Selective rotational energy transfer from individual Λ-doublet levels of highly rotationally excited CN( )

Optical–optical double resonance was employed to investigate inelastic transitions between highly rotationally excited CN(A2Π) Λ-doublet, fine-structure levels, induced by collisions with argon. Rotationally excited CN was prepared by 193 nm photolysis of BrCN diluted in Ar at total pressure ∼1 Torr. Dramatically different final state distributions were observed for initial Λ-doublet levels of A′ and A″ symmetry. These propensities are believed to arise from approach in a `helicopter' orientation on the more attractive Ar–CN(A) A′ potential energy surface, followed by curve crossing to the A″ potential energy surface correlating with the next lower rotational asymptote.