Effect of vibrational excitation in reactant ions

The effect of vibrational excitation of reactant ions on ion—molecule reaction rates has briefly been discussed in Section 5.2.1 in connection with reactions (100) and (101). 

The effect of ionic vibrational energy was first examined for the reaction 

a very fine investigation was carried out on this reaction by Chupka et al. [204]. Producing their reactant H2 ions in selected vibrational states, or even in selected rotational states, by the use of their high-resolution photoionization mass spectrometer, they studied the cross-section of reaction (123) as a function of vibrational and kinetic energy of the reactant ions. The found that the cross-section decreases as vibrational excitation increases when ion kinetic energies are low, but the reverse is true when ion kinetic energies are high. 

The decrease in the cross-section with increasing vibrational energy at low energies was only in qualitative agreement with that observed by Weingartshofer and Clarke [203], Quantitatively, the decrease observed by Chupka et al. [204] was much less than that by Weingartshofer and Clarke, the ratio of the apparent cross-section for ions produced by 720 A photons (v = 0 l) to the cross-section for i = 0 ions being only 0.83 (since direct ionization predominates at 720 A, the vibrational distri- 

An interesting finding in the study of Chupka et al. is that the H3 ions are also produced at photon energies below the H2 threshold. This was shown to be due to chemi-ionization reactions 

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