Single-photon fluorescence steady-state detection

The great sensitivity of fluorescence spectral, intensity, decay and anisotropy measurements has led to their widespread use in synthetic polymer systems, where interpretations of results are based upon order, molecular motion, and electronic energy migration (1). Time-resolved methods down to picosecond time-resolution using a variety of detection methods but principally that of time-correlated single photon counting, can in principle, probe these processes in much finer detail than steady-state techniques, but the complexity of most synthetic polymers poses severe problems in interpretation of results. 

Flourescence spectrometers can be divided into either lifetime or steady-state instruments, depending on whether they resolve the temporal behaviour of the emission (or more correctly the excited state), or not, respectively. In both cases there are strong similarities with single beam absorption instruments. Thus, much of the preceding sections is equally relevant to them. However, the levels of photons detected in fluorescence (or equally phosphorescence) are typically much lower than those in absorbance in the former one is detecting the few photons that are emitted by the sample, in the latter one is detecting those of the light source attenuated by the number absorbed by the sample. As a consequence certain features are optimized differently for fluorescence. 

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