Reaction Cross Section Atom-Molecule Systems

Reaction Cross Section Atom-Molecule Systems 

Most gas-phase reactions of chemical interest involve neutral species. Because of the difficulties of producing monoenergetic beams of neutral molecules few systematic studies of cross section vs. energy have been carried out when both reactants are neutral. The available evidence, both experimental and theoretical, indicates that the general features of Q(E) are much the same whether or not the reactants are charged. The important consideration is whether there is a barrier. 

Although cross sections have been measured for many reactions, few have been studied over a range of energies. One for which data are available is the reaction 

The cross section is plotted in Fig. 8.5. The threshold is small ( 3 kJ) and the cross sections large, comparable to those in the low-energy domain of the Ar+ + D2 system (Fig. 8.3). The studies of the K + CH3I reaction were not continued to energies where there is a precipitous drop in Q E). 

Even larger low-energy cross sections have been observed. Studies of reactions of alkali metals with halogens are even more dramatic, as long as there is no barrier. For the K + Br2 system a cross section of 2 nm has been measured assuming each collision to be reactive, 

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Reaction Cross Section Atom-Molecule Systems... 

Figure 10 The cross section of the potential surface of the H+ HF(v) —> H2 + F reaction taken at the HF bond length of 1.3 A. The potential felt by an H atom at coordinates X and Y approaching the H of HF positioned at the origin of the coordinate system is plotted as a function of the location of the attacking H. The H-F bond points down along the Y axis. Contour lines are spaced 0.5 kcal mof from each other. The energy is referred to an HF molecule expanded to 1.3 A and an H atom at the infinity.

The halogen atom—halogen molecule exchange reactions have large cross sections (4—40 ) for systems that involve predominantly short-... 

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