The effect of solvents on the absorption and emission spectra of 1,4-bis(5-phenyl-2-oxazolyl)benzene (POPOP) laser dye has been studied in various solvents at 298 K. A bathochromic shift was observed in absorption and fluorescence spectra upon increase of solvent polarity, which indicates that this transition is π-π *. The ground and excited state dipole moments were calculated as 2.23 and 6.34 Debye, respectively. The dye solution in MeOH, n-heptane, and methyl isobutyl ketone gives laser emission in the blue region upon excitation by a 337.1 nm nitrogen pulse; the gain coefficient and emission cross section as well as normalized photostability have been determined. Excitation energy transfer from POPOP to rhodamine B and fluorescine was studied to improve the laser emission from these dyes. Such an energy transfer dye laser system (ETDL) obeys a long range columbic energy transfer mechanism with a critical transfer distance, R 0 , of 25 and 33Å and k q equal to 10.4 × 10 12 and 26.2 × 10 12 M −1 s −1 for the POPOP/RB and POPOP/fluorescine pair, respectively. The POPOP dye is highly photostable in polar protic and polar aprotic solvents, while it displays photodecomposition in chloromethane solvent via formation of a contact ion pair. The photochemical quantum yield and rate of photodecomposition depend on the electron affinity of solvent.

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Laser light scattering Scattering parameter Polyvinyl alcohol Zimm plot a b s t r a c t Static laser scattering (SLS) is one of the most efficient techniques used for the molecular weight determination of polymer. Among of all polymers, polyvinyl alcohol (PVA) polymer was selected. A polystyrene (PS) was used as a standard polymer. A polyvinyl alcohol was investigated for its importance in pharmaceutical, biomedical, and industrial applications.Different concentrations in the range (3e9) Â 10 À3 g mL À1 were prepared. SLS data were obtained by using a self-built of the laser scattering system. The angular behavior of those selected polymer solutions with different concentrations were studied by plotting the intensity of the scattered light against the scattering angle in the range of 40e140 . By using Zimm plot the weight-average molecular weight M w , radius of gyration R G, and second virial coefficient A 2 of the standard PS and PVA were determined.

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