PARITY-RESOLVED STATE-TO-STATE CROSS-SECTIONS FOR INELASTIC-SCATTERING OF NOX(2)PI(1 2) (UPSILON=20, J=0.5, ELF) FROM HE - A COMPARISON BETWEEN CROSSED MOLECULAR-BEAMS EXPERIMENTS AND AB-INITIO THEORY/

Relative state-to-state cross sections for inelastic collisions betwee n NO X (2) Pi(1/2) (upsilon = 20, J = 0.5, e or f) and He have been de termined in a crossed molecular beam setup at a collision energy of 19 5 cm(-1). Efficient initial state preparation of the NO molecules is a chieved by stimulated emission pumping by way of the NO B (2) Pi(1/2) (upsilon = 5) state. The rotational state distribution of the inelasti cally scattered NO molecules is probed by laser induced fluorescence t hrough the A (2) Sigma(+) (upsilon = 3) state. The measured cross sect ions are compared with close-coupled quantum scattering calculations o n both a recently reported ab initio potential energy surface (PES) an d an extension of this PES which takes into account the elongation of the NO bond in upsilon = 20. In the latter calculations, the highly vi brationally excited molecule is treated as a rigid rotor with a bond l ength equal to the average of the upsilon = 20 level. Even with such a n apparently drastic assumption, agreement between the experimental re sults and the scattering calculations on the latter PES is excellent.