Transit time spread measurement

A number of typical detectors are described under Sect. 6.4, page 242. The main selection criteria are the transit-time spread and the spectral sensitivity. Together with the laser pulse shape, the transit-time spread determines the instrument response function (IRF). As a rule of thumb, lifetimes down to the FWHM of the IRF can be measured without noticeable loss in accuracy. For shorter lifetimes the accuracy degrades. However, single-exponential lifetimes down to 10% of the IRF width are well detectable. Medium speed detectors, such as the R5600 and R7400 miniature PMTs, yield an IRF width of 150 to 200 ps. The same speed is achieved by the photosensor modules bases on these PMTs (see Fig. 6.40, page 250). 

It should be noted that the IRF recorded in the cuvette is not identical with the laser pulse recorded directly. The horizontal path length of the laser in a 10-mm cuvette adds about 45 ps of transit-time spread to the IRF. If a large beam diameter is used, there are also path length differences for different depth in the cuvette. Moreover, reflection at the cuvette walls and scattering at the sample holder contributes to the IRF. However, these effects are essentially the same for IRF measurement by a dilute scattering solution and fluorescence measurement of a trans-... 

The results show that the full resolution of a fast TCSPC system cannot be exploited for measurements in the eommonly used 10 mm cuvettes. The optical transit-time spread can be reduced by recording only from a small spot in the cuvette. Another solution is a thin cuvette under front illumination. 

The electrons emitted by the photocathode are subsequently accelerated to 50 kV and focused on to a toroid-shaped anode. The anode is made of oxygen-free, high conductivity copper and is maintained at a high positive potential. The electron pulses interact with the copper anode forcing the emission of Cu-Ka x-ray photon pulses, which exit the vacuum chamber through a thin beryllium-foil window. A bend germanium crystal monochromator disperses and focuses the x-rays onto the sample. The duration of the x-ray pulses is measured by a Kentech x-ray streak camera fitted with a low density Csl photocathode. The pulse width of the x-rays at 50 kV anode-cathode potential difference is about 50 ps. This value is an upper limit for the width of the x-ray pulses because the transit time-spread of the streak camera has to be taken into consideration. A gold photocathode (100 A Au on 1000 A peiylene) is used to record the 266-nm excitation laser pulses. The intensity of the x-rays is 6.2 x 10 photons an r (per pulse), and is measured by means of a silicon diode array x-ray detector which has a known quantum efficiency of 0.79 for 8 kV photons. 

For steady-state measurements, the time response of a PMT is not important. However, some understancUng of the time response of PMTs is essential for an understanding of their use in lifetime measurements. There are three main timing characteristics of PMTs the transit time, the rise time, and the transit time spread (Figure 2.29). The transit time of a PMT is the time interval between the arrival of a photon at the caftiode and the arrival of the amplified pulse at the anode. Typical transit times are near... 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

Equation (4.4) has the qualitative interpretation that Ef evolves on the ground state surface until time t = t2 when it makes a transition to the excited electronic surface. It then propagates for a time t2 to tx at which time it makes a transition back to the ground electronic state. The wave function then evolves on the groimd state surface until time t when we measure its overlap with the product state. Since the fields are spread out over time, we have to integrate over both /, and t2. 

On the other hand, despite the information about long chain sulfates, sulfonates, phosphates, and carboxylates that indicates stronger interaction with Ca2+ than with Mg2+ (i.e., in apparent harmony with the sequence of the Hofmeister (44) series), several difficulties remain. For example, while Miyamoto s data for DS (10) indicate the interaction sequence Mg < Ca < Sr < Ba from solubility measurements (as well as from temperature/CMC measurements if one accepts the Mg—Ca sequence of the present paper), this sequence, with the exception of the position of Mg and Ca, is the opposite of that found by Deamer et al. (33) from condensation effects on the force/area curves of ionized fatty acids. At the same time, the ion sequence obtained by these authors from phase transition temperatures of spread fatty acids (33) differs from that deduced from the above-mentioned condensation effects, and the latter depended strongly on pH. Lastly, definite differences in ion sequence effects exist for the alkaline earth metals in their interaction with long... 

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