Effect of kinetic energy on exothermic reactions

The effect of relative kinetic energy of reactants on exothermic ion—molecule reactions was investigated in the l-ObOs with conventional single-source mass spectrometers by changing the repeller-field strength E, or the source temperature T. For example, Gutbier [150] and Stevenson and Schissler [151] found that the phenomenological cross-sections Q for the reactions 

Although many other reactions, particularly those involving or H2, have since then been found to exhibit the same and T dependence, many more reactions have been found not to obey eqn. (96). Franklin et al. [13,152], for instance, in studying ion—molecule reactions in more complex systems (hydrocarbons), found that the dependence of the cross- 

In order to explain these experimental facts, Boelrijk and Hamill [154] proposed new expressions for Q(Ff) based on the idea that nearly head-on collisions should be treated separately from glancing collisions. They assumed that above a certain transitional energy F, the microscopic reaction cross-section a becomes equal to an energy-independent cross-section, Ok, whose value approximates the gas kinetic collision cross-section. Ok. Ft is the energy at which the Langevin impact parameter becomes comparable to the impact parameter corresponding to Ok. The microscopic cross-section for the case of 0 F F, is written as 

Comparison with the experimental data of Stevenson and Schissler [151] showed that the result fits the data for the reaction Ar + ArH + H 

Stevenson [157] obtained neither the nor r dependence. Later, Stevenson [7] showed that the data of Field et al. cannot be interpreted as indicating the E dependence. Kubose and Hamill [158], on the other hand, obtained a result which tends to follow the E, dependence predicted by the G—S theory, although the absolute values of the cross-sections were smaller than the predicted ones. 

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