Reactant orientation-product polarization correlations. Collision energy dependence in the Ba + N2O ? BaO*+ N2 reaction

Abstract

The correlation between reactant collision geometry and product angular momentum alignment is determined over a range of collision energies (0.08–0.16 eV) for the Ba + N₂O → BaO^*+ N₂ reaction. Hexapole fields state-select and orient a beam of N₂O in a single (J, I, M)=(1, 1, 1) rovibrational state. The polarization of BaO^* emission resulting from the crossed-beam reaction with Ba atoms is measured. ‘Favourable’ collisions, where Ba approaches the ‘O’-end of N₂O, produce strong polarization perpendicular to the initial relative velocity vector while ‘unfavourable’ collisions produce nearly isotropic emission. At high-collision energies both effects are more pronounced. The observations are analysed in terms of two orientation-dependent cross-sections for the yield of (chemiluminescent) product molecules: σ_tot(cosγ₀) counting all products irrespective of the polarization of photoemission and σ_∥(cosγ₀) counting the products emitting photons polarized parallel to the relative velocity, v_r, of the impinging reactants. The orientational dependence of both cross-sections has been approximated by two models; a truncated Legendre series model and a trapezium opacity model. From the resulting orientation dependence of the alignment, A, of the rotational angular momentum, J′, of the product molecule BaO^* it has been found that for favourable angles of attack (cos γ₀→ 1)J′ points preferentially perpendicular to v_r, whereas for unfavourable angles of attack (cos γ₀→–1) this preference tends to be absent. Our analysis shows that the dependence of A upon cos γ₀ becomes appreciably stronger for increasing collision energy.