Previous work involved detection of retroreflected signals, focusing on polarization as a discriminator. We continue to explore the formalisms of the Stokes vector and Mueller matrix for characterization of retroreflection for common sensors such as commercial cameras. The Mueller matrix data is collected via a commercial (Scatterworks, Inc.) Complete Angle Scatter Instrument (CASI) scatterometer.
As an imaging dye absorbs a single photon, there exists a non-zero probability that the dye will bleach as the result of a photochemically induced reaction. Two possible mechanisms, which can cause this photochemical event, are radical formation and/or radiative energy transfer. A photophysical investigation was completed in order to identify a dominate mechanism governing the lightfast properties of imaging inks. Time and spatial domain spectroscopic studies were conducted on an azo class dye, Cibacron Brilliant Red 3B-A, and Rhodamine 6G bound to humic acid - polymer phenol with known radiative and binding characteristics to polyaromatics. Stern-Volmer analysis illustrated that Rhodamine 6G binds strongly to humic acid with a binding constant of approximately 3000 L/g - C, as opposed to, Brilliant Red which binds about 100 times less strongly to the polymer additive. Both dyes were found to bind statically to humic acid. Accelerated photo degradation studies revealed that dye bound to humic acid bleached at a rate proportional to the binding association of each dye. Time resolved fluorescence decay results indicated this bleaching is not the result of polaron or long distance radiative energy transfer. Radical quenching studies suggest that the primary mechanism involves a ground state reaction with a radical intermediate.
Ga contains two tryptophans. Tryptophan fluorescence decay of Ga at 25°C in 10 mM Tris-HC1 buffer pH=7.3 was triexponential with lifetimes of 3.62, 1.06 and 0.l7ns. Contrary to expectation, the shorter lived (lower quantum yield) components were blue-shifted relative to the longer Denaturation with 8M urea diminished the 1.O6ns component and increased net intensity which became dominated by a lifetime of 5. iOns. Emission of a tryptophan species was identified at 450nm with lifetime of 20-4Ons, similar to that reported earlier by Vanderkooi J. et al' . This long-wavelength emission was also eliminated by 8M urea denaturation, making it seem possible that the low intensity of the 1 .O6ns blue shifted component may result from competing excited state processes that give rise to the 450nm emission. Transient absorbance spectra of Ga show no evidence for triplet states in denatured protein while the native protein showed phosphorescence at 442nm of lifetime between 0. 1 and 1 .Oms.
The fluorescence decay kinetics in the copolymer, poly(phenylmethyl-co-p- dimethylanilinemethyl silane) have been studied by steady state and time correlated fluorescence spectroscopy and three absorbing and emitting species are proposed to explain the complex fluorescence decay behavior. In the case of poly(phenyl methyl silane) homopolymer, excitations have been shown to delocalize over several tens of monomer units, however, in the copolymers discussed here the extent of the delocalization has been shown to be dependent on the degree of substitution of the dimethylaniline side group. In the more highly substituted copolymers the excitations are shown to be confined to almost a single monomer unit by the electron donating nature of the dimethylaniline side group. A simple geometric distribution of chain lengths is used to predict the delocalization length of the excitations.
Polarized two-photon fluorescence excitation spectra were obtained for the nucleotides TMP, CMP, GMP, and AMP in neutral aqueous room temperature solution (0.05M). The wavelength range was 400- 600 nm, corresponding to states reached with 200-300 nm single UV photons. INDO/S-CI predictions were carried out for the corresponding methyl bases, and were somewhat useful for the 260 nm bands with certain parameterizations. Theory failed to find the strong two-photon absorption seen below 450 nm, which may be of Rydberg origin.
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