Photoluminescence Fluorescence and Phosphorescence

Qualitative aspects of photochemistry were known long before the development of chemistry as a science. For example, the history of chemical luminescence (or chemiluminescence) goes back to 1603 when Vincenzo Cascariolo, an alchemist 

F Fluorescence (allowed, fast), U Unfavorable (slow), P Phosphorescence (forbidden, slow). 

To place in context this first recorded discovery of a phosphor, we must recall (from Sect. 6.3) that Gustav Kirchhofif in 1859 realized that the observed frequencies of the various elements emission lines in their bright line spectra corresponded to the frequencies observed in Fraunhofer s dark line spectra. Kirchhoff concluded that the dark lines were due to the absorption of the characteristic frequencies of the elements present in the cooler outer layers of the sun s atmosphere, and that these were the same frequencies that these elements emitted when excited by an energy source such as a flame. In other words, he observed the resonance lines, radiation absorbed by a substance and immediately emitted at the same wavelength. 

Other substances, such as barium sulfide, can absorb a certain wavelength of light, but because of the molecular energy level scheme and the quantum mechanical selection rules which govern energy level transitions, the absorbed radiation may be emitted at a longer wavelength and there may also be a time delay in the transition. An examination of Fig. 8.3 will help clarify this statement and also depict the difference between phosphorescent and fluorescent materials. 

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