Excited ions exoergic reactions

Several examples of exoergic charge-transfer reactions that proceed at different rates with ground-state and electronically excited ions are listed in Table I. In some cases the cross section for the excited-state reaction may be smaller than that for the ground state, as is the case for the reactions Xe+(02, Xe)02+ Kr+(N20, Kr)N20+ Kr + (C02, Kr)COz+, whereas in other instances the excited state is more reactive, as for the processes N+(Kr,N)Kr+, N+(CO,N)CO+, 02+(Na,02)Na and 0/ (NO, 02)N0 +. The differences in reactivity are often more pronounced in the region of low ion translational energies1 lb (Fig. 10). The role of excited-state ions in charge-transfer reactions was reviewed by Hasted some time ago,175 but much more experimental data has been obtained recently, as indicated by the data shown in Table I. 

Up to this point we have discussed collisional deactivation of vibration-ally excited ions formed by ionization or as products of exoergic particle-transfer ion-molecule reactions. A somewhat different situation prevails with larger vibrationally excited ions, such as those formed as intermediates in ion-molecule association reactions. Reactions in which such excited intermediates are formed generally demonstrate a third-order dependence of the rate on the concentrations of the reactants at relatively low pressures. The general reaction mechanism may be represented as... 

Fullerton and Moran (1971) considered the role of dispersion and short range forces in reactions of He+ + N2, in the context of the phase-space model, and also (1972) reactions of C+ with 02 and N2. The same authors (Moran and Fullerton, 1971) discussed collision-induced dissociation of excited 02 and NO+ ions, and in another paper (1972) rotational excitation of N + produced in thermal reactions A + + N2 -> N + + A, with A a noble-gas atom. Phase-space results in the last paper compared favourably with experiment, with some discrepancies observed for the moderately exoergic reactions. 

Impressive excitation functions have been measured using the photo-ionization/grided-chamber technique discussed in Chapter 3. Once again, there may be uncertainty in the product ion detection efficiency for exoergic reactions. 

Figure 3.1 Translational energy dependence of the reaction cross-section, ctrIEt) for the Hj"(v = 0) -h He HeH -f- H reaction [adapted from T. Turner, O. Dutuit, and Y. T. Lee, J. Chem Phys. 81, 3475 (1984)]. For this ion-molecule reaction the observed threshold energy is equal to the minimal possible value, the endoergicity of the reaction. Exoergic ion-molecule reactions often have no threshold. By exciting the vibrations of the Hj reactant the cross-section for the reaction above can be considerably enhanced.

The [Ru(bipy)3] ion reacts with [e ]" to yield the luminescent excited state of [Ru(bipy)3] ", owing to the great exoergicity of the reaction. ... 

On the other hand, in the reaction of Ar++( P) with methane, only nondissociated CH4 was observed as a product this was interpreted as formation of the other product Ar+ in the excited Ar+ ( So) state, which takes up a substantial amount of the available energy and makes the exoergicity of the process too small to make the dissociative charge transfer possible (Smith and Adams, 1980). Such a formation of an excited projectile final state has been observed in other reactions, too, notably in collisions of He " " with some molecular targets (NO, NHj, H2S) in which the reaction window concept directs the process to formation of the excited He ( P) state and the ground state of the molecular product ion, thus making determination of its vibrational energy distribution possible (Famik et al., 1993 Herman, 1996). 

The nomenclature is confusing and its use confused, as an examination of the literature reveals. By exoergic charge-transfer reaction, we imply that the formal chemical reaction A B,A)B is exoergic for ground-state products. If the transfer of the electron is resonant or accidentally resonant, the ionic product will normally be formed in an excited electronic and/or vibrational state such that the actual electron transfer will be thermoneutral or essentially so. The exoergicity is released subsequently by the decay of the excited B product ion. 

A. The rare gas ion Ar+ reacts with H2 to form ArH+. The reaction is exoergic. Provide one physical model to illustrate why the nascent product will be vibra-tionally excited. To continue with this reaction go to Problem B and then to C. The F + H2 reaction also leads to nascent HF molecules that are vibrationally excited. What chemical argument can be used to make this observation fiirther support the model ... 

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