THE AVAILABILITY OF SOLAR RADIATION BELOW 290 nm AND ITS IMPORTANCE IN PHOTOMODIFICATION OF POLYMERS

Abstract— On a percentage basis, ozone is a very minor component of the atmosphere; at STP it would make a layer only about 2 mm thick. On almost every other basis (biological, meteorological, paleontological, photochemical, etc.) it is a major component, due mainly to the tremendous reduction in solar ultraviolet flux which it causes in the 220–290 nm region. Since no data are available for Λ < 285 nm, a rational basis for estimating the flux reaching the earth's surface in this region is discussed. Variations in ozone concentration are of great importance, and it is possible to have more radiation with Λ < 270 nm fall on a surface in one extreme day than in several years of typical days. Often, persons involved in studies of polymer degradation by sunlight mention that a negligible fraction (1 ppm) of the radiant flux reaching the earth's surface is associated with wavelengths below 290 nm and infer that studies at shorter wavelengths will not be of much practical value. Such inferences are questionable for at least two reasons. (1) The quantum flux density below 290 nm is about 10¹⁶ photons cm^-2 month^-1, so that considerable long-term damage is possible since most of the flux will be absorbed in a layer only a few microns thick. (2) Even if solar radiation below 290 nm were completely absent, the existence of correlations between absorption peaks in the near ultraviolet and visible, and in the infrared with ionization potentials typically 6–12 eV or 200-100 nm) is evidence that we may expect studies in the ultraviolet and extreme ultraviolet (EUV) to provide important clues to the problem of improving the resistance of polymers to sunlight.     

EFFECT OF ULTRAVIOLET RADIATION ON THE ENERGY METABOLISM OF THE CORNEAL EPITHELIUM OF THE RABBIT

The present research was directed at quantifying possible alterations in corneal epithelial metabolic activity secondary to in vivo exposure to ultraviolet radiation (UVR). Microfluorometric energy metabolite assays on microgram (microgram) sized, freeze-dried tissue samples were used as an in vitro means of assessing overall metabolic activity in the epithelium of control rabbit corneas and in the epithelium of UVR-exposed rabbit corneas 2 min after discontinuation of exposure. The specific assays were for glucose, glycogen, adenosine triphosphate (ATP), and phosphocreatine (PCr). The radiant exposures were kept constant at 0.05 J cm-2 for all UVR wavelengths utilized (290, 300, 310 and 360 nm). Experimental UVR exposure conditions served to increase epithelial glucose and glycogen concentrations. Although the epithelial ATP concentrations were unchanged, the epithelial PCr concentrations (a high energy phosphate bond reservoir) decreased as a result of UVR exposure. Overall, the data demonstrate a decrease in corneal epithelial metabolic activity, which may be wavelength-dependent, as a result of UVR exposure. It is suggested that immediate metabolic stress can be responsible for the pattern of epithelial cell loss seen in photokeratitis.     

CHANGES IN TOTAL DIFFUSE SPECTRAL REFLECTANCE OF AMINO ACIDS AND PROTEINS AFTER ULTRAVIOLET IRRADIATION

Abstract— Total diffuse reflectance spectra of air-dried surfaces of free and neutralized amino acid preparations before and after irradiation in vitro are reported. It was found that some free or neutralized amino acid surfaces underwent modification which resulted in changes in the diffuse reflectance spectra after U.V. exposure. It is suggested that these reflectance changes result from transformations in the side chains of the amino acids and that the transformations may differ from those occurring when amino acids in solution are irradiated. Histidine, cystine, hydroxyproline and some protein surfaces showed changes in reflectance of 330–400 nm light similar to those reported in skin after U.V. irradiation in viuo.     

Salt-inclusion synthesis of two new polar solids, Ba6Mn4Si12O34Cl3 and Ba6Fe5Si11O34Cl3

A new family of salt-containing, mixed-metal silicates (CU-14), Ba6Mn4Si12O34Cl3 (1) and Ba6Fe5Si11O34Cl3 (2), was synthesized via the BaCl2 salt-inclusion reaction. These compounds crystallize in the noncentrosymmetric (NCS) space group Pmc2(1) (No. 26), adopting 1 of the 10 NCS polar, nonchiral crystal classes, mm2 (C2v). The cell dimensions are a = 6.821(1) A, b = 9.620(2) A, c = 13.172(3) A, and V = 864.4(3) A3 for 1 and a = 6.878(1) A, b = 9.664(2) A, c = 13.098(3) A, and V = 870.6(3) A3 for 2. The structures form a composite framework made of the (M(4+x)Si(12-x)O34)9- (M = Mn, x = 0; M = Fe, x = 1) covalent oxide and (Ba6Cl3)9+ ionic chloride sublattices. The covalent framework exhibits a pseudo-one-dimensional channel where the extended barium chloride lattice (Ba3Cl1.5)(infinity) resides, and it consists of fused eight-membered meta-silicate rings propagating along [100] via sharing two opposite [Si2O7]6- units to form an acentric lattice. Single-crystal structure studies also reveal the ClBa4 unit adopting an interesting seesaw configuration, in which the lone pair electrons of chlorine preferentially face the oxide anions of the transition metal silicate channel, thus forming the observed polar frameworks. Similar to the synthesis of organic-inorganic hybrid materials, the salt-inclusion method facilitates a promising approach for the directed synthesis of special framework solids, including NCS compounds, via composite lattices.     

Electrical-optical analysis of a GaN/sapphire LED chip by considering the resistivity of the current-spreading layer

The effects of current distribution in LED chips on the electrical potential and optical light extraction efficiency are investigated by a numerical simulation. The results show that when the resistivity of the current-spreading layer is decreased there is current-crowding near the n-contact. On the other hand, when the resistivity in the current-spreading layer increases, there is current-crowding near the p-contact. When the current is crowded near the n-contact due to less resistivity of the current-spreading layer, the input power is lower because of the smaller series resistance in the chip, and the light extraction efficiency is higher since the shadowing effect of the p-contact can be avoided. For L _p = 50 μm in this study, the light extraction efficiency at ρ _ITO = 0.1 × 10⁻³ Ω·cm is 1.4 times better than that when L _p = 100 μm, even though the driving voltage is raised 1.02 times.