Pulsed LEDs

If the width of the excitation flash (nowadays typically from a pulsed LED or laser diode) is not shorter than approximately 1/3 of the fluorescence lifetime to be measured, signal deconvolution should be used to extract meaningful values from the kinetic data fit. 

Excitation of TPE in hexane with a 305-nm, 0.5-ps laser pulse led to the appearance of absorption bands at 423 and 650 nm during the time duration of the excitation pulse. The 630-nm band decays and the 423-nm band shifts to 417 nm with a lifetime of 5 1 ps, and the 423-nm band shifts to 417 nm on this time scale. The 417-nm absorption then decays with a lifetime of 3.0 0.5 ns. The 630-and 417-nm bands were assigned to an electronic transition from the vertical singlet excited state (5iy) and to an electronic transition from the nonfluorescent S p, respectively. ... 

An oxygen sensor array using these principles was developed by Liebsch et al. [60]. A modification of the applied imaging set up is illustrated in Fig. 7. It consists of a CCD camera with a fast mechanical shutter, an array of 96 pulsed LEDs, a computer-steered pulse generator, a constant power supply, a light-guiding adapter with 96 optical fibers, and the corresponding excitation and emission filters. 

Information regarding the solution conformation of 13 was derived from the pyropheophorbide ring current induced shifts in the resonance positions of the carotenoid and quinone moieties. These two species were found to be extended away from the tetrapyrrole, rather than folded back across it. The absorption spectrum of 13 was essentially identical to the sum of the spectra of model compounds. The pyropheophorbide fluorescence, however, was strongly quenched by the addition of the quinone. This implies the formation of a C-Phe -Q state via photoinitiated electron transfer from the pyropheophorbide singlet state, as was observed for C-P-Q triads (see Figure 4). Excitation of the molecule in dichloromethane solution at 207 K with a 590 nm laser pulse led to the observation of a carotenoid radical cation transient absorption. Thus, the C-Phe -Q " state can go on via an electron transfer step analogous to step 4 in Figure 4 to yield a final C -Phe-Q state. This state had a lifetime of 120 ns. The quantum yield at 207 K was 0.04. At ambient temperatures, the lifetime of the carotenoid radical cation dropped to about SO ns, and the quantum yield could not be determined accurately because of the convolution of the decay into the instrument response function. 

Excitation using high-intensity 532 nm laser pulses led to photoionization of these dyes [67]. The transient absorption recorded following the laser flash excitation of 3 is shown in Fig. 5. The linear dependence of the absorption at... 

In contrast to the large pulse-to-pulse variation of a Xe flash lamp, the energy of a pulsed LED is very stable. This might make the correction circuitry of the excitation intensity unnecessary, which would lower the complexity and cost of the instrumentation even more. 

For quadnipolar nuclei, the dependence of the pulse response on Vq/v has led to the development of quadnipolar nutation, which is a two-dimensional (2D) NMR experiment. The principle of 2D experiments is that a series of FIDs are acquired as a fimction of a second time parameter (e.g. here the pulse lengdi applied). A double Fourier transfomiation can then be carried out to give a 2D data set (FI, F2). For quadnipolar nuclei while the pulse is on the experiment is effectively being carried out at low field with the spin states detemiined by the quadnipolar interaction. In the limits Vq v the pulse response lies at v and... 

As one important example, the introduction of the prism-controlled, colliding-pulse, mode-locked (CPM) dye laser [12,13] led almost innnediately to developments in measurement teclmique with pulses of less than 100... 

However unlike H which is the most abundant of the hydrogen isotopes (99 985%) only 1 1% of the carbon atoms m a sample are Moreover the intensity of the signal produced by nuclei is far weaker than the signal produced by the same number of H nuclei In order for NMR to be a useful technique in structure deter mination a vast increase in the signal to noise ratio is required Pulsed FT NMR pro vides for this and its development was the critical breakthrough that led to NMR becoming the routine tool that it is today... 

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

Romanchenko and Stepanov (1981) recognized that the impulse imparted at the spall plane to material downstream, because of the elastic-plastic nature of this material, led to an attenuating tensile stress pulse propagating toward the sample-window interface as is illustrated in Fig. 8.9. Thus, the maximum tension inferred from the measured spall signal should be adjusted for this attenuation in estimating a material spall strength at the spall plane. 

The carbon-containing catalyst was treated by ultra-sound (US) in acetone at different conditions. The power of US treatment, and the time and regime (constant or pulsed), were varied. Even the weakest treatments made it possible to extract the nanotubules from the catalyst. With the increase of the time and the power of treatment the amount of extracted carbon increased. However, we noticed limitations of this method of purification. The quantity of carbon species separated from the substrate was no more than 10% from all deposited carbon after the most powerful treatment. Moreover, the increase of power led to the partial destruction of silica grains, which were then extracted with the tubules. As a result, even in the optimal conditions the final product was never completely free of silica (Fig. 12). 

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

The development of pulse radiolysis techniques have led to the determination of a number of the important kinetic processes of the species produced by the irradiation of H2O. The results that have been obtained for a number of the most important reactions are presented in Table II. These results demonstrate that the net effect of radiation is H2O decomposition in the absence of any reactive substrate. 

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