STATE-TO-STATE CROSS-SECTIONS FOR ROTATIONAL-EXCITATION OF ORTHO-NH3 AND PARA-NH3 BY HE AND H-2

Parity resolved state-to-state cross sections for rotational excitation of ortho- and para-NH3, colliding with He and H-2, have been obtained in a crossed molecular beam experiment. State preparation of the 0(0)+ state for ortho-NH3 and the 1(1)- state for para-NH3 is achieved by adiabatic expansion of ammonia, seeded in argon, followed by electrostatic state selection in a hexapole electric field. The NH3 rotational state distribution is probed by (2+1) REMPI spectroscopy via the B approximately state. A comparison with calculated cross sections shows a reasonably good agreement for pure rotational excitation. However, strong deviations from predicted parity propensity rules are observed.