Walter L. Borst

Intermolecular energy transfer in binary systems of dye polymers

We present results and physical interpretations for the energy transfer mechanisms in two-component dye polymer systems. The data consist of fluorescence emission spectra and decays. Two dyes were embedded in an epoxypolymer base, and only they participated in the energy transfer. Following pulsed laser excitation of the donor dye, energy transfer took place to the accept dye. The possible transfer paths considered here were nonradiative and radiative transfer. The latter involves two steps, emission and absorption of a photon, and therefore is relatively slow, while nonradiative transfer is a fast single step resulting from direct Coulomb interactions. A predominantly nonradiative transfer is desirable for applications, for instance in wavelength shifters in high energy particle detection. We studied the concentration effects of the dyes on the energy transfer and obtained the relative quantum efficiencies of various wavelength shifters from the fluorescence emission spectra. For low acceptor concentrati...

Effect of solvent polarity on a rotational isomerization mechanism of rhodamine‐B in normal alcohols

Experiments were performed to investigate the effects of solvent polarity and solvent viscosity on the nonradiative rates of rhodamine‐B in a series of normal alcohols. The fluorescence quantum yields and lifetimes for rhodamine‐B in these solvents were measured and used to calculate the nonradiative rates. From a modified kinetic model, the nonradiative rate can be deduced to be the conversion rate for crossing over a potential barrier from an excited planar state to an excited twisted state. Both solvent viscosity and solvent polarity can play key roles in this mechanism because the former may hinder the rotational motion and the latter may change the barrier height. Our results show that the variation in the nonradiative rates is due to solvent polarity changes rather than to solvent viscosity changes. The effect of solvent viscosity on the excited‐state lifetimes of rhodamine‐B in a series of methanol–poly(ethylene oxide) mixed solvents was also investigated. Our results indicate that the measured lif...

Pulsed laser fluorescence microscopy of coal macerals and dispersed organic material

Abstract A new pulsed laser microscopy technique was applied to the study of fluorescence from liptinite macerals in the time and wavelength domains. Useful parameters for coal characterization are the fluorescence decay times and percentage contributions of the individual fluorescing compounds to the total intensity. Characteristic decay times in the nanosecond and sub-nanosecond range are obtained for sporinite, fluorinite, cutinite and resinite of the Lower Kittanning coal seam and alginite of the New Albany shale group of the Illinois Basin. Time-resolved emission spectra for the individual fluorophores are obtained and generally found to be broad but specific for each maceral type. The total sporinite fluorescence comes from fluorophores whose individual spectra peak in the blue-green and yellow regions. The percentage contributions from each fluorophore vary systematically with increasing coal rank. The previously observed red-shift in the conventional composite spectra is interpreted, especially for sporinite. The new laser microscopy is described in some detail. The reliability of the method was tested with mixtures of selected organic compounds in solution and subsequent analysis. The fluorescence decay times and time-resolved spectra agree with those for the pure substances under certain conditions. These conditions apply to the coal macerais studied. Generally, one-, two- and three-component mixtures can be resolved if the component lifetimes differ by a factor of two or more and the individual contributions to the total intensity exceed 10%. Fluorescence decay times of 200 ps and greater can be measured with an accuracy of about 100 ps.

Fast Analog Technique For Determining Fluorescence Lifetimes Of Multicomponent Materials By Pulsed Laser

This paper describes a system capable of measuring fluorescent lifetimes and time-resolved spectra of microscopic particles, thus substantially enhancing the usefulness of fluorescence microscopy as a characterizing tool. Tests and applications of this technique for identifying component lifetimes (in the nanosecond and subnanosecond range) and spectra in organic model compounds also are described. A multi-exponential decay fit to the data yields fluorescence decay times, percentage contributions (fractional intensities) and time-resolved spectra for each decay. Two-component non-interacting mixtures are resolved if the decay times differ by a factor of two and the percentage contribution from each fluorophore is more than 10%. For three-component mixtures, it is necessary for the resolution of the lifetimes that the decay times differ by at least a factor of three and the percentage contribution from each component be more than 15%.

