Time-dependent chemiluminescence from the surface-catalysed recombination of O and NO on polycrystalline Ni

Abstract

The near-infrared chemiluminescence produced by the surface-catalysed recombination of nitric oxide with a supersonic beam of atomic oxygen on polycrystalline nickel has been studied. The dependence of the luminescence on time, substrate temperature, O and NO fluxes and pre-exposure to the O atom beam has been investigated. The spectrum of the luminescence has also been determined. Strong initial chemiluminescence was observed decaying with time to a steady state. The use of argon-ion bombardment to clean the sample prior to each experiment was essential to the observation of this time-dependent luminescence. The initial luminescence shows a strong dependence on the substrate temperature, increasing by a factor of 80 when the substrate temperature is reduced from 300 to 195 K. The luminescence showed an initial first-order dependence on the atomic oxygen and nitric oxide fluxes, before levelling off at higher fluxes. The spectrum of the luminescence shows intensity rising from a threshold of 530 nm to a maximum at ca. 800 nm, which is shifted significantly towards longer wavelengths compared to that from the termolecular gas-phase recombination of O and NO. Pre-exposure of the substrate to the O-atom beam suggests that a monolayer of chemisorbed oxygen, which reduces both the binding energy and dissociative adsorption of NO on the substrate, is responsible for the production of a weakly bound electronically excited NO₂ which may leave the surface and emit light. A mechanism involving surface poisoning by NO₂ product molecules has been suggested.