Empirical rovibrational energy levels of ammonia up to 7500 cm⁻¹

The most recent version of the MARVEL (Measured Active Rotational-Vibrational Energy Levels) protocol and code is used to update and extend the list of accurately known empirical rovibrational energy levels of 14NH3 falling in the range of 0 − 7500 cm−1. Compared to an earlier similar study covering all the measured transitions of 14NH3 [22], the present investigation is limited to transitions with an upper energy level below 7500 cm−1, considers 82 sources (28 new) of transitions data, corrects several older assignments, and utilizes an improved validation procedure based partially on high-quality first-principles rovibrational energies. This study yields 4936 uniquely labelled empirical rovibrational energy levels, the set is complete up to 2413 cm−1. We confirm the overall high accuracy of the most recent “spectroscopic” (fitted) 14NH3 potential energy surface, C2018, up to 7500 cm−1. Investigation of energy values resulting from previous effective-Hamiltonian fits forms an important part of this study. It is established that previous effective-Hamiltonian fits, using high expansion orders in the Hamiltonian, are inaccurate for certain high rotational excitations of all vibrational parents considered. Employing the ground vibrational state as a test case, it is shown that the origin of the problem is that the observed 14NH3 spectroscopic network formed by rotational transitions and used for the effective-Hamiltonian fit contains floating components. A much improved effective-Hamiltonian fit is presented whereby the floating components are connected to the principal components of the spectroscopic network by carefully selected first-principles links. It is recommended that for all molecules the experimental set of transitions employed for an effective Hamiltonian fit should be checked for floating components and extra care be exercised if they exist, as in these cases the adequate reproduction of the transitions does not automatically mean that the energies derived from the effective-Hamiltonian parameters are correct as well. The connectivity problem of measured spectroscopic networks becomes more and more pronounced as the order of the Hamiltonian is increased