Degenerate vibrations higher vibrational levels

From these selection rales, only the + — rule is strictly valid. In addition to these rales, perturbation allowed transitions are possible in ammonia. For example, the ground vibrational-inversion state of NH3 is mixed by a higher-order vibrational-rotational interaction with the doubly degenerate U4 level (Fig. 8). As a result of these mixings, there are the following transitions allowed by these interactions (see Fig. 8)... 

For the selective enhancement of the wanted reaction channel by laser excitation of the reactants, the time span At between photon absorption and completion of the reaction is of fundamental importance. The excitation energy n hco (n = 1,2,...) pumped by photon absorption into a selected excited molecular level may be redistributed into other levels by unwanted relaxation processes before the system ends in the wanted reaction channel. It can, for instance, be radiated by spontaneous emission, or it may be redistributed by intramolecular radiationless transitions due to vibrational or spin-orbit couplings onto many other nearly degenerate molecular levels. However, these levels may not lead to the wanted reaction channel. At higher pressures collision-induced intra- or intermolecular energy transfer may also play an important role in enhancing or suppressing a specific reaction channel. 

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