The influence of the orientation of the NO molecule upon the chemiluminescent reaction NO+O3→NO2*+O2

A beam of state-selected NO molecules (J = Ω = 3 2 ) has been produced by an electrostatic hexapole and has been collided with O3 molecules in a scattering chamber. The E-field dependence of the chemiluminescent cross section, σhr, has been investigated and resulted in the determination of the M-dependence of σhr: σhr (M)/σ0 = 1.192±0.009, 0.0848±0.015, 1.177±0.015, 0.783±0.009 for M = 3 2 , 1 2 , −1 2 and −3 2 , respectively. Application of the Legendre expansion technique and the density matrix formalism provided a deconvoluted σhr(γ), for a single angle of attack γ of the NO axis, expressed in simple model functions with adjustable parameters. From this analysis it is concluded that chemiluminescence only occurs when cos γ ≈ 1, the “end-on-head” orientation of NO yielding ≈ 30% of all collected light, and when cos γ ≈ −0.275, the “broad-side-tail” orientation of NO yielding the remaining 70%. The steric factors belonging to these reactive orientations have been estimated and are S1 = 0.25±0.07 and S2 = 0.40±0.09, respectively. The observed dependence of σhr has been confronted with the rules of Woodward and Hoffman. Although there are indeed two symmetries (bpl and cpl) correlating the electron orbitals of the reactants and the products, these rules do not lead to an explanation of the steric effects of the NO+O3 reaction.