Mass spectrometric sampling of negative ions from flames of hydrogen and oxygen: the kinetics of electron attachment and detachment in hot mixtures of H 2 O, O 2 , OH and HO 2

Abstract

This is a study of the negative ions formed in premixed oxygen-rich flames of H 2 + O 2 + N 2 , with traces of an alkali present to produce free electrons. The ions were observed by continuously sampling the burnt gases of such a flame into a quadrupole mass spectrometer. The negative ions are OH - and O - 2 ; they form when free electrons attach to O - 2 molecules in e - + O 2 + M —> O - 2 + M, (IV) early in the flame, where M is any molecule in the flame gases capable of removing energy from the colliding electron and molecule of O 2 . Very soon in the flame, after a residence time - 2 becomes equal to its rate of disappearance via O - 2 + H -> e - + HO 2 , (—II) O - 2 + OH -> OH - + O 2 , (V) and (—IV), the reverse of process (IV). Likewise, the other negative ion, OH - , rapidly (after a residence time - —>H 2 O + e - . (-I) The rates of all the above reactions are very much increased by the fact that the concentrations of the free radicals H, OH and O are initially high in a flame’s reaction zone and later fall, becoming close to their equilibrium concentrations some 20 mm from the burner. In addition, these negative ions attain concentrations for thermodynamic equilibrium at the final temperature of the burnt gases. During the sampling of a flame, the relative concentrations of OH - and O - 2 are perturbed, as a result of free electrons being lost to the metal around the sampling orifice, causing in turn an equilibrium or steady-state concentration to be shifted. In addition, the sample adjusts its composition on being cooled by first heat transfer to the colder sampling nozzle and secondly in the near-adiabatic supersonic expansion of gas into the vacuum inside the instrument. Such perturbations of the observed ion concentrations are considered in detail, and, in fact, enable values for the rate coefficients of the forward and reverse steps in reactions (I), (II), (IV) and (V) to be deduced.