The infrared spectrum of the rare gas-tetrahedral molecule ionic complex Ar-NH+4 has been detected. In the region of the v3 (t2) triply degenerate vibration of the NH+4 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 NH+4 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 ≈ 220 cm-1 are substantial
