Finite Coherence Length Research Articles

Influence of finite coherence length of incoming light on enhanced backscattering.

We have investigated the effects of the finite coherence length of the incoming light on multiple scattering in a random medium. Large optical fluctuations observed with coherent light gradually decrease their intensity as the coherence length of the incoming light is shortened. When the coherence length is shortened below the elastic mean free path, the enhanced backscattering peak is revealed without ensemble average of the fluctuations. This variation of the fluctuations as a function of the coherence length of the incoming light has been analyzed on the basis of the diffusion approximation.

Lineshape Analysis of X-ray Diffraction Profiles: Polyethylene and Model Copolymers

Studies of the shapes of X-ray diffraction peaks from synthetic polymers are still rather uncommon. One probable cause of this situation is the small peak-to-background ratio in most polymer diffraction experiments; it is difficult to achieve precise line profiles for quantitative analysis. Increased utilization of automated data collection/analysis systems and more intense X-ray sources should alleviate this restriction. We suspect, furthermore, that confusion about nomenclature has impeded the acceptance of lineshape analysis for polymers. The peak broadening mechanisms which are generally considered are finite coherence length or crystal size, lattice parameter fluctuation, and displacement disorder of the second kind. Both latter mechanisms have, unfortunately, been referred to as “strains” or “microstrains”. Metallurgists have traditionally expressed displacement disorder as a (length dependent) “microstrain”, and this convention has been adopted in some studies of polymer diffraction. Other work on polymers, however, has termed lattice parameter fluctuation as “microstrain“. The inconsistent use of this term can imply a nonexistent relation between two distinct phenomena.

Multiplicity distributions in e+-e- and μ-p reactions and their rapidity dependence from a quantum-statistical point of view

The methods of quantum statistics (QS) including a finite coherence length are applied to multiply distributions P(n) in e+-e- and mu -p reactions. The rapidity dependence of P(n) gets a simple explanation as a consequence of a scaling regularity derived from QS.

Rapidity scaling of multiplicity distributions in a quantum-statistical approach.

Quantum statistics including a finite coherence length is applied to the study of multiplicity distributions from high-energy hadronic collisions measured in finite rapidity windows. A new scaling property in rapidity (''..beta.. scaling'') is derived. It contains forward-backward correlations as a special case. Agreement with data is satisfactory. We find strong indications for an increase of chaoticity with energy. Further experimental tests are suggested.

Ultrasonic attenuation in incommensurate solids due to order parameter fluctuations

Abstract A theory for the dispersion and attenuation of sound waves due to amplitude and phase fluctuations of the order parameter in incommensurate solids is developed. Using Langevin equations for acoustic and order parameter modes we obtain a Debye behavior for the elastic response at “high” frequencies and a strongly non-Debye behavior at “low” frequencies due to infrared divergences of simple perturbation theory. The divergences are treated by summing up the leading diagrams of a 1/N expansion where N is the number of components of the order parameter. At low frequencies the dispersion of sound waves is only little affected by the divergences; however their absorption increases dramatically in pure systems, leading to diverging viscosities throughout the incommensurate phase and an anomalous ω3/2 law for the ultrasonic attenuation coefficient. We discuss also how the anomalous elastic response at low frequencies changes if a finite coherence length due to pinning effects is taken into account. Finall...

Use of a position sensitive detector for data acquisition of synchrotron X-ray diffraction from adsorbed gas monolayers on graphite

The structure of monolayers of gases adsorbed on graphite has been studied by synchrotron X-ray diffraction [1–3]. Exfoliated Union Carbide ZYX graphite was used as a substrate. A spectrometer using a position sensitive detector and a slit configuration increased the data acquisition efficiency compared to that of the triple axis spectrometer on which the measurement was initiated. The origin of this comes from the finite coherence length of the graphite flakes, which is [4] 1600 Å. This is much less than the 10 000 Å which can be studied by the triple axis spectometer using Ge(111) both as monochromator and analyser [4]. So the resolution of the triple axis spectrometer is far too good for the object studied. The reason why this is bothering us is that we are studying monolayers of atoms, thus having very few atoms to scatter from, and even though we are working at the storage ring DORIS some of the experiments are on the limit of the doable with the intensity available. As no monochromator and analyser crystal exists which can fill out the gap between 1600 and 10 000 Å the slits configuration was invented. Although the introduction of slits limited the part of the synchrotron beam we utilized, this was well compensated for by the use of a position sensitive detector. However, this clearly illustrates the need for focusing elements. In some of the experiments the intensity need will also require wiggler lines.

Absolute Parametric Excitation by an Imperfect Pump or by Turbulence in an Inhomogeneous Plasma

Finite-bandwidth effects are investigated by using the random phase approximation. The conditions of validity are checked by solving exactly a particular model. It is found that a finite coherence length for the pump wave can favor the existence of absolute instability.

Wavefront Reconstruction with Light of Finite Coherence Length*

Wavefront Reconstruction with Light of Finite Coherence Length*