High-resolution spectroscopic probing of allowed and forbidden ortho- and para-nuclear spin-isomers of NH₃

Ammonia (NH₃) is a pyramidal symmetric top molecule with inversion motion and it exists in two distinct nuclear spin isomeric forms, ortho-NH₃ (K = 3n) and para-NH₃ (K ≠ 3n). Here, a pair of electric dipole-allowed [^RQ(4,3) and ^PP(2,2)] and weak forbidden [^OP(3,3) and ^SQ(8,1)] transitions of gas-phase NH₃ were probed in the ν₄ fundamental vibrational band occurring at 6.2 µm mid-IR interference-free spectral region using an external-cavity quantum cascade laser coupled cavity ring-down spectrometer. We have experimentally achieved the ortho-to-para ratio (OPR) of (1.03 ± 0.24) and (1.08 ± 0.14) at room temperature (296 K) for the allowed and forbidden transitions of NH₃, respectively. Furthermore, our experimental findings confirm that the nuclear spin-symmetry is conserved under the nonreactive perturber-induced collisional processes. This study paves the way to directly probe the nuclear spin-isomers of gas-phase NH₃ and thus may lead to an in-depth understanding of nuclear spin-isomerism associated with various physical and chemical processes.

This work shows the high-resolution probing of ortho- and para-nuclear spin-isomers of NH₃ in the gas phase at room temperature using quantum cascade laser coupled cavity ring-down spectroscopy at 6.2 µm mid-IR spectral region.