Transient absorbance

Both the jackshaft and spindle are designed to absorb transient increases or decreases in torsional power caused by twisting. In effect, the shaft or tube used in these designs winds, much like a spring, as the torsional power increases. Normally, this torque and the resultant twist of the spindle are maintained until the torsional load is... 

Figure 18.3. Time evolution of the photomultiplier output as an absorbing transient is generated and decays.

Figure 3 shows representative single-wavelength absorbance transients for three dyes electrostatically bound to colloidal Sn02. The transients correspond to photoinitiated bleaching and recovery of the respective MLCT absorbances. From Fig. 3, it is clear that (1) injection is rapid in comparison to backET, (2) back ET is complex kinetically, but (3) the complex recovery rates depend strongly on the identity of the dye, at least in the first few hundred nanoseconds of the recovery. In order to isolate the shorter-time recovery kinetics, transients were fit, somewhat pragmatically, to a bi-exponential decay function ... 

The observation (112-115) that neutral aqueous solutions of [Ni11H 3G4]2 consume molecular oxygen with the appearance of a strongly absorbing transient at 350 nm lead to detailed investigations and discovery of nickel(III)-peptide complexes (113). The oxidized nickel complexes have absorption maxima around 325 and 240 nm (e = 5240 and 11,000 M I cm-1, respectively for [NiM1H 3G4]-). Reduction potentials (116) (Table II), measured by cyclic voltammetry, show a small dependence on ligand structure which can be correlated... 

It was also found that the electrical resistivity of ceramics based on silicon carbide, and, more recently, zinc oxide could be made sensitive to the applied field strength. This has allowed the development of components that absorb transient surges in power lines and suppress sparking between relay contacts. The non-linearity in resistivity is now known to arise because of potential barriers between the crystals in the ceramic. 

Some triplet photosensitizers also undergo electron transfer with iodo-nium and sulfonium salts. Manivannan et al. [104] have demonstrated that ketocoumarins sensitize photolysis of diphenyliodonium salts exclusively from their triplet states. An electron transfer mechanism was inferred from observation of a light absorbing transient assignable to the ketocoumarin radical cation, on laser flash photolysis of a ketocoumarin onium salt mixture in methanol. 

For electrolyses involving time scales shorter than about 500 /is, the diffusion layer is of the same order as S, and the absorbance is sensitive to the evolving concentration profile of R (6, 46, 47). The resulting optical transients can be useful for characterizing rather fast electrochemical processes, which are otherwise complicated severely by nonfaradaic contributions to current and charge functions. Theoretical absorbance transients can be computed from (17.1.13), once the diffusion-kinetic equations defining the concentration profile of R have been solved, either analytically or by numeric methods such as digital simulation. 

Additional work soon discredited this postulate. The 386 nm transient could not be detected in other solvents. No transient was detected upon LFP of 10 in benzene or hexafluorobenzene. In cyclohexane, a transient with X = 367 nm was formed 144 ns after the laser pulse. Only in nitrile solvents were transient absorption bands observed near 400 nm. The definitive experiment was performed by Linda Fladel who demonstrated that the pseudo-first order rate constant of formation of the 386 nm absorbing transient was linearly dependent on the concentration of acetonitrile. This proved that the transient is formed by reaction of an invisible species which reacts with acetonitrile. The observable transient was ylide 12 formed by capture of 11. The same transient was formed by LFP of azirine 13. 

Spectral Identity of the Seawater Transient. Initial survey work showed a single transient in the UV region, with no other detectable absorptions in the range 300-700 nm. Figure 1 compares the spectra of the UV-absorbing transients formed in seawater under various conditions with one another and with the literature spectra of dihalide ion-radicals. All spectra have been divided by that of authentic dibromide ion-radical and normalized to 1.00 at 360 nm. With the exception of the 390 nm point at 100 vb in nitrous oxide-saturated seawater, the data are consistent with the dlbromide ion-radical rather than the other species. Similar comparisons (not shown) of the UV transient generated in seawater using pulse radiolysis lead to the same conclusion. 

If weakly-absorbing transients not detectable by optical filter tests are formed and react in our system, they should be detectable by their subsequent reactions with Br2. However, in C02-free seawater Br2 showed a clean self-decay reaction and negligible first-order decay constants (Table VI) as summarized for the second-order component in Figure 2. 

