Activation of Unimolecular Reactions by Ambient Blackbody Radiation

Abstract

The historical “radiation hypothesis” as a mechanism for activating unimolecular thermal dissociation of gas-phase molecules, long discredited on the authority of Langmuir, has been revitalized by the discovery and characterization of the process of thermal dissociation of trapped gas-phase ions by the ambient blackbody radiation field surrounding the ions. This development was made possible by improvements in Fourier transform ion cyclotron instrumentation that allowed long-time trapping of weakly bound cluster ions at extremely low pressures. Binding energies can be derived from measurements of these dissociation rate constants both by detailed kinetic modeling and by simpler Arrhenius temperature-dependence approaches, although the latter require special considerations for small molecules. These approaches have been applied to thermal dissociations of molecules, including cluster ions and large biomolecule ions.