In building on the work of Robinson-Bixon-Jortner, Plotnikov suggested three approximations : the vibrational structure is decoupled from medium, the system is assumed to be at zerotemperature condition, such that vibrational energies are smaller than thermal energies , and the initial electronic state of the transition is in the zeroth-vibrational state. Understanding that fast vibrational loss is typically on the order of ∼10 11 s -1 offers important simplifications to the model , since radiative and non-radiative processes normally occur between electronically excited states in vibrational equilibrium. For ultra-fast internal conversion, the latter approximation may not be valid, since internal conversion will be competing directly with vibrational relaxation , and may therefore be multiple orders of magnitude faster than fluorescence, like in low pressure vapours . From these rationalisations, the rate of internal conversion can be expressed as :