MASS-SPECTROMETRIC SAMPLING OF NEGATIVE-IONS FROM FLAMES OF HYDROGEN AND OXYGEN - THE KINETICS OF ELECTRON-ATTACHMENT AND DETACHMENT IN HOTMIXTURES OF H2O, O-2, OH AND HO2

This is a study of the negative ions formed in premixed oxygen-rich fl ames of H-2 + O-2 + N-2, with traces of an alkali present to produce f ree electrons. The ions were observed by continuously sampling the bur nt gases of such a flame into a quadrupole mass spectrometer. The nega tive 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 flam e, where M is any molecule in the flame gases capable of removing ener gy from the colliding electron and molecule of O-2. Very soon in the f lame, after a residence time < 1 mu s, the rate of formation of O-2(-) becomes equal to its rate of disappearance via O-2(-) + H --> e(-) HO2, (-II) O-2(-) + OH --> OH- + O-2, (V) and (-IV), the reverse of pr ocess (IV). Likewise, the other negative ion, OH-, rapidly (after a re sidence time < 1 mu s) attains a steady-state concentration, governed by its rate of production in (V) being equal to its rate of consumptio n in H + OH- --> H2O + e(-). (-I) The rates of all the above reactions are very much increased by the fact that the concentrations of the fr ee radicals H, OH and O are initially high in a flame's reaction zone and later fall, becoming close to their equilibrium concentrations som e 20 mm from the burner. In addition, these negative ions attain conce ntrations for thermodynamic equilibrium at the final temperature of th e burnt gases. During the sampling of a flame, the relative concentrat ions of OH- and O-2(-) are perturbed, as a result of free electrons be ing 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 trans fer to the colder sampling nozzle and secondly in the near-adiabatic s upersonic expansion of gas into the vacuum inside the instrument. Such perturbations of the observed ion concentrations are considered in de tail, and, in fact, enable values for the rate coefficients of the for ward and reverse steps in reactions (I), (II), (IV) and (V) to be dedu ced.