IONIZATION OF THE GROUP-3-METALS LA, Y AND SC IN H-2-O-2-AR FLAMES

Four pairs of premixed, fuel-rich/fuel-lean (FR/FL; equivalence ratio phi = 1.5/0.75), H-2-O-2-Ar flames at four temperatures in the range 1900-2425 K, all at atmospheric pressure, were doped with about 10(-6) mole fraction of the group 3 metals La, Y and Sc using atomizer techniques. The metals produce solid particles in the flames and gaseous metallic species. The latter include free metallic atoms, A, near the flame reaction zone, but only the monoxide AO and the oxide-hydroxide OAOH further downstream at equilibrium; the [OAOH]/[AO] ratio varies in FR/FL flames. Metallic ions (<1% of the total metal) were observed by sampling a given flame along its axis through a nozzle into a mass spectrometer. All of the observed ions can be represented by an oxide ion series AO(+) . nH(2)O (n = 0-3 or more) although their actual structures may be different; e.g. A(OH)(2)(+) for n = 1, interpreted as protonated OAOH. A major objective was to ascertain the ionization mechanism, principally that of La. The ionization appears to receive an initial boost from the exothermic chemi-ionization reaction of A with atomic O to produce AO(+); further downstream, the ionization level is sustained by the thermal (collisional) ionization of AO to produce AO(+) and/or the chemi-ionization of OAOH with H to produce A(OH)(2)(+). The ions AO(+), A(OH)(2)(+) and higher hydrates are all rapidly equilibrated by three-body association reactions with water. Ions are lost by dissociative electron-ion recombination of A(OH): and possibly higher hydrates. The chemical ionization of the metallic species by H3O+ was investigated by adding a small quantity of CH4 to the flames. The ion chemistry is discussed in detail. An estimate of the bond dissociation energy D-0(o)(OLa-OH) = 408 +/- 40 kJ mol(-1) (4.23 +/- 0.41 eV) was obtained.