Observation of the infrared spectrum of the ν3 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 ν3 (t2) triply degenerate vibration of the NH4+ monomer, a parallel and perpendicular band are observed. Tunnelling splittings of the Q branches of the latter band are attributed to subtantially 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 ≈220 cm−1 are substantial.