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

The most recent version of the MARVEL (Measured Active RotationalVibrational 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 (Al-Derzi et al., J. Quant. Spectrosc. Rad. Transfer 161 (2015) 117–130), the present investigation is limited to transitions with an upper energy level below 7500 cm−1 , considers 27 new sources 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 (EH) fits forms an important part of this study. It is established that previous EH fits, using high expansion orders in the Hamiltonian, are inaccurate for certain high rotational excitations for all vibrational parent states 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 Preprint submitted to Elsevier March 6, 2020 network used for the EH fit contains floating components. A much improved EH fit is presented whereby the floating components are connected to the principal components of the spectroscopic network by carefully selected first-principles “bridges”. It is recommended that for all molecules the experimental set of transitions employed for an EH fit should be checked for floating components and extra care is exercised if they existed, as in these cases the adequate reproduction of the transitions does not automatically mean that the energies derived from the EH 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. Keywords: ammonia (14NH3), rovibrational transitions, rovibrational energy levels, MARVEL, database, spectroscopic networks, effective