Detection and Characterization of a 475 nm Absorbing Transient In Humic Substance Extracts (HSX) and Concentrated Natural Waters (CNI. ... 

Nanosecond absorbance transients were measured with a single beam instrument (11). Excitation was provided by a 6-8 ns, 1 mJ 610 nm pulse from a NdrYAG pumped rhodamine dye laser. Absorption transients were either detected with a Hamamatsu R928 (A<900 nm) or R406 (A>900 nm) photomultiplier operating with a 2.5 ns response time. Picosecond absorbance transients were measured with a double beam apparatus (11). 1.5 ps, 1 mJ, 610 nm excitation pulses were generated by using the output of a mode-locked Ar" laser to synchronously pump a rhodamine dye laser. 

Transient states generated from PH Hj. Absorbance transients generated from the P%Hm state provide a calibration for the extent of the absorbance changes which can be expected for a charge separation with a quantum yield of 1. Fig. 1 shows transient spectra in the 760 to 900 nm region recorded with 1.5 ps time resolution from the PHlHj Q state at 90 K. The multichannel detection was not sensitive beyond 900 nm. 

These experiments find that absorbance transients are detected on the ns time scale from reaction centers in the PHl Hj state. Significantly, a small amount of triplet P is formed. The spectrum of the precursor to the P state differs from that associated with the P Hl state. The extent of the P990 nm bleaching in the precursor state corresponds to a quantum yield of 0.09 . 06 compared to normal photochemistry. This yield agrees with that measured in room temperature photochemical trapping of and supports the assignment of the precursor state as P Hj ". Since in these experiments electron transfer to is blocked by its pre-reduction, electron transfer to must only compete with the decay of P. The lifetime of P has been measured to be 20 ps at room temperature (10) under conditions in which both bacteriopheophytins are reduced (11). By taking 20 ps as an estimate of the decay time P at 90 K, the measured yield of 0.09 for the state corresponds to an electron transfer time... 

Flash-induced absorbance transients were detected at both 820 and 700 nm. Saturating activation flashes, 8-9 ns FWHH, were provided by a Q-switched, frequency doubled Nd YAG laser. 

Figure 2 shows flash-induced absorbance transients due to Pyoo - The decay halftime in the control was ca 30 ms and assumed to reflect recombination between PTOO" and F Fb" ( ) After UV-irradiation and depletion of phylloquinone the optical transient still showed a slow recombination in confirmation of (9) but the decay halftime was longer. This behavior is different from that in solvent-extracted preparations where a fast recombination was observed between Pyoo" 0 (6) We suggest that UV-irradiation results in extensive modification of PSI and that the optical flash transients do not support the conclusion (9) that PSI electron transfer function is unimpaired following such treatment. 

This work attempts a systematic exploration of the role of FNR in thylakoid membranes. Flash induced absorbance transients linked to the electron transfer processes involving FNR have been identified in thylakoid membranes between 300 and 590 nm and its kinetics studied in the time range from about 20 us to a few sec at various experimental conditions. In this paper our attention is focussed on the mechanism of photoreduction of FNR by PSl. The absorbance transient spectra of FNR , the neutral singly reduced form, was identified between 430 and 550 nm and the kinetics of the absorbance changes were studied in membrane preparations of different FNR and Fd content. Our results show that photoreduction of FNR to FNR by PSl does not require the presence of soluble or membrane-bound Fd. 

FIGURE 1. Gramicidin insensitive absorbance transients in thylakoid membranes at 515, 530 and 545 nm average of 32 transients. 

The absorbance transient can be unequivocally assigned to PSl, since it persists in the presence of DCMU under anaerobic conditions favorable for cyclic electron transport around PSl (11) or in the presence of DCMU and 5-10 mM ascorbate (data not shown). 

Absorbance transients have been observed in isolated thylakoids which could be identified to originate from the reduction of FNR to the semireduced FNR by PSl. Generation of FNR appears to occur unperturbed in the absence of Fd. Role of Fd and the Fd FNR complex, its dynamic properties under various experimental conditions as well as the role of ferredoxin quinone reductase (FQR) are currently investigated. 

Absorbance Transients of Ferredoxin NADP+ Reductase in Isolated Thylakoid Membranes 667... 

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