Chemical kinetics of yttrium ionization in H2-O2-N2 flames

Trace amounts (less than or equal to 10(-6) mol fraction) of yttrium were introduced into six H-2-O-2-N-2 flames in the temperature range 1820-2400 K at atmospheric pressure. Metallic ions were observed by sampling the flames through a nozzle into a mass spectrometer, and were measured as profiles of ion concentration vs. distance (i.e. time) along the flame axis. The ions observed conform to a series YO+ . nH(2)O (n = 0-3); YO+ is barely detectable, Y(OH)(2)(+) with n = 1 is the dominant ion, and the higher hydrates are produced by cooling that occurs during sampling. The major yttrium neutral species are the oxide-hydroxide OYOH (> 80%) and the oxide YO (< 20%). By the addition of 0.25 mol % of CH4 and a trace of potassium, it was possible to boost the ionization level appropriately to make electron-ion recombination of Y(OH)(2)(+) the dominant process; values of the recombination coefficient (9.7 +/- 1.5)T-2.3 +/- 0.6 cm(3) molecule(-1) s(-1) were obtained. A radical species such as H in these flames can overshoot its equilibrium concentration by a factor gamma of 100 or more in the reaction zone but gamma decays downstream towards equilibrium. Neutral OYOH and YO, whose concentrations depend on gamma, are linked by the balanced reaction YO + H2O = OYOH + H having an equilibrium constant K = exp(-49,000/RT). This leads to a bond energy value D-o(OY-OH) = 449 +/- 20 kT mol(-1). Analysis of the ion profiles as a function of gamma is helpful in ascertaining the ion formation processes. Yttrium ions are produced by the chemi-ionization reaction of OYOH with H; because the ionization energy of YO is low, a small contribution (< 1%) might arise from collisional (thermal) ionization of YO. The rate constant for chemi-ionization was found to be (2.7 +/- 0.8) X 10(-12) exp(-22452/T) cm(3) molecule(-1) s(-1). This rate constant should be regarded as a lower limit because a fraction of the total yttrium, presumably small, may be present in these flames as solid particles. (Int J Mass Spectrom 176 (1998) 1-12) (C) 1998 Elsevier Science B.V.