Measurement of emission characteristics fluorescence, phosphorescence and chemiluminescence

2 MEASUREMENT OF EMISSION CHARACTERISTICS FLUORESCENCE, PHOSPHORESCENCE AND CHEMILUMINESCENCE 

Emission characteristics of a molecular system can be expressed by three types of measurements (1) observation of emission and excitation spectra, (2) measurement of quantum efficiencies, and (3) determination of decay constants or radiative lifetimes. 

For recording of the emission spectrum, the emitted radiation is focussed on the slit of a monochromator and intensities measured attach wavelength. Since sensitivities of photocells or photomultipliers are wavelength dependent, a standardization of the detector-monochromator combination is necessary for obtaining true emission spectrum This can be done by using a standard lamp of known colour temperature whose emission characteristics is obtained from Planck s radiation law. The correction term is applied to the instrumental readings at each wavelength. Very often substances whose emission spectra have been accurately determined in the units of relative quanta per unit wavenumber intervals are 

Wavelength dependence of detector response can also be compensated by using a fluorescent screen in front of the photocell or photomultiplier. This screen acts as a quantum counter. A concentrated solution of Rhodamin B in glycerol (3g per litre) or fluorescein in 0.01N NajCO, has been used for this purpose. Quantum counters work on the principle that whatever be the wavelength of radiation incident on the screen, if completely absorbed, the photodetector sees only the wavelength distribution of fluorescence from the dye. It requires that the fluorescence yield of the counter material be independent of wavelength of excitation and therefore that its emission intensity is directly proportional to the incident intensity. 

For atomic systems the integrated areas under the absorption and emission spectral curves provide values of t (Section 3.9). For molecules, various sources of error vitiate the result, specially when electronic hands overlap and one band can borrow intensity from the other. Existence of vibrational peaks quite often present problems in measuring the area under the absorption curve accurately. But if one can measure t/ and tj f, t0 can be calculated from the expression (10.2). 

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