Observation of the infrared spectrum of the nu(3) band of the argon-ammonium ionic complex

The infrared spectrum of the rare gas-tetrahedral molecule ionic complex Ar-NH4+ has been detected. In the region of the nu(3) (t(2)) triply degenerate vibration of the NH4+ monomer, a parallel and a perpendicular band are observed. Tunnelling splittings of the Q branches of the latter band are attributed to substantially hindered internal rotation of the NH4+ unit. A recently developed model of atom-spherical top complexes is applied to estimate anisotropic terms in the intermolecular potential; both the vibrational anisotropy of the order of 90 cm(-1) and the rotational anisotropy approximate to 220 cm(-1) are substantial.