The intermolecular potential of NH4+-Ar II. Calculations and experimental measurements for the rotational structure of the nu3 band

The mid-infrared spectrum of the upsilon3(t2) transition of the NH4+-Ar complex has been recorded at rotational resolution using photofragmentation spectroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor states. The P and R branches of the strongest perpendicular subbands are rotationally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwidth of 0.02cm-1, giving a lower limit of 250ps for the lifetime of the excited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for the contribution due to Coriolis coupling between the internal and total rotational angular momenta. The absolute energies, rotational and distortion constants for the first few intermolecular bending and stretching levels of the ground intramolecular vibrational state are determined in a numerical solution to the rotation-intermolecular vibration Hamiltonian, employing a three-dimensional ab initio intermolecular potential. The results are compared with the experimental constants in order to assess the accuracy of the calculated potential. The relative energy levels from this calculation are also compared with those from a two-dimensional representation of the potential energy surface (ʹfixed-Rʹ model) in order to judge directly the influence of the radial dependence of the potential.