Dark decay

The abihty to accept and hold the electrostatic charge in the darkness. The photoconductive layer should support a surface charge density of approximately 0.5-2 x 10 C/cm. The charge also has to be uniformly distributed along the surface, otherwise nonuniformities can print out as spot defects. The appHed surface potential should be retained on the photoreceptor until the time when the latent electrostatic image is developed and transferred to paper or, if needed, to an intermediate belt or dmm. In other words, the "dark decay" or conductivity in the dark must be very low. The photoconductor materials must be insulators in the dark. 

Dark Decay of UDMH in Air, UDMH was observed to undergo a gradual dark decay in the 30,000-liter Teflon chamber at a rate which depended on humidity. Specifically, at 41 C and 4% RH the observed UDMH half-life was " 9 hours (initial UDMH 4.4 ppm) and at 40 C and 15% RH, the half-life was -6 hours (initial UDMH 2.5 ppm). The only observed product of the UDMH dark decay was NH3, which accounted for only -5-10% of the UDMH lost. In particular, no nitrosamine, nitramine, or hydrazone were observed. Formaldehyde dimethyIhydrazone was observed in previous studies which employed higher UDMH concentrations and reaction vessels with relatively high surface/volume ratios (, ) ... 

The mechanism of the UDMH dark decay is unknown, but it is presumed to be heterogeneous in nature. It is probably not wall adsorption, since much slower decay rates were observed previously in the absence of O2 ( ) ... 

Dark Decay of UDMH in the Presence of NO, When 1.3 ppm of UDMH in air was reacted in the dark with an approximately equal amount of NO, 0.25 ppm of UDMH was consumed and formation of -0.16 ppm HONO and -0.07 ppm N2O was observed after -3 hours. Throughout the reaction, a broad infrared absorption at -988 cm" corresponding to an unidentified product(s), progressively grew in intensity. The residual infrared spectrum of the unknown product(s) is shown in Figure 2a. It is possible that a very small amount (50.03 ppm) of N-nitrosodimethylamine could also have been formed but the interference by the absorptions of the unknown product(s) made nitrosamine (as well as nitramine) detection difficult. No significant increase in NH3 levels was observed, in contrast to the UDMH dark decay in the absence of NO. Approximately 70% of the UDMH remained at the end of the 3-hour reaction period this corresponds to a half-life of -9 hours which is essentially the same decay rate as that observed in the absence of NO. 

As with the UDMH-air dark decay, the UDMH-NO-air decay reactions are unknown and probably heterogeneous in nature. The fact that the presence of NO does not significantly change the UDMH decay rate but changes the products formed suggests that the initiation reaction is the same in both cases, but that the NO reacts with the intermediates formed. Additional investigation is required to characterize this process. 

The use of short (fs) laser pulses allows even highly transient ion-radical pairs with lifetimes of t 10 12 s to be detected, and their subsequent (dark) decay to products is temporally monitored through the sequential spectral changes. As such, time-resolved (ps) spectroscopy provides the technique of choice for establishing the viability of the electron-transfer paradigm. This photochemical (ET) mechanism has been demonstrated for a variety of donor-acceptor interactions, as presented in the foregoing section. 

During storage time in the dark there is a small loss of voltage, AV, (dark decay or discharge)... 

Most phthalocyanines show inferior dark decay to azo pigments, an exception being the titanyl phthalocyanines, where in the best cases, e.g. the a- and Y- forms, they show comparable dark decay coupled with superior photosensitivity. A comparison of the sensitivity of azo pigments and selected polymorphs of phthalocyanines is shown graphically in Figure 4.12. 

Ten Brink, H. M., and H. Spoelstra, The Dark Decay of HONO in Environmental (Smog) Chambers, Atmos. Environ., 32, 247-251 (1998). 

Analysis of the time and temperature dependent decay of the surface voltage on an amorphous film after charging, but prior to exposure (xerographic dark decay), and of residual decay after exposure can (in combination) be used to map the density of states. 

Dark discharge rate must be sufficiently low to maintain an ample amount of charge on the photoreceptor during the exposure and development steps. A high dark decay rate will limit the available contrast potential. The residual potential remaining after the xerographic cycle must be small enough that it does not impair the quality... 

