SPECTROSCOPIC CRITERIA FOR WAVELENGTH SHIFTING, FAST, AND RED-INFRARED SCINTILLATORS

An outline is presented of the distinguishing characteristics of polyatomic molecule electronic excitation, including internal conversion, intersystem crossing, spin-orbitally-determined transitions, and orbital character of excited state configurations, with the resultant effects on fluorescence quantum efficiency. The mean lifetime of fluorescence in its frequency dependence and the corresponding oscillator strength of the related absorption is analyzed in terms of the Einstein coefficient A21/B-12 ratio. A solvent cage spectroscopic model is discussed in relation to internal torsional modes of certain molecules, as well as twisting-intramolecular-charge-transfer (TICT) and excited-state-intramolecular-proton-transfer (ESIPT). The role of the latter as large-magnitude wavelength-shifters is stressed. Criteria for wavelength-shifting, short mean lifetime, red-to-infrared molecular fluorescence are given, with special consideration of solid phase effects as a guide to plastic scintillator development.