Time-correlated single photon counting sensitivity

Time-correlated single photon counting (TCSPC) [28] is one of the most sensitive methods for studying time-resolved emission. In this technique, single photon events are detected after excitation and a statistical distribution of photons representing the decay of the excited state is built up over time. 

Until recently we were unable to determine k for 1(4) and 1(6) via this method, since this not only requires a time resolution better than 10 ps, but especially since the quenching of the donor fluorescence, that accompanies the electron transfer, makes the measurements extremely sensitive to the presence of minor, fluorescent impurities. After careful recrystallization a sample of 1(6) was now obtained for which the level of impurity fluorescence is sufficiently low to detect the very short lived ( 3-4 ps) residual donor fluorescence. A typical fluorescence decay as observed for this sample in ethylacetate via picosecond time correlated single photon counting (Bebelaar, 1986) is shown in Fig. 3. Via a biexponential reconvolution procedure the lifetime of the short component was determined to be 3.5 0.5 ps, while that of the impurity background is comparable to the lifetime of the isolated donor (-4500 ps) and thus probably stems from one or more species lacking the acceptor chromophore. Similar results were obtained in tetrahydrofuran (3 1 ps) and in acetonitrile (4 lps). 

Time-correlated single-photon counting (TCSP) has proven to be a much-used method for measuring fluorescence lifetimes. It is highly sensitive in that it requires only one photon to be incident on the detector per excitation cycle, and statistical analysis of the experimental data gives lifetimes with well-defined error limits. Commercial systems are available which allow lifetimes from 50 ps to many tens of nanoseconds to be measured with relative ease and high precision. 

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. 

In order to overcome this obstacle, we used a synchronously pumped, mode-locked dye laser, cavity-dumped at 4 MHz and time-correlated single-photon counting detection (18). Because of the higher sensitivity of this experimental system we were able to work at low e, using aqueous rhodamine B solutions with concentrations down to 10" M. To examine the dependence of the fluorescence decays on we chose to work with surface-solution interfaces, so as to minimize the problems associated with inhomogeneous surface coverage, which arise with dry surfaces... 

Fluorescence lifetimes have been measmed directly by time-correlated single-photon counting for pentacene in p-terphenyl [100]. This experiment requires careful selection of the laser pulse eharaeteristies sueh that the pulse duration is short enough to resolve the 23 ns decay time, yet has a bandwidth narrow enough to allow speetral selection of individual molecules. Four different molecules had the same lifetime to within experimental uncertainty, indicating that the principal contributions to the Sj state decay (radiation and internal conversion to Sq) are not strongly sensitive to the local environment in this relatively homogeneous crystalline matrix. 

Of special interest for time-correlated single photon counting are the linear fo-cused dynodes, which give fast single electron response and low transit-time jitter, and the fine mesh and metal channel types, which offer position sensitivity when used with an array of anodes. Moreover, PMTs with fine-mesh and metal channel dynodes can be made extremely small, which results in low transit time, low transit-time jitter, and a fast single-electron response. 

Steady-state and time-resolved fluorescence spectroscopy Absorption and fluorescence spectra were measured with a Hitachi 557 spectrophotometer and a Hitachi 850 spectrofluorometer, respectively. The time-resolved fluorescence spectra were measured with the apparatus reported previously [4,6] in principle, the time-correlated single photon counting system under a low excitation condition. The pulse intensity (540 nm, 6 ps (fwhm)) was in a range of 10 to 10 photons/cm. The time resolution of our optical set-up was 6 ps. Correction of spectral sensitivity and data treatment were carried out as reported previously [4,6]. 

Obviously, other spectroscopic time-resolved methods have been applied, although less general. Most often, EPR has been used for triplets and radicals (see for instance [29, 30]). Time-correlated single-photon counting, on the other hand, has proven to be a sensitive and informative method for mechanistic smdies of singlet reactions [31, 32], besides than a technique useful for analytic applications, e.g. for determining the composition of mixmres of aromatics, when both lifetime and spectrum shape were required for obtaining a reasonable picture. 

For fluorescence measurements, by far the most versatile and widely used time-resolved emission technique involves time-correlated single-photon counting [8] in conjunction with mode-locked lasers, a typical mo m apparatus being shown in Figure 15.8. The instrument response time of such an apparatus with microchannel plate detectors is of the order of 70 ps, giving an ultimate capability of measurement of decay times in the region of 7 ps. However, it is the phenomenal sensitivity and accuracy which are the main attractive features of the technique, which is widely used for time-resolved fluorescence decay, time-resolved emission spectra, and time-resolved anisotropy measurements. Below ate described three applkations of such time-resolved measurements on synthetic polymers, derived from recent work by the author s group. 

The technique known as time correlated single photon counting is based on the probability that one (single) photon emitted by a luminescent sample could be detected by a highly sensitive photomultiplier. This probability is statistically bound with the change in the emitting excited state concentration with time by a specihc operative procedure. 

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