Intermolecular dynamics of NH-rare gas complexes in the ν umbrella region of NH investigated by rovibrational laser jet-cooled spectroscopy and ab initio calculations.

High resolution jet-cooled spectroscopy experiments have been realized to investigate the intermolecular dynamics of van der Waals (vdW) heterodimers between NH and rare gas (Rg) atoms in the ν umbrella mode region of NH. With respect to a previous study dedicated to NH-Ar [Asselin et al. Mol. Phys. 116, 3642 (2018)], the sensitivity and spectral resolution of our laser spectrometer coupled to a pulsed supersonic jet have been significantly improved to derive more accurate excited state spectroscopic parameters from rovibrational analyses. In addition, we calculated the ground and ν excited vibration-rotation-tunneling (VRT) states of these complexes on the four-dimensional ab initio potential energy surfaces from Loreau et al. [J. Chem. Phys. 141, 224303 (2014), ibid. 143, 184303 (2015).] Transition frequencies and intensities of the allowed ν = 1 ← 0 transitions obtained from the calculated energy levels and wave functions agree well with the experimental data and are helpful in their analysis. By means of a pseudodiatomic model with the assumption of weak Coriolis coupling, the rovibrational analysis of both the Π(j = 1,k = 0) ←Σ(j = 0,k = 0) and Σ(j = 1,k = 0) ←Σ(j = 0,k = 0) transitions in ortho NH-Rg (Rg = Ne, Ar, Kr, Xe) complexes enabled us to determine reliable excited state parameters and derive accurate values of the effective vdW bond length R, force constant k, and vdW stretching frequency ν. Comparison between the experimental structural parameters and those from the ab initio calculated VRT levels shows good agreement for NH-Ne, NH-Ar and NH-Kr, and a similar variation of R, k, and ν with the polarizability of Rg in the ground and ν excited states. Anomalously small values of ν and k derived for NH-Xe in the Π(j = 1,k = 0) state suggest that the applied model is not valid in this case, due to the presence of another state coupling to the perturbed Π state. Such a state could not be found, however.