Counterpropagating pulsed molecular beam scattering of NH3–Ar. II. State resolved differential cross sections

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Abstract
State resolved differential cross sections (DCSs) for the rotational excitation of NH3 in collisions with Ar for various final states at a collision energy of 158 meV have been determined in a counterpropagating pulsed molecular beam scattering experiment. Simultaneous state and velocity specific detection of scattered products is achieved through resonance enhanced multiphoton ionization with subsequent ion time-of-flight analysis. The supersonic expansion ensures the preparation of well-defined initial states in both nuclear spin modifications. While measured DCSs for para-NH3 are parity averaged, state resolved cross sections could be determined for ortho-NH3. All DCSs are dominated by small or intermediate angle scattering, indicating the importance of the anisotropy in the long range part of the potential surface. The maxima in the DCSs are shifted towards larger scattering angles with increasing energy transfer, a behavior characteristic of a rotational rainbow. Excitation probabilities to states which are forbidden in the centrifugal sudden approximation are characterized by a significant decrease in intensity in the backward direction as well as a steeper exponential fall off at small angles.