Irreversible bleaching

The charge transfer kinetics in the above example is a diffusion-controlled process and is completed within 100 p,s following the laser pulse excitation. The two traces shown in Fig. 2B can thus be utilized to monitor the rate of reaction between trapped electrons and dye molecules in solution. The quantum efficiency for the photocatalytic reduction of these two dyes as measured from the magnitude of irreversible bleaching was -4.5%. 

Figure 2B Absorption-time profile showing the irreversible bleaching of the azo dyes. Left Acid Orange 7. Right Direct Blue 1 recorded at monitoring wavelengths 485 and 620 nm, respectively. The traces were recorded following 308-nm excitation of W03 colloids in aqueous dye solution. (From Ref. 99.)...

Intensity dependent bleaching of poly(methylmethacrylate) doped with acridine or diphenyl butadiene and an acridine-containing methacrylate copolymer, has been observed using excimer lasers (XeF, KrF). Threshold-like irreversible bleaching was seen with the copolymer more gradual transient bleaching with the mixtures. The implications for submicron microlithography are discussed. 

A large number of papers has appeared on the subject of excimer laser exposure of polymer films (16-21). Most of these have dealt with the phenomenon of photoablation. A few have observed intensity dependent photochemistry (22,23). The latter authors were concerned with the effect of exposure intensity on resist development characteristics. The utility of nonlinear photochemistry for image modification has not been explored except in our earlier communication, in which strongly nonlinear irreversible bleaching was observed for KrF laser irradiation of acridine/PMMA films with lOnsec pulses (5). 

We have not yet explored the mechanism of the irreversible bleaching in any detail. Some film removal is visually evident. 

Illumination under the donor-limited condition, either in the absence or presence of oxygen, produces absorbance changes quite different from those under the acceptor-limited condition. There is, e.g., a prominent irreversible bleaching of (3-carotene and the accessory chlorophyll absorbing at 670 nm [see Fig. 4 (B)]. As a result of the oxidation of d> by the secondary acceptor SiMo, the lifetime of P680 is greatly increased and thus intensifying its destructive effect. Of course, this donor-limited photoinhibition can be readily prevented if an artificial secondary electron donor such as DPC or Mn is also added. 

A EXPERIMENTAL FIGURE 5-6 Fluorescence recovery after photobleaching (FRAP) experiments can quantify the lateral movement of proteins and lipids within the plasma membrane, (a) Experimental protocol. Step H Cells are first labeled with a fluorescent reagent that binds uniformly to a specific membrane lipid or protein. Step B A laser light is then focused on a small area of the surface, irreversibly bleaching the bound reagent and thus reducing the fluorescence in the illuminated area. Step B In time, the fluorescence of the bleached patch increases as unbleached fluorescent surface molecules diffuse into it and bleached ones diffuse outward. The extent of recovery of fluorescence in the bleached patch is... 

Abeles and co-workers have studied the inactivation reaction of 3-dimethyl-amino-1-propyne with bovine liver MAO-A (93). Again, the inactivation reaction was found to be stoichiometric and irreversible, with covalent modification of the flavin cofactor suggested by irreversible bleaching of the flavin spectrum. Based on the chemical and spectral properties and pK of the enzymically de-... 

Telfer A, De Las Rivas J, and Barber J (1991) j5-Carotene within the isolated Photosystem II reaction centre photooxidation and irreversible bleaching of this chromophore by oxidized P680. Biochim Biophys Acta 1060 106-114 Telfer A, Dhami S, Bishop SM, Phillips D and Barber J (1994)/3-carotene quenches singlet oxygen formed by isolated Photosystem II reaction centers. Biochemistry 33 14469-14474... 

The redox potential of the medium may change in the relatively anaerobic growth conditions of many cultures. This can cause irreversible bleaching of some indicators. 

The e.p.r. signal of pj also decreases with the time of illumination at room temperature. After the total P755 band is irreversibly bleached, no e.p.r. signal is detected, in accord with the production of BPh dianions. 

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