STATE-TO-STATE CROSS-SECTIONS FOR ROTATIONAL-EXCITATION OF ORTHO-NH3 AND PARA-NH3 BY ORTHO-H2 AND PARA-H2 - EXPERIMENT VERSUS THEORY

State-to-state cross sections for inversion-rotational excitation of ortho- and para-NH3, colliding with para-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 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 state. From a comparison with previous NH3-normal-H-2 scattering results relative cross sections are obtained for both NH3-para-H-2 and NH3-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.