Modeling the spectrum of the 2ν₂ and ν₄ states of ammonia to experimental accuracy

The vibrational spectrum of ammonia has received an enormous amount of attention due to its potential prevalence in hot exo-planet atmospheres and persistent challenges in assigning and modeling highly excited and often highly perturbed states. Effective Hamiltonian models face challenges due to strong coupling between the large amplitude inversion and the other small amplitude vibrations. To date, only the ground and ν₂ positions could be modeled to experimental accuracy using effective Hamiltonians. Several previous attempts to analyze the 2ν₂ and ν₄ energy levels failed to model both the microwave and infrared transitions to experimental accuracy. In this work, we performed extensive experimental measurements and data analysis for the 2ν₂ and ν₄ inversion-rotation and vibrational transitions. We measured 159 new transition frequencies with microwave precision and assigned 1680 new ones from existing Fourier transform spectra recorded in Synchrotron SOLEIL. The newly assigned data significantly expand the range of assigned quantum numbers; combined with all the previously published high-resolution data, the 2ν₂ and ν₄ states are reproduced to experimental accuracy using a global model described here. Achieving experimental accuracy required inclusion of a number of terms in the effective Hamiltonian that were neglected in previous work. These terms have also been neglected in the analysis of states higher than 2ν₂ and ν₄ suggesting that the inversion-rotation-vibration spectrum of ammonia may be far more tractable to effective Hamiltonians than previously believed.