Endothermic Charge Transfer

Efficient conversion of translational kinetic energy into internal energy has also been discovered for endothermic processes other than atom transfer, such as charge transfer [176—179], dissociative charge transfer [94,176, 180,181] and collision-induced dissociation [98, 169]. 

Another interesting endothermic charge transfer in which translational energy is converted into electronic energy is 

The reaction was studied by Uttervack and Broida [177] by optically detecting the comet-tail band emission CO (A tt — 2 ). The relative 

The distribution of population in the vibrational states of the CO (A 7t) state was different from that expected on the basis of the Franck—Condon principle. From this, Utterback and Broida [177] consider that some intermediate states are involved in the charge transfer in this energy range. 

There is the possibility of an ion—atom exchange for the apparent charge transfer reaction represented by eqn. (116). In an attempt to distinguish between electron transfer and ion—atom exchange, an experiment was carried out with 0, and both C O and C 0 were observed. The result showed that the apparent ratio of charge transfer to 

---

Endothermic Charge Transfer Reactions. One of the most interesting categories in studying the dependence of ion-molecule reactions on translational energy is that involving endothermic charge transfer... 

Another method used alkali metal beams (AMB) to give a negative ion and a positive ion (equation 2.15). The threshold is the sum of the ionization potential of the alkali metal and the electron affinity of the molecule. A related procedure is the determination of the threshold for endothermic charge transfer (EnCT) from one anion to a neutral to form a second anion. The threshold is equal to the difference in the electron affinities of the species so that the electron affinity of the reactant must be known. These procedures yielded accurate electron affinities of the halogen molecules [18-19, 28, 29]. 

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

Endothermic charge transfer. Also called energetic ion beam electron transfer. 

Fig. 20. Cross-sections for the endothermic charge transfer reaction Kr + D2 Kr + D2 as a function of Kr kinetic energy. The arrow indicates the probable error for the two points closest to the theoretical threshold energy. (From ref. 176.)...

Finally, a comprehensive study of the thermochemistry of sulfur fluoride cations and neutrals was performed (Fisher et al., 1992). Endothermic charge-transfer reactions of several of the SF+ ions were examined. Analyses of these cross sections yielded the ionization energies of SF from reactions with Xe of SF3 from reactions with CH3I, NO, and C HsBr of SF4 from reactions with Xe and O2 and of SF5 from reactions with NO, C6H5CF3, and CH3I. IE(SF2) was not measured in our work, as a precise photoelectron value (10.08 0.05 eV) was already available in the literature (DeLeeuw et al., 1978). Some of our values... 

Several negative-ion-neutral reactions have been studied, principally in an attempt to determine limits for the electron affinities of important diatomic and triatomic molecules. In the more widely used method, one looks for charge transfer between a low-energy (preferably thermal), negative ion X and the neutral molecule Y. Alternatively, the kinetic energy threshold for an endothermic charge transfer reaction between X and Y is measured. 

Endothermic charge-transfer reactions are particularly interesting because information concerning electron affinities can be derived directly from threshold measurements. If we consider a reaction of the type... 

Fig. 16. Kinetic energy dependence of the endothermic charge-transfer reaction Cr + CI2—Cl2 + CL...

---