Knowledge Bank

The problem of coupling between a large amplitude motion state and a small amplitude vibration state remains an area of considerable interest and few conclusions in molecular physics. The $\nu 4$ and $2\nu 2$ states of NH$3$ provide an excellent opportunity to study the coupling of large and small amplitude motions in an inversion system where the quantum mechanics have been worked out in detail . In spite of the well established Hamiltonian, a large body of high quality spectra remains to fit to experimental accuracy. Several recent studies have resulted in 2-10 times the infrared accuracy over restricted ranges of quantum numbers and have either been unable to fit or have completely ignored the 162 reported microwave transitions . Assessment of the perturbations details provides some insight into why previous analyses might have not completely succeeded; however, the ability of current Hamiltonian to completely model the large amplitude inversion coupled with small amplitude vibration remains to be adequately addressed. We report extensive new measurements of the rotation-inversion transitions in the $\nu 4$ and $2\nu 2$ states, a global analysis of existing data and provide a complete assessment of the inversion-rotation-vibration Hamiltonian in NH$3$.