Use of an ``ordered'' electron spin state as a probe in molecular spectroscopy: Optically detected adiabatic demagnetization in excited triplet states in zero field

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Abstract
An ``ordered'' electron spin ensemble in molecular excited triplet states has been created by adiabatic demagnetization in the rotating frame. The ``ordered'' state can be viewed as an explicit spin sublevel population distribution dependent upon the triplet's local field. The state is created isentropically from the initial value of the triplet's spin alignment and is investigated by a series of optically detected spin‐echo experiments in which the phosphorescence intensity is made to reflect quantitatively the order remaining in the ensemble. Experimental results on the 3ππ* state of 1,2,4,5‐tetrachlorobenzene in a durene host are presented. In this system the order persisted for a time approaching the triplet lifetime and was independent of temperature over the range investigated. The results of these initial experiments suggest that the ordered state can be used to monitor many features associated with excited triplets such as the communication between localized triplet states at different energies, migration of triplet excitons, and as a general probe of the dynamics of any process that alters the excited state's environment. Several specific experiments are outlined to illustrate these principles.