Gate time

Figure 3.14. Picosecond Kerr gated time-resolved resonance Raman (ps-K-TR ) spectra of the ICT state of DMABN (a), DMABN-N (b) and DMABN-dg (c) obtained by 267 nm pnmp, 330nm probe in methanol at 50ps delay time. (Reprinted with permission from reference [28]. Copyright (2001) American Chemical Society.)...

Temporal characteristics at early stages were elucidated by measuring fluorescence intensity with the gate time of 1.74 ns as a function of the delay time. Compared to the laser pulse, the time where the maximum intensity is attained shifts to the early stage as the laser fluence becomes high. Of course, we could not find out any decay component with intrinsic fluorescence lifetime of 17 and 35 ns. It is concluded that an Si - Si annihilation occurs quite efficiently during the pulse width. 

Figure 22 Time-resolved absorption spectra measured at 30,220, and 2000 nsec (gate time 30 nsec) after e during PR of DMP (5.0 x 10 M) in DCE.

Gating obviously can be used to fine-tune between selectivity and sensitivity On increasing the gating time from 0 to 100 ps, the normalized intensity [defined as (F-Fo)/Fo] of all species is increased (Fig. 16b) except for KG. On increasing the gating time to 250 ps, oxaloacetate is widely suppressed and citrate is reduced by 40%, while isocitrate, fumarate, and malate remain much less affected. Obviously, L-malate and oxaloacetate, and citrate and isocitrate can be nicely discerned via different delay times. 

Fig. 6. Left Deviation of the averaged beat note between the two frequency chains from the expected value for various counter gate times. Right Measured Allan standard deviation between the two chains as a function of the counter gate time...

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

In order to increase the resolution, a heterodyne circuit may be used, which consists of two oscillators the quartz crystal microbalance and a second reference crystal. The frequency difference in the kilohertz range can be measured with a universal counter and the same resolution and gate time discussed above are valid however if the period of the differential wave is measured, the accuracy can be highly increased and a 1 Hz resolution is easily attainable for a 1 ms gate time. This approach has been used by Sheng-li Chen et al. [19] to measure the kinetics of silver oxide formation on polycrystalline silver. These authors reported a resolution of 0.1 Hz in 1 ms which corresponds to 0.44 ng cm"2 in the mass sensitivity. 

Figure 13.30 Luminescence spectra of Eu-68 (50 JiM) at pH 7.4 (lOOmM HEPES buffer) upon addition of increasing amounts of Zn + (O-lO.Oequiv) with a delay time of 50p.s and a gate-time of 1.00 ms (Lex = 320mn). Inset the changes in luminescence intensity at Xem = 614mn [95]. (Reproduced with permission from K. Hanaoka et al, Development of a zinc ion-selective luminescent lanthanide chemosensor for biological applications, Journal of the American Chemical Society, 126,12470-12476, 2004. 2004 American Chemical Society.)...

Fig. 5. Threshold effect (facing page, top) and fluorescence spectra at different ,. Facing page, bottom decay of anthracene-like emission and build-up of product emission. This page fluorescence spectra at early and long gating times after the pulse. Note the evolution of the product emission with time. (A) All times, 1400 cm (B) all times,...

Here the averaging time is identical to the connter gate time. Becanse the dead time between connter readings was mnch larger than the inverse connter bandwidth jnxtapositioning of say Is gate time data to derive the AUan deviation for longer times wonld prodnce false resnlts [43]. 

Fig. 23. Gated time-resolved fluorescence spectra of atactic poly (styrene) in dichloromethane solution, (a) Early gated spectrum, delay St = 0 ns, gate width St = 3 ns. (b) Late gated spectrum, St =45 ns, St = 3 ns...

It will be noted that the kinetic scheme adapted above reverse dissociation of excimer sites, the experimental evidence for this being the triple exponential decay of the monomer fluorescence and the gated time-resolved fluorescence spectra, reported here and seen earlier " in the homopolymer. 

Figure 9. Fluorescence spectra and decay characteristics of POS containing polystyrene. M, styrene monomer region, dual decay kinetics. D, styrene excimer region, triple decay characteristics (double fit shovm does not correlate with other wave lengths, thus meaningless). P is POS fluorescence, triple decay characteristics when styrene excited (see box), but single, t = 1.68 ns when excited directly. EGS is early-gated time-resolved spectrum which matches closely spectrum of P excited directly, and difference between late-gated spectrum LGS and known spectrum of D. ...

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