Linear photobleaching

In addition, for reasons that we cannot explain, the curve of Figure 3 is sharper than the theory of reference 13 would predict, and is sharper than would be obtained from a linear photobleaching system of the same initial optical density (28). Thus there appear to be significant advantages in this type of nonlinear transient bleaching, although more work is needed to understand it. 

The rate of photobleaching of unisotropic dye molecules in solid polymer matrices has been investigated by Kaminov et al. I65a) bleaching rate is linearly proportional to the intensiy of the incident radiation from an argon laser, indicating a one-photon process. 

A series of 3-alkyl- and 3-aryl-7/7-furo[3,2- ]-l-benzopyran-7-ones 78 (linear furocoumarins) was synthesized and evaluated for their photochemical and nonphotochemical crosslink formation with DNA as well as for their spectro-photometric and fluorescent properties, lipophilicity, and ability to photobleach A, A -dimethyl-/)-nitrosoaniline (RNO) after irradiation with UVA light <2002AP187>. The synthesis of the linear furocoumarins (Scheme 10) was a modification of a previously published method in which 7-hydroxy-2//-l-benzopyran-2-ones 76 were converted into / -ketoethers 77 by alkylation with haloketones under phase-transfer catalysis conditions. Base-catalyzed intramolecular condensation and subsequent acidification gave the corresponding 78. A molecular complex between each one of these fluorescent furocoumarins and DNA was observed, but only compounds with a 3-Me or 3-Ph group showed UVA irradiation-induced crosslink formation. 

Figure 7. Linear curves obtained from Figure 6 by linking together the reflectance values (Roo) from either the photoyellowing or photobleaching periods as a function of radiation time.

Upon UV irradiation, fulgide 35 is converted to compound 36, with a quantum yield of 0.20 in toluene at room temperature. It is wavelength independent between 313 and 366 nm. In contrast, the quantum yield of photobleaching of compound 36 was found to be temperature and wavelength dependent. The linear-dependent relation in toluene at 21°C is shown in Eq. 4.1. 

The direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

Up to now, most proposals for photobleaching image enhancement have relied on linear photochemistry, in which the transmittance is a function only of total dose, and not on the rate at which that dose is delivered. The kinetics of such linear photochemistry are well understood and have been described analytically (28). The exposure depends solely on a single parameter which is the product of extinction coefficient, quantum yield, intensity, and time. No increase in contrast can be obtained by changing extinction coefficient or quantum yield, since this merely scales the dose. Contrast can be increased only by increasing the initial optical density, which increases the dose requirement. Only with nonlinear (intensity dependent) photochemistry can one obtain steeper bleaching curves at a specified optical density. 

Figure 6. Spectral slope of CDOM from two lakes (Hargreaves, unpublished). S (nm ) is an exponential parameter from the relationship agjo, =ae The value of S can be computed as the absolute value of the slope when Ln(acd<, j) is plotted against wavelength over the UV and blue range. Such plots tend to be linear over UV wavelengths when DOC is high (upper curve) but can sometimes be separated into a steeper UV-B slope (280-320 nm) and shallower UV-A slope (320-380 nm) when substantial photobleaching has occurred (lower curve). These lake samples are from the upper mixed layer, June 2001 (particles removed with GF/F filter, Shimadzu UV-1601 spectrophotometer, 10 cm quartz cuvette, low DOC deionized water spectrum subtracted small glitch at 345-350 nm in lower curve is caused by spectrophotometer imperfection).

In another experiment, the irradiated sample was photobleached with IR radiation and then maintained at room temperature in the absence of oxygen until 90% of the radicals had decayed. If the sample was then re-irradiated, the maximum concentration of 4.2 x 1016 electron g 1 was restored after absorption of 3.0x1019 eVg-1. The decrease of the trapped electron concentration was thus assigned by Keyser et al. to a reaction of free radicals with trapped electrons. If the G value for alkyl radicals obtained by Waterman and Dole [214] for linear polyethylene at 77°K is used (G = 3.3), it can be calculated that the concentration of free radicals is 1.5 x 10-3 mole l-1 if the absorbed dose is 3 x 101 9 eV g-1. Scavenging of electrons in hydrocarbon glasses by biphenyl shows that, in this case, 1.5 x 10-3 mole l-1 biphenyl scavenges 50% of the ejected electrons [215]. 

The important point is that is proportional to the optical penetration depth, 5, but depends only logarithmically on the photosensitizer concentration, light fluence and threshold value. Hence, doubling any one of the latter factors does not double the effective treatment depth a 7-fold increase is required. This is a limitation, in that it makes it more difficult to extend the treatment depth, but conversely it means that these factors do not have to be known as accurately as they would if depended linearly on them. (Note that Equation (7) must be modified to include effects like photobleaching and oxygen depletion, but the essential conclusions remain valid.)... 

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