Unimolecular reactions in the gas phase have been known for nearly a centur
y. Unimolecular ionic reactions form the basis for mass spectrometry. This
review article covers three major aspects: (1) Activation of unimolecular r
eactions by ambient blackbody radiation and the resultant work on biomolecu
les. (2) The definition of activated complex parameters through RRKM calcul
ations on ab initio potential energy surfaces. (3) Competing decay mechanis
ms of large hot carbon cluster ions-radiative decay and thermionic emission
.