Chemical kinetics of lanthanum ionization in H-2-O-2-N-2 flames

Trace amounts (less than or equal to 10(-6) mol fraction) of lanthanum were introduced into five, premixed, fuel-rich, H-2-O-2-N-2 flames at atmospher ic pressure in the temperature range 1820-2400 K. Aqueous salt solutions of the metal were sprayed into the premixed flame gas as an aerosol using an atomizer technique. The concentrations of the major neutral species present in flames, believed to be LaO and OLaOH, are linked by the balanced reacti on LaO + H2O = OLaOH + H having an equilibrium constant K = 0.1859 exp(-114 64/T), based on a crude estimate of the bond dissociation energy for lantha num oxide-hydroxide D-0(0)(OLa-OH) = 408 +/- 40 kJ mol(-1). Metallic ions w ere observed by sampling the flames through a nozzle into a mass spectromet er and, for kinetics purposes, were measured as profiles of ion concentrati on versus distance (i.e. time) along the flame axis. The ions detected can be represented by an oxide ion series LaO+.nH(2)O (n = 0-3 or more); the ge nuine flame ions are thought to be LaO+ (n = 0) and La(OH)(2)(+) (n = 1), e quivalent to protonated OLaOH. The two ions are linked by the fast balanced reaction LaO+ + H2O + M = La(OH)(2)(+) + M, where M is a third body. The m ajor ion production processes appear to be the chem-ionization reaction OLa OH + H reversible arrow La(OH)(2)(+) + e(-), and thermal (collisional) ioni zation LaO + M reversible arrow LaO+ + e(-) + M because the ionization ener gy IE00(LaO)= 4.90 eV is low. Ion loss processes involve dissociative elect ron-ion recombination. When electron-ion recombination of LaO+.nH(2)O was m ade dominant by the trace addition of potassium, values of the global recom bination coefficient are given by (1718 +/- 515)T-3.0 +/- 0.2 cm(3) molecul e(-1) s(-1). The rate constant for thermal (collisional) ionization of LaO was found to be 8.0 x 10(-10) T-1/2 exp(-IE00/RT) cm(3) molecule(-1) s(-1). The pre-exponential factor or cross section is only 8% of that determined for the alkali metals. The rate constant for the chemi-ionization of OLaOH is given by (5.6 +/- 1.7) x 10(-12) exp(-17350/T) cm(3) molecule(-1) s(-1). Although no direct evidence was found for particle formation in these flam es, both rate constants for ion production would be lower limits if appreci able lanthanum were present as solid particles. (Int J Mass Spectrom 188 (1 999) 213-224) (C) 1999 Elsevier Science B.V.