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8537181 
Journal Article 
Direct Determination of Frenkel Exciton Coherence from Ronchi Ruling and Transient Grating Experiments 
Kenkre, VMN 
2021 
Springer Science and Business Media Deutschland GmbH 
Lecture Notes in Physics 
982 
87-113 
English 
Applications of the memory function formalism were discussed here from the viewpoint of experiments of the gentle kind, in which the system being studied is not modified strongly by the probe used to make the measurements. A simple discussion of exciton coherence was given at the beginning of the chapter with emphasis on theoretical concepts underlying its quantitative measurement. It focused on the ratio of the exciton mean free path to the lattice constant, and avoided philosophical distractions. A theory of coherence in the context of experimental studies using Ronchi rulings pioneered by Ern and others in the Dupont laboratories was developed next. The analysis addressed the time evolution of an initially periodic distribution of triplet excitons. It was followed by a similar analysis of transient grating investigations for singlet excitons introduced in the field by Fayer. This latter method uses the same concept as the method of Ronchi rulings but is conceptually cleaner as it excites initially just one, or perhaps more accurately two, spatial modes of the exciton density by crossing laser beams into the crystal. These two classes of experiment, Ronchi ruling and transient grating, are particularly suitable to probe exciton coherence. In the last part of this chapter, an interesting analysis of possible effects of initial phases on grating and ruling phenomena was provided. It took as its starting point, an early study (Kenkre 1978b), of the initial condition term in the Zwanzig derivation of the GME, carried out long before transient gratings entered the molecular crystals field. The connection between initial pair states and initially random states was elucidated. Explicit tables of coherence parameters were made available through the analysis of a series of reported experiments, in several crystals at room temperature on triplets and in anthracene crystals at several temperatures on singlets. © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG.