Recent progress in chemi-ionization kinetics

A review of cherni-ionization reactions based on knowledge acquired since 1968 is given. Cherni-ionization reactions are considered to be: 'Reactions by which the nurnber of elementary eh arge carriers is increased as the direct result of the formation of new chemical bonds'. This includes both associative (A + B ~AB++ e-)andrearrangement(A + BC--+ AB++ C + e-)ionization reactions. Rate coefficients for these reactions are often on the order of 10- 12 to 10- 9 ml molecule- 1 sec- 1 and decrease with increasing temperature (relative collision velocity). Fora given reactant A tbe rate coefficients also tend to decrease with an increasing nurnber of atoms in B. A nurober of examples of exoergic cbemi-ionization reactions between ground state reactants have now been definitely established. More detailed knowledge of reactions involving an electronically excited collision partner has become available and meaningful comparisons to Penning ionization can be made. However, our understanding of the details of the processes occurring are based primarily on Observations of noble gas metastables, which, as is shown, do not in all respects apply to reactions of other species. I. INTRODUCTION Chemi-ionization is a process by which the number of elementary charge carriers is increased as a direct result of the formation of new chemical bonds. Two elementary reaction types are covered by this definition: Associative ionization : (1) Re-arrangement ionization: A + BC ~AB+ + (C + e-) (2)t A few years ago ( 1968-1969), this author summarized 1 knowledge of the hundred or so chemi-ionization reactions which had then been studied. It t Work sponsored by Project SQUID which is supported by the Office of Naval Research, Department of the Navy, under Contract N00014-67-A-0226-0005, NR-098-038. tThe parentheses in Reaction (2) indicate that the reaction may Iead directly to the negative ion. 287 PAC 39 l 8 ARTHUR FONTUN is the purpose of the present paper to discuss the developments in this area since then. In environments where chemi-ionization is observed energy transfer ionization often can occur as weil. This process is commonly referred to as Penning ionization . Penning ionization: (3) The species B may be simply ionized in Reaction (3) or it may be ionized with subsequent dissociation to a smaller ion and neutral fragments 2 ~ / ,. / / / / / / / A + e;case a A+ B'!'case b lnternuclear distance Figure 1. Schematic potential energy curves illustrating associative ionization paths for reactants in their ground internal energy states. The arrow indicates electron ejection. Case (a), solid line: endoergic reaction. Case (b), dashed line: exoergic reaction. dissociation of B to a positive and a negative ion (ion-pair production) is another possibility (see, e.g., Ref. 3), as is A + + B- formation (Ref. 1 b ). Penning ionization and other ionization processes are sometimes included in the term chemi-ionization4 • 5 . While there is some theoretical justification for this, e.g. common entrance reaction channels are sometimes involved in Penning and 'true' chemi-ionization processes, it appears undesirable to broaden a term which has acquired a very specific meaning, as a result of a specific need among workers in the various fields of chemistry involving ionized gases. This paper is concerned with Penning ionization only to the extent that it bears on the discussion of reactions of types (1) and (2). 288 RECENT PROGRESS IN CHEMI-IONIZATION KINETICS Several classes of chemi-ionization reactions can be distinguished on the basis ofthe electronic excitation energy, (E), of reactant A and the ionization potential, (I.P.), of reactant B. This may be shown on the basis of a few simplified potential energy curves for associative ionization. Electron ejection is in general a Franck-~Condon process and is indicated as such by the arrows in the following figures. Figure 1 illustrates the reactions (discussed in Section II) in which both reactants are in their ground state. With the AB+ curve positioned as in case (a) associative ionization is an endoergic process and requires a high relative velocity of the reactants. In case (b) the AB+ curve lies partly below the AB curve. In this region associative ionization is an exoergic process. When reactant A is electronically excited (Sections AB* A+B lnternuc distance Figure 2. Schematic potential energy curves illustrating associative ionization involving an excited reactant, .E(A) < /.P.(B). (After Herman and Cermak6 ). III and IV), two situations can be recognized, E(A) < J.P.(B) (Figure 2) and E(A) ;;?.: J.P.(B) (Figure 3). In the former case, exemplified by the formation of homonuclear diatornie ions [see, e.g., Reactions (31) and (32) below], Penning ionization cannot occur. In the latter case, Penning ionization competes with associative ionization. Two transitions are shown in Figure 3 : In case (a) the energy ofthe ejected electron exceeds E(A) - I.P.