Figure 5.5 Typical photoreceptor behavior through xerographic cycles showing dark decay, first-cycle residual potential V i, and cycled-up residual potential after many cycles [2].

The origin of the deep localized states in the mobility gap that control the dark decay has been attributed to structural native thermodynamic defects [12]. Thermal cycling experiments show that the response of the depletion time to temperature steps is retarded, as would be expected when the structure relaxes toward its metastable liquid-like equilibrium state. As the structure relaxes toward the equilibrium state, t(j decreases further until the structure has reached equilibrium. The only possible inference is that must be controlled by structure-related thermodynamic defects. The generation of such defects is, therefore, thermally activated. We should note that because the depletion discharge mechanism involves the thermal emission of carriers... 

Returning to the depletion discharge, it should be stressed that with thick films and a good blocking contact between a-Se and the preoxidized aluminum substrate, the depletion discharge process dominates. There are several reasons that amorphous selenium possesses good dark decay characteristics ... 

Typical dark discharge characteristics for pure Se and Sb Sei photoreceptors are shown in Fig. 7.1 for compositions noted in the figure. It is apparent that for pure a-Se, the decay of the surface potential is relatively slow. Comparison of the respective characteristics for a-Sb -Sci with the dark discharge behavior of pure a-Se shows clearly that alloying a-Se with antimony increases the dark-decay rate. The discharge rates in a-Sb -Sei were not constant but decreased with time. 

There are several physical processes that can lead to the decay of the surface potential. The currently accepted model for the dark decay in a-Se-based films involves [3,14,15] ... 

In a series of experiments carried out on a composition series of glassy Sb cSei- c alloys, it was found that the time-dependent dark-decay rate of the potential to which... 

It is found that in a-Sb cSei- c alloys, electrons (the mobile carrier species) are depleted (n-type system) during dark decay, leaving behind a deeply trapped positive space charge. Note that the same situation prevails in alkali-doped a-Se [16]. 

Figure 7.3 shows a dark-decay curve and a PIDC for the a-Sbo.osSeo.gT film. It can be seen that the sample exhibits relatively little dark decay. Nevertheless, in order to... 

Figure 73 Dark-decay curve (solid circles) and photoinduced discharge curve (open circles) for Sb0.03Se0.97-...

Fig. 5. Light-induced formation and dark decay of P7001 in the reaction centre of the photosystem 1 of subchloroplasts at T < 294 K [45], The arrows indicate the moments of switching the light on ( ) and off (t).

Fig. 8. Dark decay [45] of P700 1 in the reaction centre of photosystem 1 of subchloroplasts at T < 240K.

Metal chelates of 8-hydroxyquinoline such as (111) with photoconductive properties are reported to be useful in electrophotographic systems.233 The incorporation of a tin complex into a photo-conductive zinc oxide layer is stated to reduce dark decay . In other words, the electrostatic charge applied to the photoconductor has a longer lifetime. Two of the complexes disclosed for this application are (112) and (113). These compounds are prepared from dibutyltin oxide by reaction with 2-mercaptopropionic add and thioglycolic acid, respectively 234... 

The analysis of the curves of e, photobleaehing in the presence of a number of acceptors by means of Eq. (30) made it possible to determine the ratios VJx (Table 2). From Eq. (30) these ratios are seen to serve as effective rate constants for photobleaehing processes. Knowing the values of ve and ae from the experiments on the dark decay of e,7 by reactions with the same acceptors (see Ref. [31]), one can further determine from these ratios the time t. Since x is independent of the kind of acceptor, upon determining x in a given matrix for one of the acceptors, one can calculate the values of V, for other acceptors from the photobleaehing curves provided the bleaching conditions are identical. 

After [9], numerous papers were published related to the low-temperature photooxidation of P700 in the PS1 reaction centres of plants and the reverse process of recombination of P700+ and a reduced electron acceptor (see e.g. Refs. [208-211]). In these works controversial data on the kinetics of the dark decay of P700 + were obtained. Therefore in Ref. [212] the kinetics of charge recombination in the PS1 reaction centres was investigated in detail over a broad range of times and temperatures. 

Fig. 28. Dark decay [212] of P700+ in the reaction centre of photosystem 1 of subchloroplasts at T < 240 K (a) and at T St 240 K (b). Proposed schematic structures of the reaction centre at low and high temperatures are shown at the bottom of the Fig. (c)...

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