Electron-jump mechanism

The reactions of Ba, Ca, Sr with F2 are found [354] to have large reaction cross sections ( 115—160 A2), indicating an electron jump mechanism. Visible chemiluminescence, which represents a minor (<0.5%) reaction channel, has been observed from several electronic states of MF. For Ca + F2, the vibrational population for CaF( 2E+) is strongly inverted. Emission is also observed [354, 357] from the dihalides, BaF2, CaF2 and SrF2, but whether these arise from the radiative two body recombination process... 

The formation of the alkaline earth cyanide is the major pathway in the reaction M + BrCN. The other channel (giving MBr + CN) is observed for the reactions of Ba and Sr. The ratio of the cross section is o(BaCN)/ a(BaBr) 25-100 and a(SrCN)/o(SrBr) 250-1000 [363]. It was not possible to measure internal state distribution for the alkaline earth salts, but for the CN product of the minor channel, the vibrational distribution was found to be N(d = 1)/N(p = 0) <. 0.2 and Txot = 1250K for Ba + BrCN and TIot = 750 K for Sr + BrCN. The reaction dynamics appear to be consistent with an electron jump mechanism which would favour the breakup of the M+(BrCN) ion pair to give MCN + Br. 

The vibrational state distribution for the ground state CaO product from the reaction Ca + C02 shows a similar monotonic decrease with increasing vibrational quantum number to that from Ca + 02 and, again, only 10% of the reaction energy appears as CaO internal excitation [383]. No information exists about the amount of CO excitation. There is also evidence that low-lying excited states of CaO are produced in the reaction which is again assumed to proceed via an electron-jump mechanism. 

CH3C1 (90, 34 and 8 A2, respectively). These values suggest a similarity between these reactions and the corresponding ground state alkali atom reactions. The ionisation potential of Hg (3P2) is 4.974eV which is similar to those for the alkali atoms and so an electron jump mechanism is proposed for these chemiluminescent reactions of Hg (3P2 ) In contrast, the reaction of another spin-orbit state of metastable mercury with bromine, Hg (3P0) + Br2, has a much smaller chemiluminescent cross section [3 A2 compared with 150 A2 for Hg (3P2) + Br2] [406], which cannot be reconciled with an electron jump, suggesting the existence of a barrier to reaction of Hg (3P0) which is not present in the case of Hg(3P2). 

The chemiluminescent reaction A1 + 03 yields AlO (A, B) with preferential population of high vibrational levels in both states [415, 416]. In contrast, a Boltzmann vibrational state distribution is observed [415] for the A10 (B) product from A1 + N20, suggesting a different reaction mechanism in this case. An electron-jump mechanism operates for A1 + 03 giving the observed preferential population of AlO (A) [417] with a high degree of vibrational excitation. For A1 + N20, reaction takes place at shorter range, allowing production of the B state of AlO. ... 

The reactions of Sn with Cl2 and Br2 show forward scattering of the SnX product, with about 30—45% of the reaction energy appearing as translation of the products in the case of Sn + Cl2 [424]. The exact contribution to the reaction of the various spin-orbit states of tin, Sn(3P012), is unknown. Similarities between the results for Sn + Cl2 and those for Li + Cl2 [297] suggest an electron-jump mechanism, although the ionic—covalent curve crossing radius is quite small for Sn + Cl2 (— 2.9 A). 

Both M and MX are scattered strongly forward relative to the incident M2 beam, and it appears that these reactions occur by an electron jump mechanism similar to that for the M + X2 series of reactions (see Section II.C.l). 

It was found that the reaction cross section for the dimers of CO2 was between four and eight times larger than that of the monomers.This effect suggests a different mechanism for the dimeric process versus the monomeric one, probably because of the positive electron affinity of the dimers. The reaction in this case occurs via an electron jump mechanism like reaction (4.4). It was also established that the product BaO in both dimeric reactions is much colder rotationally than in the monomeric case (Fig. 11). This phenomenon seems to be quite general, both in reactions of vdW molecules and in their dissociation. It results from the multitude of channels available for dividing the total angular momentum in the reaction complex. 

Electron-jump in reactions of alkali atoms is another example of non-adiabatic transitions. Several aspects of this mechanism have been explored in connection with experimental measurements (Herschbach, 1966 Kinsey, 1971). The role of vibrational motion in the electron-jump model has been investigated (Kendall and Grice, 1972) for alkali-dihalide reactions. It was assumed that the transition is sudden, and that reaction probabilities are proportional to the overlap (Franck-Condon) integral between vibrational wavefunctions of the dihalide X2 and vibrational or continuum wave-functions of the negative ion X2. Related calculations have been carried out by Grice and Herschbach (1973). Further developments on the electron-jump mechanism may be expected from analytical extensions of the Landau-Zener-Stueckelberg formula (Nikitin and Ovchinnikova, 1972 Delos and Thorson, 1972), and from computational studies with realistic atom-atom potentials (Evans and Lane, 1973 Redmon and Micha, 1974). 

Besides the above potentials, several potentials have been proposed to describe reactions such as K + Br2 which proceed by means of an electron-jump mechanism.239-242 Of these surfaces, the one with the greatest physical content is that of Karplus and Godfrey240 which incorporates an ionic surface based on the Rittner model and a reasonable covalent surface (for large K-Br2 separations). The jump from the covalent to the ionic surface is accomplished in a continuous manner by means of an energy dependent switching function. None of these surfaces are completely satisfactory for the description of K + Br2 type reactions.169... 

Fig. 4. Potential energy curves for molecules Br2, HgCl2 and their anions formed in the electron jump mechanism (from D. R. Hardin et al.15 by permission of Taylor and... 

The electron jump mechanism, (see 1), has long been invoked71,2 in a highly simplified manner to explain the stripping dynamics of alkali atom-halogen molecule reactions M + X2. Electron transfer occurs in the entrance valley of the covalent M + X2 potential surface near the intersection with the ionic M+ + X2 potential surface. The M - X2 internuclear distance R at the intersection Rc is roughly estimated... 

The role of X2 vibrational quantization in the M + X2 electron jump mechanism has also received recent theoretical attention. It has been suggested179 that this may be treated classically to a good approximation and criteria for the validity of this have been determined180 from semi-classical theory. Potential energy surfaces have been calculated semi-empirically181 for K + Cl2 and ab initio182 for Li + F2. 

The electron affinity of the oxygen atom is 34 kcal mole", which leads to a cross-section of 13 A which is in good agreement with the experimental result. A similar spectator stripping treatment for Rb,Cs + NOCl CsCl + NO by Grice et al. [176], however, suggests that the electron jump mechanism, for which = /(M) — (RX), is operative in this case and not a spectator stripping mechanism. 

The large cross-sections can be explained in terms of an electron jump mechanism [233, 265]. The chlorine atom p-orbital which is out of the plane of the three atoms ( P) participates in the reaction and this then must lead to an excited sdkali metal atom ( P). Theoretical studies [266, 267] indicate that such reactions involving out-of-plane p- or 7r-orbitals will have large cross-sections. 

Theory was compared with experimental results on ion—dipolar charge transfer [60, 61] and proton transfer [62, 63] reactions, and was found to predict very satisfactorily the rate coefficients of proton transfer reactions which are considered to proceed by a capture mechanism. The energy dependence of the proton transfer was also correctly represented by the theory. As to the charge transfer reactions, the comparison is difficult because the observed rate coefficients are considerably larger than the capture limit. This is explained by the contribution of a long range electron-jump mechanism to the rate coefficients. Even in this case, however, the trend of the dipole effect is predicted correctly by the theory. 

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