A high resolution study of the ν3 band of the ammonium ion (NH+4) by velocity modulation laser absorption spectroscopy

Abstract

Over 200 transitions in the ν₃ band of the ammonium ion (NH⁺₄ ) have been measured near 3 μm with a color center laser spectrometer to an accuracy of 0.005 cm⁻¹ using the technique of velocity modulation laser absorption spectroscopy. These have been assigned to strongly allowed Q⁰, P⁺ and R⁻ branches in the triply degenerate asymmetric stretching mode of a spherical top molecule, and analyzed by nonlinear least–squares methods with an effective sixth order Hamiltonian. Ground state constants were fixed at values obtained with a recent ab initio molecular potential surface. Twenty‐six molecular parameters were determined in the fit. The fact that terms in the Hamiltonian beyond fourth order are required to fit transitions with J as low as 12 indicates the existence of a perturbation involving the ν₃=1 levels. It is suggested that a Fermi resonance between ν₃ and ν₂+ν₄ is an important source of this perturbation, as in the case of CH₄. The frequencies of ‘‘forbidden’’ transitions, required in order to determine the ground state parameters, are predicted to high accuracy with these new constants.