(B) + EK (the relative kinetic energy of A * and B) and an associative ion AB+ is formed. In case (b) the energy of the electron is less than this quantity and a Penning ion B + is formed, the additional energy becoming kinetic energy of the separating species A and B +. Since Ref. 1 was prepared, further review papers have appeared which cover selected areas of chemi-ionization in detail. Wexler 9 has reviewed ion (including chemi-ion) production in endoergic reactions of ground state reactants in molecular beams, where the necessary energy is provided by the relative kinetic energies of the reaction partners; interaction energies up to 289 ARTHUR FONTUN A*+B ----------~---lo I nternuclear d istance Figure 3. Schematic potential energy curves for the situation E(A) ~ /.P.(B). Case (a), solid line: associative ionization. Case (b), dashed line: Penning ionization. (After Hotop and Niehaus 7 • 8 ). Dis the dissociation energy of AB+ formed in transition (a). For the sake of clarity. EK is shown too large and is given for case (b) only. about 20 eV, the upper Iimit for processes involving the formation of new chemical bonds, are covered. The production of ions in flames and the subsequent reactions of these ions have been discussed by Miller 10, Calcote 11 , Calcote and Miller 12, and Peeters, Vinckier and van Tiggelen 13 . Lampe's review2 covers essentially the same period as Ref. 1 and is restricted to reactions of electronically excited species at near room temperature; it includes Penning reactions and emphasizes aspects other than those covered in Ref. 1, to which it thus represents a valuable complement. The article by Rundei and Stebbings 5 on collision processes involving metastable excited species includes a review of che-mi-ionization and Penning ionization reactions of these species. The paper by Stedman and Setser 14 on the chemical reactions of the noble gas metastables provides a useful background for understanding the chemi-ionization reactions of these species. The theoretical understanding of chemi-ionization has been summarized by Berry4 . Since then, Miller et al. 15- 17 have published a series of papers concerned with the theory of Penning ionization and associative ionization involving excited species. Ho top and Niehaus 7 • 8 , Micha, Tang and Muschlitz 18 , Olson 19, and Cohen and Lane20 have published in the same area. Myers and Young21 have discussed the product energy distribution of endoergic atom-atom associative ionization reactions. In much of the recent experimental work the state of the reactants has been known and the reaction products have been identified, sometimes including informatiot:t on their excitation state. Such important details are mentioned below in the discussions of individual reactions. Most experiments have been performed in thermal systems in which the temperature was not specifically controlled but can be assumed to have been in the range of 300 to 500 K. Self-sustaining flame data were obtained predominantly at 290 RECENT PROGRESS IN CHEMI-IONIZATION KINETICS temperatures in the 1800 to 3000 K range. Wherever other temperatures apply, specific mention is made of this fact. All rate coefficients are given below in ml-molecule-second units. Data from beam sturlies are usually quoted in the original articles as cross sections, Q(cm2). The Q-data resulting from thermal (i.e. not velocity-selected) beam work have, for the sake of uniformity, been converted in this paper to rate coefficients via the standard formula k = Qv = Q(8kT/nJ1)! = 1.46 x 104 Q( Tj Jl)-t ml molecule- 1 sec- 1, where v is the mean relative velocity of the reactants. However, for (the few) velocity selected beam sturlies only cross sections are meaningful and the data are reported in cm2 ; energies from such sturlies are reported in centre ofmass units, corresponding to relative collision energies in bulk kinetics experiments. In this review, the Iiterature available up to May 1973 has been considered. II. REACTIONS WITH BOTH REACfANTS IN THE ELECTRONIC GROUND STATE A. Metallic species reactions Since for most compounds the ionization potential exceeds the bond energies, the number of exoergic or near thermoneutral chemi-ionization reactions that can be anticipated between ground state neutral species is relatively small. Until recently no such reactions bad been uniquely identified, though the formation of chemi-ions in hydrocarbon-oxygen reactions may be reasonably attributed to such a process (cf. Section II.B). Also, the presence ofMeOH + (Me denotes a metal atom) in alkaline earth seeded flames is due 1 c to either Me + OH ~ MeOH+ + eor MeO + H ._. MeOH+ + e- (4) (5) where Reaction (4) is about thermo