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Dielectric measurements on samples consisting of banana-shaped molecules with a polymorphism I/SmCPA/B7 have been performed for the first time. The dielectric increment for the reorientation of the molecules about the long axis, and the corresponding relaxation time, surprisingly decrease at the SmCPA/B7 transition. This effect is discussed in terms of the undulated structure which partially destroys the ferroelectric short range order. Decreasing temperature gives an even stronger hindrance for this reorientation and could explain controversial results obtained in the B7 phases of different samples. At lower temperatures a further mesophase was detected. This was additionally observed by calorimetric and atomic force microscopy methods. 相似文献
74.
Ulrike Mammel Manfred Brun Dieter Oelkrug 《Fresenius' Journal of Analytical Chemistry》1992,344(4-5):147-152
Summary The optical absorption and fluorescence of scattering samples is investigated theoretically and experimentally under the condition of steady-state and pulsed irradiation, taking into account inhomogeneous distribution of the absorbing species. Based on the Kubelka-Munk model, numerical iterative expressions for the steady-state and time-resolved light flues of excitation and emission are derived. The intensities of reflected, transmitted and forward or backward emitted radiation in layered samples with vertical concentration gradients are discussed. Modification of fluorescence spectra and decay curves due to self absorption, background absorption, secondary emission and time of flight dispersion are calculated and compared to experimental data of hydrocarbons adsorbed on polycrystalline silica gel. 相似文献
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Giuseppe Chirico Ulrike Kapp Konstanin Klenin Werner Kremer Jörg Langowski 《Journal of mathematical chemistry》1993,13(1):33-43
Dynamic light scattering measurement data of DNA topoisomers are compared with two different computer models: a Monte Carlo model and a Brownian dynamics model. First results on the kinetics of formation of a superhelix and on the influence of curved sequences on the dynamics of superhelical DNA are reported.To whom correspondence should be addressed 相似文献
77.
Ulrike Mohr 《Optics Communications》1982,41(1):21-26
The change of photon statistics of a nearly coherent single-mode light beam travelling through a saturable m-photon-amplifying or absorbing medium (m = 1, 2, …) is theoretically investigated. With the help of an approximation scheme valid for ? 1, Δn2 ? 2 ( mean value and Δn2 variance of the photon number) an analytical expression is derived for the quantity Δn2/. The results holds for arbitrary values of the initial population inversion and of the degree of saturation. It turns out that for both amplification and attenuation processes after sufficiently strong interaction Δn2/ tends to an asymptotic value. 相似文献
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A previous approximation scheme [6] which allowed to calculate, in a simple manner, the quantity Δn2/n (n mean value and Δn2 variance of the photon number) in the course of a multi-photon absorption process, is extended to study the change of the shape of the photon distribution, too. It turns out that there is a tendency to symmetrize the distribution. Moreover, an approximative expression for the asymptotic distribution to which the distribution tends after a sufficiently strong absorption, irrespective of the initial distribution, is found in form of a Gaussian. 相似文献