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The temperature difference ΔT = T* -T (where T* is the local temperature and T the ambient tem-perature) is studied as a function of the excitation wavenumber v̂̃ cxc  in the longwave region of the absorption band of 4-amino-4'-nitrostilbene (ANS) and 4-dimethylamino-4'-nitrostilbene (DNS) as well as rhodamine S, rhodamine 6G and Na-fluorescein in poly(vinyl alcohol) (PVA) films heated up to 403 K. Local temperatures T* are determined by a universal Kennard-Stepanov relation. A linear dependence of ΔTon v̂̃ cxc  has been found for all investigated luminescent compounds. For dyes, due to the big spectral overlaps between the absorption and fluorescence bands, it was possible to excite also in the anti-Stokes region. In this case for ΔT = 0, i.e. when T* = T, one has v̂̃ cxc  . = V ()-()  . In view of the slow geometrical relaxation of excitation energy excess in PVA polymers, the relaxation time τ R  for thermal equilibration between the luminescent molecule and the vicinal polymer exceeds distinctly the mean fluorescence lifetime τ F .

The Local Temperature Dependence of Fluorescent Centres in PVA Films on the Excitation Wavenumber