Absorption and emission spectroscopy in scattering media: modelling of fluorescence decay curves and photometric laws for systems with inhomogeneous absorber distribution

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.     

Ten microseconds in the life of a superhelix

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     

Photoproduction of positive pions from hydrogen with PHOENICS at ELSA

The differential cross section of the reaction γp→π⁺n has been measured with the PHOENICS detector at ELSA in Bonn. For the first time this cross section has been determined simultaneously over a large range of photon energies (E_γ = 220−900 MeV) and pion angles (Θ^c.m._pi = 35°−135°) with a tagged photon facility. The experimental set-up allowed a considerable kinematic overdetermination of the investigated reaction. Accordingly, the background contributions have been suppressed to below 1%. The measured differential cross section is in good agreement with existing data. The comparison with different model calculations is presented.     

Photoproduction of π+ mesons on polarized protons at photon energies between 0.5 and 2.2 GeV

The target asymmetry T = (σ↑ ? σ↓)/(σ↑ + σ↓) for the reaction γp → π⁺n has been measured at the Bonn 2.5 GeV electron synchrotron for a pion c.m. angle of 40° and γ energies between 0.5 and 2.2 GeV. Butanol was used as the target material. About 35% of the protons could be polarized using the dynamic-polarization method in a continuous-flow cryostat operating at 1°K and 25 kG. The π⁺ mesons were detected in a magnetic-spectrometer system. Considerable structure in the asymmetry was observed.     

Photon statistics in saturated m-photon amplification and attenuation of coherent light

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 $\text{n}$ ? 1, Δn² ? $\text{n}$² ($\text{n}$ mean value and Δn² variance of the photon number) an analytical expression is derived for the quantity Δn²/$\text{n}$. 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/$\text{n}$ tends to an asymptotic value.     

Positive pion photoproduction from hydrogen at photon energies between 500 MeV and 1,400 MeV in forward direction

The differential cross section of the reactionγ+p→π ⁺ was measured at pion CM-angles of 20° and 30° for photon energies between 500 MeV and 1,400 MeV. The pions were detected in a magnetic spectrometer. By measuring each pion trajectory and by offline calculation of the initial pion parameters an energy resolution of about 2.5% FWHM was achieved. The results complete a set of data which were measured in recent years at the Bonn 2.5 GeV synchrotron. In comparison to photoproduction analyses two effects were revealed: The η cusp appears in the energy dependence of the cross section as a sharp drop atK _γ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.     

The Influence of Multi-Photon Absorption on Photon Statistics

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.