The Determination of the Kinetics of Unimolecular Ionic Dissociations by Field Ionization Mass Spectrometry with the Use of a Sharp Edge

The kinetics of very fast dissociations of positive ions can be determined by field ionization mass spectrometry using a sharp edge as a source of field ions. The broadening of fragment peaks in a mass spectrum was studied, and was mainly due, in favourable cases, to the time required for the parent ion to dissociate. Broadening due to other causes was removed from an observed peak by a deconvolution procedure before the kinetic analysis was carried out. The required knowledge of the potential distribution in the ion source was found b y conformal transformations. Dissociations occurring within times of 10-12 to 10-9 s after the initial ionization were accessible to study. Protrusion growth (which is undesirable) during such experiments was investigated. The kinetics were found for the formation of the tert-butyl carbonium ion from three separate precursors, the 2,2-dimethylpropane ion, the 2,2-dimethylbutane ion and the 2,2-dimethylpentane ion. The times required were some picoseconds for most of the ions dissociating, and rate constants were obtained. Some of the molecular ions formed by field ionization were, however, stable for over 10-5 s, in sharp contrast to the same ions produced by electron or photon impact.