Enhancement Of Fluorescent Fingerprints By Time-Resolved Imaging

Laser detection of latent fingerprints tends to fail when prints are located on certain types of surface, such as brown cardboard and wood, because the generally weak fingerprint fluorescence is overwhelmed by background luminescence from these substrates. The fingerprint and background fluorescence lifetimes, which are of nanosecond order, were measured for samples of the above surface types and for fingerprints treated by the most successful current detection procedures. Our results indicate that time-resolved imaging can in these situations improve image contrast by about an order of magnitude.

Photochemistry of an unusual exsudatinite in Permian Basin shales

Abstract An unusual variety of exsudatinite is found in four Middle Paleozoic shales of the Permian Basin. Its strong blue to blue-green (λmax = 450–470 nm) fluorescence highlights it prominently along sedimentary bedding partings and microfractures. Unlike most primary liptinites, fluorescence spectra of this microfracture exsudatinite are characterized by additional structure in the form of competitive peaks in the green wavelengths (480–500 nm) and a broad secondary structure in the yellow wavelengths. Subtle flow structures are common along the extent of the maceral which may reach maximum widths of approximately 10 μm and lengths of 100 μm. Time-resolved fluorescence data indicate a complex fluorescence of several nanosecond and sub-nanosecond lifetimes. Unperturbed fluorescence lifetimes (3.0–7.4 ns) are significant in Woodford and Wolfcamp shales, but are minor or absent in Atoka and Canyon Group shales. These lifetimes are interpreted to represent aromatics and heteroaromatics (3–5 rings) in relatively unperturbed fluorescence microenvironments where the radiative rates are high. In contrast, two other groups of shorter lifetimes suggest that the natural decays of molecules are influenced by photochemical interactions that are shorter than natural fluorescence decay. We speculate that intermediate lifetimes (1.3–2.6 ns) indicate charge transfer perturbations to various degrees by peripheral electron-donating functionals. These exciplexes represent the inter- and intramolecular transfer of non-bonded electrons from ground state donors to proximal and coplanar excited acceptors. The shortest lifetimes (〈0.30 ns) may be ascribed to several processes that severely perturb the fluorescence.

Primary scintillant fluorescent decay times in binary and ternary scintillators by near UV pulsed laser excitation

Abstract Primary scintillant fluorescence decay times and spectra of seven commercially available binary and ternary, plastic and acrylic scintillators are presented. Excitation is by a pulsed, near ultraviolet laser. Thus the decay times are those from the final energy transfer stage following nuclear particle excitation. Various fitting functions previously used in describing the emitted pulse shape are reviewed, and the fluorescence decay time contribution of the primary scintillant is discussed.

Fluorescence decay kinetics of “Polyester yellow” in solutions and in polymers

Abstract We present some studies of the fluorescence decay kinetics of “polyester yellow” in benzene, glycerol and polymethyl methacrylate. The fluorescence of the dye in glycerol showed the presence of three dominant components, which correspond to three conformers differing in the orientations of the malononitrile group with respect to the dialkylaminophenyl group. At high concentrations of the dye, a ground state dimerization was observed for the first time in nonpolar solvents and in polymers. Two different configurations were identified from their fluorescence decay kinetics.

Time-resolved fluorescence spectroscopy of crude oils and condensates

Abstract A pulsed laser fluorescence spectroscopy system, previously developed for coal characterization, was used to characterize thirteen crude oils and condensates. From time-resolved fluorescence spectra, three individual components (fluorophores) with decay times in the nanosecond and sub-nanosecond range were resolved. A good correlation was obtained between the fluorescence decay times of these components and API gravity. The fluorescence decay of normal crude oil was different from biodegraded crude oil. Two of the three resolved component spectra and decay times resembled those of anthracene and pyrene in solution.

Fluorescence Decay Times Of Multicomponent Microscopic Materials By Pulsed Laser Excitation

A fast analog technique developed for the determination of fluorescence emission parameters of microscopic geological particles is applied to a variety of samples. Presented here are fluorescence data from liptinite coal macerals, analysis of fluorescence emitted from crude oils, applications to fiber analysis and plastic scintillators. Characteristic decay times in the nanosecond and subnanosecond range are given along with time-resolved emission spectra.