Selective Excitation and Energy Transfer

The problems which have just been referred to can be illustrated by considering experiments on the quenching of electronically excited atoms (A ). If they are removed only by spontaneous emission and in collisions with an added quencher (Q), i.e.. 

With continuous illumination, a steady-state analysis leads to the celebrated Stern-Volmer relationship °  

Both the continuous and pulsed experiments, as they have been described, have a serious defect. No information is obtained about the nature of the quenching process, unless its products are identified and their concentration related to that of the atoms which have been excited. This is a deficiency which is common to a wide variety of kinetics experiments. 

Because of the complex, multilevel nature of the NO2 system, simplifying assumptions have to be made in analyzing the quenching. Keyser et chose a step-ladder model to represent the internal relaxation, in which each bimolecular collision is supposed to remove the same amount of rovibrational energy. The experimental observations could then be matched with roughly 3 kcal/mole being removed per collision. 

The problems in analyzing the results of photochemical (and chemical) activation experiments on unimolecular reactions (see Section 1.4) are closely related to those which arise in studies of fluorescence quenching. Excitation of a polyatomic molecule with monochromatic radiation can produce species with a well-defined energy (e) which exceeds the critical energy (e°) for some unimolecular chemical change. The usual goal in such experiments is to find the specific rate constant, k e defined by 

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