Introduction and Historical Remarks

In Chap. 6, we discussed low-energy optical excitation states, the singlet and triplet excitons and energy transfer. The primary experimental method applied there was optical spectroscopy in the visible, in the near IR and in the UV spectral ranges. In the present chapter, we treat the structure and the dynamics of localised triplet states, of triplet mini-excitons, and of triplet excitons in molecular crystals. The primary experimental method for the investigation of the lowest-energy triplet level Ti is electron-spin resonance (ESR) (Fig. 7.1). 

In all the systems dealt with here, the Ti states have three notable properties their lifetime is many orders of magnitude longer than that of the Si states, their total spin quantum number is S = 1, and they consist even without an applied 

Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA. Weinheim 

In an applied magnetic field Bo 0, the triplet states have in addition a Zeeman energy. The Zeeman splitting then consists of three non-equidistant terms. For technical reasons, the ESR spectrum is usually measured at a constant microwave frequency, e.g. 9.4 GHz (X band) or 35 GHz (Q band). It exhibits a fine-stracture splitting between the resonance fields of the Amj = 1 transitions which is also termed the fine stmcture (Fig. 7.1) this is a direct result of the zero-field splitting. 

ODMR is as a rule possible when an optical transition probability coupled to Ti depends on the orientation of the spin when for example the phosphorescence probabilities of the three components of the triplet state ( T ), Ty), and Tz) at Bo = 0 or T+), I To), and T ) at Bq 0) are different. Frequently, the sensitivity of optical detection is much greater than that of detection via the microwave absorp- 

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Introduction and Historical Remarks on Thin Polymer Films. 30... 

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