Steady-state emission spectra and their correction

4 Steady-State Emission Spectra and their Correction 

Excitation spectra are especially useful for analysing mixtures. If the emission spectrum is (in part) due to an impurity, the excitation spectrum is likely to differ markedly from the absorption spectrum of the sample. This may be used in low-temperature work (Section 3.10) to determine whether an absorption spectrum produced by irradiation is due to a single photoproduct. 

Phosphorescence spectra are commonly weaker than fluorescence spectra, even when solid sample solutions are probed. For unperturbed detection of phosphorescence, the fluorescence must, therefore, be eliminated. This is readily achieved by surrounding the sample with a metal cylinder that is rotated at variable speed by a motor ( rotating can ). A hole in the cylinder exposes the sample to the excitation beam for a short period, during which the detector does not see the sample. Rotation of the can subsequently opens the sample to the detector, long after the fluorescence has decayed. The phosphorescence lifetime can then be determined from the intensity dependence of the phosphorescence on the rotational speed. In modem instalments, the elimination of fluorescence is commonly achieved by replacing the continuous light source with a pulsed source (a stroboscope or a pulsed laser) and the photomultiplier is electronically deactivated during and shortly after the excitation pulse. 

The more convenient method of phosphorescence excitation was introduced by Kearns in 1965. Because emission spectroscopy is much more sensitive than absorption, particularly for phosphorescence that can efficiently be separated from stray light and fluorescence, the determination of singlet triplet absorption spectra by monitoring the resulting phosphorescence emission has often been successful with comparatively little effort. 

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