Picosecond Fluorescence Lifetimes
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MEASURING PICOSECOND FLUORESCENCE LIFETIMES Using a Chromacity 520 in line with an Edinburgh Instruments FLS1000 Equipped with a Hybrid Photodetector Experimental Configuration The measurement of fluorescence lifetimes provides a wealth of information on the sample under study, from charge-carrier lifetimes in semiconductors to the local environment of biomolecules in living cells. In many of these studies, short fluorescence lifetimes on a picosecond timescale are required to be measured. Fluorescent lifetimes in the FLS1000 are measured using the time-correlated single photon counting (TCPSC) technique. In an ideal TCSPC system, the temporal width of the excitation pulse and the temporal response of the detection system would both be infinitely sharp (delta functions). However, this is never the case since lasers have finite pulse widths and detectors have transit response times. The deviation from the ideal is characterised by the instrument response function (IRF) of the TCSPC system. The IRF can be thought of as temporal response that the TCSPC system records for an infinitely short fluorescence lifetime. In this application note, the measurement of sub 20 picosecond lifetimes is achieved by combining a Chromacity 5202 , ultrashort pulse laser source with an Edinburgh Instruments FLS1000 Photoluminescence Spectrometer equipped with a hybrid photodetector. FIGURE 1. The Chromacity 520 was coupled into the FLS1000 Photoluminesce Spectrometer to narrow the IRF and drive performance. Materials The hemicyanine dye trans-4-[4-(dimethylamino)styryl]-1-methyl-pyridiniumiodide (4-DASPI) was chosen to demonstrate the measurement of short lifetimes using the FLS1000. DASPI dyes are commonly used as short lifetime fluorescence probes and as a laser gain media. 4‑DASPI was purchased from Sigma Aldrich® and dissolved in either anhydrous ethanol or distilled water. H 3C The measured fluorescence decay is convoluted with the IRF, and the Full Width Half Maximum also known as the temporal width of IRF (FWHMIRF) determines the minimum lifetime that can be successfully measured. A useful rule of thumb is that the minimum fluorescence lifetime that can be determined through reconvolution analysis is around 1/10th of the FWHMIRF. It is therefore important to minimise the FWHMIRF when measuring short fluorescence lifetimes. The FLS1000 is a fully modular spectrometer and can be configured to suit different research requirements. By changing the configuration of the FLS1000 or the light source, the width of the IRF can be adjusted to meet the minimum measurable fluorescence lifetime required in different research applications. In this application note, the measurement of sub 20 picosecond lifetimes is achieved by combining a chromacity 520, ultrashort pulse laser source with an Edinburgh Instruments fls1000 photoluminescence spectrometer equipped with a hybrid photodetector. FIGURE 2. Molecular structure of 4-DASPI Discover More chromacitylasers.com sales@chromacitylasers.com Version 1 © Chromacity Ltd - 2

MEASURING PICOSECOND FLUORESCENCE LIFETIMES Using a Chromacity 520 in line with an Edinburgh Instruments FLS1000 Equipped with a Hybrid Photodetector 2 3 2 FWHMIRF = FWHM2 detector + FWHM laser + FWHM dispersion + FWHM electronics FIGURE 3. There are four main components which combine to give the total width of the IRF. For the measurement of 4‑DASPI in the following section, the 300 gr/mm lifetime grating was used. To lower the temporal dispersion even further a subtractive double monochromator could be used, as found in the Lifespec II Spectrometer, which has zero temporal dispersion....

MEASURING PICOSECOND FLUORESCENCE LIFETIMES Using a Chromacity 520 in line with an Edinburgh Instruments FLS1000 Equipped with a Hybrid Photodetector Results Intensity (counts) It is known that when 4-DASPI is dissolved in water its fluorescence lifetime decreases fivefold,1 and a solution of 4‑DASPI in water was therefore also measured to provide a more challenging sample to test the short lifetime capabilities of the FLS1000. The fluorescence decay and fit of 4-DASPI in water are shown in Figure 5b revealing a lifetime of 11 ps which is in perfect agreement to the 11 ps lifetime...