Fluorescence Lifetimes: Effect of Precision of Relaxation Times from Fluorescence Depolarization Data

Abstract

The fluorescence lifetime τ enters into the Perrin-Weber equation used to determine rotational relaxation times of proteins by the steady-state depolarization method. If is either too short or too long, the relaxation time cannot be measured with precision. With reasonable assumptions as to photometric precision, it has been possible to compute the precision of relaxation time assays for different lifetime:relaxation time ratios, thus also determining the “optimum” lifetimes for different systems. These results provide a basis for designing experiments requiring optimal conditions, and also provide insight into the amount of allowable deviation from optimal conditions. The short lifetimes of intrinsic protein fluorescence can often be used to determine the global relaxation rate of the macromolecule with acceptable precision.     

Quantitative analysis of crystallization kinetics by light depolarization technique. Possibilities and limitations

The kinetics of isothermal crystallization of various polymers was investigated by light depolarization technique (LDT) using the new setup with direct registration of depolarization ratio. Experimental data were analyzed using new method proposed by Ziabicki who shown that degree of crystallinity is a non-linear function of degree of depolarization, crystal thickness, and its birefringence. Other experimental methods were involved providing supplementary information on crystal thickness (SAXS) and allowing comparison of crystallization kinetics (WAXS, DSC). The advantage of LDT relies on high sampling rate allowing on-line measurements and lack of inertia effects that exist in other methods like calorimetry. The limitations of the applicability of the method are discussed. The method needs supplementary information not only on crystal thickness but also on variable optical birefringence of real crystals. Our results show that LDT can be used in a simple way for investigation of crystallization kinetics at relatively high temperatures, providing large and perfect crystals. In such a case it is sufficient to use crystal intrinsic birefringence and final crystal thickness typical at particular temperature of crystallization. On the other hand, depolarization ratio combined with measurements by other methods (crystallinity and crystal thickness) can be used for estimation of crystal birefringence.     

带壳介质椭球的等效介电常量

通过引入椭球坐标系,求解满足给定边界条件的拉普拉斯方程,得出了带壳介质椭球内外的电势分布,从而进一步讨论了带壳椭球的等效介电常量.     

ALIGNMENT INSTABILITY INDUCED BY IRRADIATION OF VISIBLE LIGHT IN FRUSTOELECTRIC LIQUID CRYSTALLINE CELLS SHOWING THE V-SHAPED SWITCHING

Abstract

Irradiation of the powerful visible light from the laser expedites the deterioration of the frustoelectric liquid crystalline cells (Inui mixture) showing the V-shaped switching. This deterioration is caused by the light absorption in the aligning layer, which strongly influences the molecule-surface polar interaction; this cannot be observed in the cell with the transparent aligning material in the used visible light region. This is observed just in the tilted smectic X* phase, not in the SmA phase. Irradiation during the switching leads to hysteresis in the V-shaped pattern and changes even the surface molecular alignment. These results can be explained by the shielding of the surface charge due to the alignment of the spontaneous polarization and the disturbed polar anchoring.     

Bound water: Evidence from and implications for the dielectric properties of aqueous solutions

Broadband dielectric spectra of a variety of aqueous solutions are evaluated as to indications of water that may be considered bound. Static permittivity decrements due to depolarizing internal electric fields, from kinetic depolarization, as well as from dielectric saturation are discussed. The latter effect reflects the preferential orientation of water permanent dipole orientations within strong Coulombic field of small ions, especially multivalent cations. Such water may be considered bound even though rapid rotations around the orientation of the electric dipole moment are definitely possible and also a fast exchange of water molecules between the hydration region and the bulk may take place. Water exhibiting large dielectric relaxation times, as typical for regions with large local concentration of foreign matter, may also be named bound. However, no clear evidence for interaction energies exceeding the hydrogen bond energy of pure water has been found. Rather enhanced relaxation times at low water content reflect the small concentration of hydrogen bonding sites and thus low probability density for the formation of a new hydrogen bond. Potential interferences of the water relaxation with relaxations from other molecules or from ionic structures are mentioned briefly.     

Scope of electron transport studies by thermally stimulated discharge current measurement

A logical approach to electron transport studies for barrier conduction in layered structures was adopted by thermally stimulated discharge current (TSDC) measurement. The scope and applicability of this technique to the evaluation of the thermoelectric parameters of relaxation time, detrapping energy and depolarization rates are demonstrated here. These are characterized by the controlling factors of layer resistance and the resultant thermal and voltage gradients which apply to the drift of electrons arising from both dipolar and interfacial charges. The methodologies used in this study are suitable for parametric evaluation of structured electronic devices. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photoinduced trion absorption in monolayer WSe2

Photoinduced trion absorption in the time- and spectrum-resolved circular dichroism (CD) of a pristine WSe₂ monolayer is manifested by a pronounced trion transition when the circular polarization of a probe pulse is opposite to that of the preceding pump pulse. Valley-polarized trion absorption enables visualization of the inter-valley scattering dynamics of spin-polarized carriers, revealing a large excitation dependence that supports the valley depolarization through carrier-carrier scattering. Moreover, the CD and the Faraday rotation, which are both dominated by the spin-polarized shift of the A-exciton absorbance and the photoinduced trion transition, are confirmed to satisfy the Kramers-Kronig relationship between each other.     

Depolarization loss of interference-pattern contrast when probing radially loaded cylindrical glass laser matrices

Results of an investigation of the depolarization loss of interference-field contrast are presented. The loss of contrast was found experimentally and simulated numerically for different interferometric schemes: with lateral and radial shifts and with a reference beam. It is found that the original polarization of the probing radiation affects substantially the integral contrast of the interference field. Baltic State Technical University, 1, 1st Krasnoarmeiskaya St., St.-Petersburg, 198005, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 62–66, January–February, 1998.     

Collisional Depolarization of the Luminescence of Asymmetric Top Molecules in the Gas Phase

To describe the collisional depolarization of the luminescence of asymmetric top polyatomic molecules, integral and integrodifferential forms of master equations, in which the effect of collisions is determined by the conditional probabilities of an instantaneous error of rotational phase variables, have been obtained. A symmetry analysis of the master equations has been performed, and it has been shown that in the general case the evolution of optically induced anisotropy is given by five independent relaxation components. The kinetic equations derived are initial equations for specific calculations of the anisotropy relaxation in various collisional models. They make it possible to study the influence of the angular momentum transfer efficiency on the orientational relaxation of anisotropy in a wide range of buffer medium densities: from free rotation to binary collisions in the gas and then to rotational diffusion.     

Lidar Multiple Scattering in Water Droplet Clouds: Toward an Improved Treatment

Although it has long been recognized that the effects of photon multiple scattering generally need to be accounted for in the analysis of lidar cloud returns, this is a difficult problem and current approaches are still rudimentary. The multiple scattering process is controlled by the size of the lidar beamwidth and the distance to the cloud, which jointly determine the lidar footprint, but cloud microphysical content (i.e., particle size, concentration, and shape) exerts a strong influence on the range distribution and depolarization of the returned energy. Since clouds are inherently inhomogeneous with height, it is our premise that vertically homogeneous cloud simulations based on idealized particle size distributions lead to misleading results. We offer a more realistic approach based on the contents of growing water droplet clouds predicted by a sophisticated adiabatic cloud model, which are offered for use as new standard vertically-inhomogeneous cloud models. Lidar returned signal and depolarization profiles derived from our analytical double-scattering method are given for inter-comparison purposes.Presented at the 7th International Workshop on Multiple Scattering Lidar/Light Experiments (MUSCLE7), July 21–23 1994, Chiba, Japan.