The ground, v2 = 1, 2 and v4 = 1 states of 14ND3 analyzed at experimental accuracy

Abstract

Ammonia is the object of extensive investigations due to the peculiar pattern of its interacting vibration states and its central role in astronomical sciences. ¹⁴ND₃ is of interest in astrochemistry because the determination of deuterium fractionation ratios in ammonia and its deuterated isotopologues contributes to a deeper knowledge of the chemistry in the cold, dense cores of the interstellar medium. Here, the ground, v₂ = 1, 2 and v₄ = 1, a, s states of ¹⁴ND₃ are analyzed thanks to new spectra recorded with the Canadian Light Source synchrotron, from 60 to 1500 cm⁻¹ at a resolution ranging from 0.00096 to 0.003 cm⁻¹. Overall, 7765 inversion, rotation-inversion, and vibration–rotation-inversion transitions in ν₂, 2ν₂, ν₄, 2ν₂ ← ν₂, 2ν₂ ← ν₄, ν₄ ← ν₂, ν₂ ← ν₂, 2ν₂ ← 2ν₂, and ν₄ ← ν₄ have been fitted simultaneously to characterize the s and a levels of the excited states. 4021 and 2388 transitions were assigned in the cold and hot bands, respectively, and 150 inversion and 1206 inversion-rotation transitions in v₂ = 1, 2 and v₄ = 1. The effective Hamiltonian adopted includes all symmetry allowed interactions between and within the studied excited states, according to the most recent results on ammonia. The transitions have been reproduced at experimental accuracy using 118 spectroscopic parameters, determined with high precision. In addition, a new analysis of the ground state has been performed. 9256 data have been fitted, 78 inversion and 837 rotation-inversion transitions, and 8341 ground state combination differences. An improved set of parameters and term values are derived thanks to the increased precision of the newly assigned rotation-inversion transitions with J/ K values up to 31/30 and to the larger number of the ground state combination differences.