Gas-phase Ion-molecule Interactions involving Atom Transfer; Limitations of the Orbiting Theory in accounting for the Variation of Cross-section with Energy

Abstract

ION-MOLECULE interactions of the type X⁺ + YH = XH⁺ + Y are of interest in gas kinetics since they are chemical reactions in which the translational energy of one of the reactants may be chosen over a wide range. This may be achieved by accelerating an ion beam to the desired velocity. However, most of the information on such reactions has in fact been derived by supplying a mixture of the gases X and YH to the source of a mass spectrometer, and, if possible, choosing the electron energy so that X (but not YH) is ionized¹. The ultimate translational energy of X⁺ is then determined by its initial position and the potential difference between the repeller plate and the exit slit of the source. This technique involves the acceleration of ions throughout the actual reaction zone, and the observed reaction cross-sections correspond to mean values for all the ions with translational energies up to the maximum. A rigorous interpretation of the effect of translational energy on the reaction cross-section is, therefore, difficult if the ‘mixed gas’ approach is used.