C 3 H 5 2ϩ , formed by electron impact ionization of propane, undergoes metastable decay into C 2 H 2 ϩ ϩCH 3 ϩ . We have monitored this reaction in a magnetic mass spectrometer of reversed geometry that is equipped with two electric sectors ͑BEE geometry͒. Three different techniques were applied to identify the fragment ions and determine the kinetic-energy release ͑KER͒ of spontaneous Coulomb explosion of C 3 H 5 2ϩ in the second and third field free regions of the mass spectrometer. The KER

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... istribution is very narrow, with a width of about 3% ͓root-mean square standard deviation͔. An average KER of 4.58Ϯ0.15 eV is derived from the distribution. High level ab initio quantum-chemical calculations of the structure and energetics of C 3 H 5 2ϩ are reported. The activation barrier of the reverse reaction, CH 3 ϩ ϩC 2 H 2 ϩ ͑vinylidene͒, is computed. The value closely agrees with the experimental average KER, thus indicating that essentially all energy available in the reaction is partitioned into kinetic energy.