The intermolecular potential of NH4+-Ar II. Calculations and experimental measurements for the rotational structure of the nu(3) band

The mid-infrared spectrum of the nu(3)(t(2)) transition Of the NH4+-Ar comp lex has been recorded at rotational resolution using photofragmentation spe ctroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor sta tes. The P and R branches of the strongest perpendicular subbands are rotat ionally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwid th of 0.02 cm(-1), giving a lower limit of 250 ps for the lifetime of the e xcited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for th e contribution due to Coriolis coupling between the internal and total rota tional angular momenta. The absolute energies, rotational and distortion co nstants for the first few intermolecular bending and stretching levels of t he ground intramolecular vibrational state are determined in a numerical so lution to the rotation-intermolecular vibration Hamiltonian, employing a th ree-dimensional ab initio intermolecular potential. The results are compare d with the experimental constants in order to assess the accuracy of the ca lculated potential. The relative energy levels from this calculation are al so compared with those from a two-dimensional representation of the potenti al energy surface ('fixed-R' model) in order to judge directly the influenc e of the radial dependence of the potential.