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8627232 
Journal Article 
The Montroll Defect Technique and Its Application to Molecular Crystals 
Kenkre, VMN 
2021 
Springer Science and Business Media Deutschland GmbH 
Lecture Notes in Physics 
982 
213-243 
English 
Experiments to probe the nature of a system may be either of the gentle kind examined in Chap. 5 or such that they modify the given system substantially. An example of the latter situation is when guest molecules such as those of tetracene are placed within an aromatic hydrocarbon crystal like anthracene to ascertain the nature of the motion of excitations in the latter. Such doping can alter the original system profoundly and require new methods of solution of the equations governing motion. The defect technique was constructed for such purposes in this chapter along lines first set out by Montroll and his collaborators. It was explained in the context of a crystal. An extension to study effects of the coherence of the motion when the detectors are placed at random positions within the crystal was explained next. It was then generalized to address high concentrations of defects via the so-called ν-function formalism. A theory was constructed to interpret measurements of the exciton diffusion constant in observations with detectors placed at the end of a crystal in what is sometimes referred to as Simpson geometry. It offered an explanation of the large discrepancy in reported values of the diffusion constant. Several experimental schemes were developed theoretically to be used along with proposed experiment. One such involved varying the penetration length of the excitons in the sample and another dealt with the time dependence of detector emission. The temperature dependence and magnitude of the diffusion constant, when extracted from annihilation experiments on the one hand and sensitized luminescence on the other, were found to be in stark conflict when interpreted in the usual way. The paradox was resolved with the theory developed in this chapter. The question of whether it is the motion or the reaction process that limits the reaction-diffusion phenomenon in a given experiment was investigated carefully and conclusions were presented. © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG.