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

Abstract

Integral cross sections for rotational transitions from the lowest orthorotational level (0_0,0) 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 K_a=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 K_a manifolds on the same scale. Within a given K_a 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(0_0,0)–He collisions.