Vibronic Fluorescence Research Articles

The binding of sodium hexadecyl sulphate to poly(N-vinylpyrrolidone) in aqueous solutions. Kinetic and equilibrium studies

Binding isotherms and kinetic measurements associated with the adsorption/desorption of the surfactant sodium hexadecyl sulphate (SHS) onto the neutral polymer, poly(N-vinylpyrrolidone)(PVP), at two polymer concentrations are reported. The surfactant monomer concentrations were estimated in situ for all solutions from e.m.f. measurements of a cell containing the surfactant ion-selective electrode. The binding isotherms are very similar to those reported for shorter-chain alkyl sulphates in the presence of PVP. In addition, spectral data associated with the 3 : 1 intensity ratio of the vibronic fluorescence bands of pyrene have also been measured at the same polymer concentrations as a function of increasing SHS. The purpose of these measurements was to use the spectral data on the pyrene as a probe to monitor the hydrophobic environment in the polymer–surfactant formulation. Finally, kinetic (relaxation) data were measured with the pressure-jump relaxation technique and revealed a single relaxation process at all concentrations. The results indicate that the driving force during the binding process is a self-aggregation of the surfactant ions which occur as a result of the initially bound surfactant, creating a nucleus for the promotion of small aggregates on the polymer chain. Application of the linear phenomenological theory to the binding and kinetic data shows clearly that the adsorption rate increases with increasing amount of bound surfactant and that the desorption rate is proportional to the amount of bound surfactant. The desorption rate constant measured for the two formulations agree well. The relaxation data were also analysed using the Aniansson and Wall kinetic treatment associated with monomer aggregate exchange.

A Molecular orbital study of the rotation about the CC bond in styrene

Abstract The geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C β C 2 C 1 C 2 dihedral angle-Φ = 0°(cis), 15°, 30°, 60° and 90° — assuming that the vinyl and phenyl groups remain planar, but otherwise with full geometry optimization. Similar calculations have been carried out for 1,3-butadiene and 3-methylene-1,4-pentadiene (MPD) where rotation about 180° generates a different and not the same conformer. The torsional potential energy curve for styrene has a very flat minimum Φ = 0, i.e. the cis structure is the most stable, whereas butadiene and MPD have minima in the region Φ = 37° to 40°, indicative of more stable gauche structures. For styrene the barrier height Φ = 90° is 131.1 KJ mol −1 . These results provide strong support for the potential function obtained by Hollas and Ridley from single level vibronic fluorescence and other spectroscopic data. The distortion of the benzene ring brought about the vinyl group substitution is discussed, also the variation of the C/C and H/C bond lenghts with Φ and the change in charge on the vinyl group and the polarity of the various bonds in the conversion of the cis into the 90° gauche conformer. The stabilization energy for styrene relative to that for benzene has been evaluated according to various criteria, and, in addition, the energy associated with the distortion of the ring.

A Highly Resolved Yibronic Spectra of Dioxido-p-terphenyl and Trioxido-p-quaterphenyl in n-Hexanc at 77 K

Abstract Highly resolved vibronic absorption and fluorescence spectra of dioxido-p-terphenyl and the fluorescence spec-trum of trioxido-p-quaterphenyl in n-hexane at 77 K were obtained. The analysis of the vibrational structure in the excited singlet Si and in the ground So state gives funda-mental frequencies in the ground state in good agreement with those from infrared (IR) and Raman spectra.

Determination of electron-phonon coupling from structured optical spectra of impurity centers

Recent developments in the theory of impurity centers are reviewed. Particular attention is devoted to the way this theory can be exploited in the quantitative analysis of structured optical spectra through the use of formulas relating the absorption band shape to the shape of the fluorescence band. Analysis of departures from mirror-image relation between these two spectra can be used to deduce the parameters of the electron-phonon coupling from the shape of the structured bands. Topics discussed include zerophonon lines, the dependence of their shape and position on temperature, departures from mirror-image relation for both phonon wings and the vibronic absorption and fluorescence spectra, the energy gap between the absorption and fluorescence spectra of certain molecules, and the nature of excimer emission in solids.

Analysis of the sideband fluorescence of Eu 2+:CaF 2

The vibronic fluorescence of Eu 2+:CaF 2 at 4130 Å is calculated be treating the associated Jahn-Teller problem in the classical static limit and extracting the usual oscillator shifts in a cluster calculation. A 1 g and E g contributions are separated via the pronounced one-phonon structure. The J-T stabilization energy and the 10 Dq value for the 5 d electron are estimated to be 600 cm −1 and 16,000 cm −1 respectively.

Spectrum and energy levels of Cr3+ ions and exchange-coupled Cr3+ pairs in lanthanum aluminate

From the optical spectra the energy levels of the Cr3+ ion in LaAlO3 were determined. In the cubic field approximation we obtained with the parametersDq=1750cm-1,B=661 cm-1, andC=2908 cm-1 a good agreement between the calculated and measured energies. For higher Cr3+ concentrations the fluorescence spectrum of the exchange-coupled first nearest (1N) Cr3+ pairs was separated from the vibronic fluorescence spectrum of the single ions by selective excitation. From the experimental data the energy level scheme of the 1N pairs is constructed for the states ¦4A2·4A2〉 and ¦4A2·2E〉. In the ground state the pairs are ordered antiferromagnetically due to an isotropic exchange interaction with an exchange integralJ=(−68 ± 1) cm-1. From the fluorescence decay the intrinsic lifetimes of the fluorescent states of the single ions, the 1N and the 2N pairs were determined to be equal to (86± 4) msec, (1.75± 0.25) msec, and (31 ± 5) msec respectively. The fluorescence decay shows an energy transfer between the single ions and the pairs.

Relaxation Theory for Modulation Excitation of Vibronic States

One method for the absolute measurement of rate constants for defined quantum states is to study their response to sharply defined pumping. Such an experiment is the observation of the relaxation process after excitation of selected vibronic levels with amplitude modulated light. The observation could either involve secondary absorption or radiative emission. A general stochastic theory is formulated here for these time-defined processes, in this case, undergoing vibronic relaxation and fluorescence emission. From the general transport matrix describing the relaxation problem via the master equation, the observables are an unambiguous consequence. The solution is given in a form suitable for direct computation. For this particular case, the theory predicts a phase angle in terms of a many-shot expansion, which is not restricted to exponential fluorescence decay.