an NaCl prism. An additional feature of interest is the presence of inversion doubling. 

Two fundamentals are of the type while two are of the doubly degenerate type e. The doubly degenerate vibrations V3 and V4 account for the fifth and sixth fundamentals. 

The a I vibrations are totally symmetric, so that the molecule remains a pyramid during the vibration. The e vibrations are asymmetric and distort the pyramid. Although an 2 vibration might be expected for a molecule having C v symmetry, no such vibration exists for NH3. All four fundamentals are allowed in both the infrared and Raman spectra. 

The fundamental V4 is perturbed by the overtone 2v2 This overtone has high intensity in the R-branch, which suggests that there is a Coriolis interaction between this overtone and the V4 rotational levels. 

The fundamental V2 shows two distinct Q-branches, with a few lines of moderate intensity between them. These lines do not belong to either Q-branch. 

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