Determination of the rate coefficients of A + X^A + + X - and AX+M^A + +X - +M where A is a metal atom, X a halogen atom and M a flame species

Abstract

Premixed laminar flat flames of H 2 , O 2 and N 2 have been burnt, with trace amounts of various metals (Li, Na, K, Rb, Cs, Ga, In and Tl) and the halogens Cl, Br or I added. Mass spectrometric measurements were made of ion concentrations and also their variation with time in each flame. These observations establish that a halogen X can cause a metal A to produce ions homogeneously in a flame through the forward steps of A + X ^ A + + X - (II) AX + M A + + X - + M, (III) where M is any molecule. The reverse processes are found to facilitate ion recombination. We have measured the rate constants k 2 and k 3 of the ion-producing steps in (II) and (III) over the temperature range 1820—2590 K; they vary with temperature according to k 2 = A 2 exp ( - A EJRT) and k 3 = A 3 T -3-5 exp ( - AE 3 /RT). The activation energies of each reaction were found to equal the appropriate endothermicity, in contrast with previous shock tube determinations of k 3 . For Ga and In with all three halogens, ion recombination rates were measured directly with a little (lt; 1 vol. %) C 2 H 2 also added to the burner supplies. These observations unambiguously gave k -2 , the coefficient for recombination in (II), because k -3 is very small for these two metals. Also, it was found that these k -2 for Ga and In did not equal the ratio of the measured forward rate constant, k 2 , and the equilibrium constant for (II). The origin of this departure from detailed balancing is apparently that the ionizing step in (II) proceeds from A and X in their ground states, but the reverse recombination of ions is to ground state Ga, In or Tl and electronically excited ( 2 P1/2) atoms of X. This conclusion was confirmed by computations of k -2 using Landau-Zener curve-crossing theory for the products being either ground state atoms or with X excited. The alkali metals K, Rb and Cs, on the other hand, participate in each direction in (II) with A and X only in their ground states (so that detailed balancing does hold), except that there are one or two cases where excited states of A are important and result in anomalously large k 2 and k -2 . Li and Na are in an intermediate position, in that ionization in (II) proceeds only from ground state A and X, but both excited and ground state halogen atoms can be the products of ion recombination. Detailed balancing does not strictly hold with Li and Na in (II), but use of it is not likely to generate errors greater than a factor of 2. We were not able to detect any systematic or significant temperature dependence of k -2 for any A and X, and conclude that curve-crossing theory gives a good general account of how k -2 varies amongst the various A and X. Detailed balancing always appears to hold for reaction (III) and our values of the three-body ion recombination coefficient, k -3 , derived indirectly from k 3 and the equilibrium constant, indicate that it varies as T -3 . Also, its magnitude is almost three times larger than predicted by Bates and Flannery’s theory, except for a few A and X, where curve-crossing considerations make k -3 undetectably small.