In order to then further investigate the nature of inter-excited state internal conversion, Manian and co-workers examined the thermal accessibility between each transition approximated as a Boltzmann factor. Transitions in which additional energy is required by the system (E m < E n ) as those high energy states are thermally inaccessible by the exciton residing on a lower electronic state. This switching function B m,n is only used in the case of an "uphill" transition, and set to unity in the case of a "downhill" transition, formally cast as :