State-to-state cross sections for rotational excitation of ortho- and para-NH 3 by Ar

Abstract

Parity resolved state-to-state cross sections for rotational and tunneling inversion excitation of ortho- and para-NH 3 colliding with Ar, have been obtained in a crossed molecular beam experiment. State preparation of the 0 + 0 state for ortho-NH 3 and the 1 − 1 state for para-NH 3 is achieved by adiabatic expansion of ammonia followed by electrostatic state selection in a hexapole electric field. The NH 3 rotational state distribution is probed by (2 + 1) REMPI spectroscopy via the B state. The scattering experiments have been performed at two collision energies, 35 meV and 56 meV, by varying the NH 3 molecular beam velocity. The experimental values for the relative cross sections are compared with the results from close coupling calculations using an ab initio potential. Also a modified potential scaled to spectroscopic data for the NH 3-Ar complex is considered. For ortho-NH 3 observed parity propensities are in good agreement with theoretical predictions. In the case of para-NH 3 discrepancies are present. For the parity averaged rotational excitation a reasonably good agreement between experiment and theory is obtained.