Transfer reaction, cross section

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

Gustafsson and Lindholm (19) have shown the effects of translational energy on charge transfer reactions with H2, N2, and CO. They observe that endothermic reaction cross-sections increase with increasing kinetic... 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

Harpoon reactions of alkaline metal atoms with halogen molecules in the gas phase seem to be the first instance of the observation of chemical electron transfer reactions at distances somewhat exceeding gas-kinetic diameters. Actually, as far back as 1932, Polanyi, while studying diffusion flames found for these reactions cross-sections of nR2, somewhat exceeding the gas-kinetic cross-sections [69]. Subsequently, more precise measurements which were carried out in the 1950s and 1960s with the help of the molecular beam method, confirmed the validity of this conclusion [70],... 

The effect of rotational energy on a reaction cross section has been studied experimentally in only one reaction, namely, Ar+(H2, H)ArH + (Table III). For this exoergic particle-transfer process, an inverse dependence of the cross section on rotational energy was observed. 

Finally, some experimental observations are discussed in which charge transfer to surface states is important. The emphasis is on methods to be quantitative in describing the role of surface states by determining their density and reaction cross sections. Some previously published observations as well as preliminary new results are used to illustrate the role of surface bound species as charge transfer surface states. 

Fig. 4.30. Angle-resolved transfer-ionization cross sections, reaction (4.35), relative to those at 0°, for positron-argon and positron-krypton collisions at impact energies of 75 eV, 90 eV and 120 eV (Falke et at, 1995, 1997). Reprinted from Journal of Physics B30, Falke et al., Differential Ps-formation and impact-ionization cross sections for positron scattering on Ar and Kr atoms, 3247-3256, copyright 1997, with permission from 10P Publishing.

Opposite to rebound reactions is the reaction Na + Ch — NaCl + Cl which proceeds via the spectator stripping mechanism. In this case, the crossing between the nonreactive covalent Na-Cl2 curve and the Na+Cl ion-pair curve, which promotes the reaction, occurs at a large distance [Re = 5.22 A, when using the chlorine adiabatic electron affinity in Magee s equation). This distance increases to 22.3 A when sodium is excited to the 3p P level. One would expect an increased reaction cross-section, but this is not observed because electron transfers at such large distance are inefficient. The overlap between the sodium HOMO and the CI2 LUMO is very small at these distances. As a result, when the crossing radius increases substantially, there is only a small effect on the dynamics of the reaction [164, 165]. 

Two routes have been followed in reaction stereodynamics. One is to orient a molecular reactant in space and see how the reaction cross-section varies with the molecular orientation. This direction has been pioneered in molecular beam experiments using focusing of an electric hexapole field to control the molecular orientation [221-223a]. Numerous studies have applied this technique to electron-transfer reactions of alkaline-earth atoms [223b]. This technique is now complemented by the so-called brute force technique, where polar molecules are oriented in extremely strong electric fields [83]. 

In order to see the threshold behaviour of such a reaction and its marked contrast with the exothermic dissociative charge transfer, the cross-section curves for the two reactions... 

At present, no technique is available which can separate individual vibrational states from a system containing ions in various vibrational states. In such a situation, the best method to study reaction cross-sections for individual levels would be to use a system in which the relative abundance of each state is accurately known. Recently developed photoelectron spectroscopy is extremely suitable for such studies since it provides accurate knowledge on relative abundances regardless of the presence of autoionization and without any assumptions on the threshold law and the Franck—Condon factors. Very recently, the hydride ion transfer reaction... 

The prototype molecule-molecule electron transfer reaction, H J + H2 — H2 + HJ, continues to attract interest particularly with respect to the distribution of vibrational states of the Hj product, and the detailed state-to-state reaction cross-sections. Reactions of the type CH3I + K can lead to net atom transfer or electron transfer as shown in equations (12a) and (12b). The preferred channel... 

Competition with this channel can occur in three ways. Firstly, energy transfer or ionization may occur from the entrance channel before the electron-jump can take place this is clearly favoured if Rc is small, but Rm is large, as for polyatomic molecules with small electron affinities. This is likely to be operative if the total quenching cross-section for A -f BC is larger than the reaction cross-section for the analogous alkali -I- BC system. 

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