CHEMICAL-KINETICS OF YTTRIUM IONIZATION IN H-2-O-2-N-2 FLAMES

Trace amounts (less than or equal to 10(-6) mol fraction) of yttrium w ere introduced into six H-2-O-2-N-2 flames in the temperature range 18 20-2400 K at atmospheric pressure. Metallic ions were observed by samp ling the flames through a nozzle into a mass spectrometer, and were me asured 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 domin ant ion, and the higher hydrates are produced by cooling that occurs d uring sampling. The major yttrium neutral species are the oxide-hydrox ide 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 ioniz ation 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 rad ical species such as H in these flames can overshoot its equilibrium c oncentration by a factor gamma of 100 or more in the reaction zone but gamma decays downstream towards equilibrium. Neutral OYOH and YO, who se 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 a scertaining the ion formation processes. Yttrium ions are produced by the chemi-ionization reaction of OYOH with H; because the ionization e nergy of YO is low, a small contribution (< 1%) might arise from colli sional (thermal) ionization of YO. The rate constant for chemi-ionizat ion was found to be (2.7 +/- 0.8) X 10(-12) exp(-22452/T) cm(3) molecu le(-1) s(-1). This rate constant should be regarded as a lower limit b ecause a fraction of the total yttrium, presumably small, may be prese nt in these flames as solid particles. (Int J Mass Spectrom 176 (1998) 1-12) (C) 1998 Elsevier Science B.V.