State resolved cross sections for rotationally inelastic collisions of NH2(X 2B1) with helium

Integral cross sections for rotational transitions from the lowest orthorotational level (00,0) of NH2 induced by collisions with helium have been measured in a crossed beam experiment. A pulsed supersonic beam of rotationally cold NH2 was produced by 193 nm photolysis of a dilute mixture of ammonia in argon seed gas at the tip of a nozzle; the helium target was also prepared as a pulsed beam. The final rotational state populations (without spin-state resolution) in the Ka=0 and 1 manifolds of NH2 were interrogated in the collision zone by dye laser fluorescence excitation in the à 2A1←X̃ 2B1 band system. Because of the rotational selection rules in this band system, it was not possible to put the cross sections for the two Ka manifolds on the same scale. Within a given Ka manifold, there is a marked preference for collisional transitions with the smallest ΔN change. These experimental results are compared to calculated close-coupling rotationally inelastic cross sections for H2O(00,0)–He collisions.