State-to-state cross sections for rotational excitation of ortho- and para-NH 3 by ortho- and para-H 2. Experiment versus theory

Abstract

State-to-state cross sections for inversion-rotational excitation of ortho- and para-NH 3, colliding with para-H 2, have been obtained in a crossed molecular beam experiment. State preparation of the 0 + 0 state for ortho-NH 3 and 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. From a comparison with previous NH 3—normal-H 2 scattering results relative cross sections are obtained for both NH 3—para-H 2 and NH 3—ortho-H 2 collisions. The experimental values are compared with the results from close coupling and coupled states calculations using ab initio potentials. It is demonstrated that the collisional behaviour of para-H 2 is strongly different from the scattering by He.