Rotational spectrum and hyperfine structure of the (para)-NH3–(ortho)-H2 van der Waals molecular complex

Abstract

The hyperfine structure of the (para)-NH₃–(ortho)-H₂ van der Waals molecular complex has been observed and analyzed for the first time. The measurements of the pure rotational transitions were carried out in the frequency range of 75-190 GHz using a newly developed millimeter-wave jet spectrometer with a molecular beam aligned coaxially with the direction of millimeter-waves propagation. ¹⁴N nuclear quadrupole coupling constant of NH₃, spin-spin and spin-rotation interaction constants of the two protons of the H₂ molecule were determined for (para)-NH₃–(ortho)-H₂ in its ground Π state. The obtained parameters contain useful information about angular orientation of the NH₃ and H₂ monomers within the van der Waals complex and provide an additional tool for evaluating the quality of the NH₃–H₂ interaction potential. The latter determines the reliability of the calculated excitation and de-excitation rates of ammonia during collisions with hydrogen in the dense interstellar clouds.