State-to-state scattering of highly vibrationally excited NO with argon at collision energies over 1 eV.

We present state-to-state differential cross sections for rotationally inelastic collisions of vibrationally excited NO XΠ ( = 9) with Ar using a near-counterpropagating molecular beam geometry. These were obtained using the stimulated emission pumping technique coupled with velocity map imaging. Collision energies well over ∼1 eV were achieved and rotational excitations up to ∼Δ = 60 recorded for the first time for inelastic collisions. This allowed us to investigate scattering of a diatomic molecule in a Π state which is initially well described by Hund's case (a) into final states well described by Hund's case (b) as the rotational level splitting becomes larger than the spin-orbit splitting. Differential cross sections for both parity-changing and parity-conserving collisions exhibit very similar structures at the high collision energies. Quantum scattering calculations have been carried out to obtain approximate integral cross sections, which confirm the high rotational excitation. These studies will take the arena of rotationally inelastic collisions to a new regime while providing insight into dynamics under extreme non-equilibrium conditions. Furthermore, these present a unique challenge to both quantum and quasi-classical scattering calculations to validate the methods and the potential energy surfaces used to assess their applicability under extreme conditions.