Influence of translational and internal energy upon the chemiluminiscent part of the exothermic reaction NO + O 3 → NO *2 + O 2 → NO 2 + hν + O 2

Abstract

The influence of internal (rotational and electronic fine structure states of NO) and translational energy upon the chemicaluminescence has been studied by crossing a seeded NO beam with an effusive O 3 beam for energies large in comparison with threshold. The translational energy (0.4< E tr< 1.6 eV) is selected by a velocity selector placed in front of the scattering region. Thus, the internal beam temperature (state distribution) remains constant, when E tr is varied by changing the rotational frequency of the velocity selector. The cross section of this exothermic process is found to increase very rapidly with translation, ν hν ∝ E 3.75 tr, with some levelling off starting at E tr ≈ 1.2 eV. The influence of the internal energy of the NO-molecules is studied by changing the pressure behind the nozzle, at a constant speed of the velocity selector; thereby the internal beam temperature is varied (100 ⩽ T| ⩽ 300 K). A linear behaviour is found: σ hν ∝ [1+α( kT|/ Ethreshold)] with Ethreshold=0.129 eV and α = 4.5 ± 0.5 for E tr between 0.62 and 0.84 eV. Assuming the rotational and electronic fine structure temperature equal to T |. we obtain (do hν /d Ē t)/(dσ hν /d Ē tr) = 4.5 ± 0.5 at E tr = 0.7 eV with Ē int = Ē rot + Ē el· E int is even more efficient than E tr in promoting chemiluminescence in the visible. If the internal energy effect can be attributed ot different cross sections for the two fine structure states involved, one derives σ hν(Π 1 2 )/σ hν(Π 3 2 ) = 0.27 ± 0.03. The exothermic chemiluminescent reaction cross section is surprisingly energy selective far from the threshold.