Search results for "RADIATION"
showing 10 items of 5298 documents
X-ray versus Ultraviolet Irradiation of Astrophysical Ice Analogs Leading to Formation of Complex Organic Molecules
2019
In astrochemistry, complex organic molecules (COMs) are defined as species with at least one C atom and six or more atoms in total. More than 70 COMs were detected toward various interstellar and c...
Probabilities, States, Statistics
2016
In this chapter we clarify some important notions which are relevant in a statistical theory of heat: The definitions of probability measure, and of thermodynamic states are illustrated, successively, by the classical Maxwell-Boltzmann statistics, by Fermi-Dirac statistics and by Bose-Einstein statistics. We discuss observables and their eigenvalue spectrum as well as entropy and we calculate these quantities for some examples. The chapter closes with a comparison of statistical descriptions of classical and quantum gases.
Blue luminescence in ZnO single crystals, nanopowders, ceramic
2007
The luminescence spectra and luminescence decay processes were studied in a ZnO single crystal, nanopowders and ceramic at liquid helium and room temperature under VUV synchrotron radiation as well as under pulsed laser excitation. The exciton-exciton and exciton-multiphonon processes were compared in different ZnO nanopowders (commercial powder, powders obtained by vaporization-condensation technique) and ceramic. The possibility of luminescence decay time modification by Al3+ doping was shown.
<title>Multiphoton-absorption-induced structural changes in fused silica</title>
1991
The basic properties (light refractive index, density, mechanical strength, etc.) of fused silica are changed by the influence of high-intensity light from the glass transparency region capable of generating excitons by multiphoton absorption. The self-trapped exciton decay near the microcavity in the fused silica structure leads to the stable elementary intrinsic defect pair (nonbridging oxygen atom and three-fold-coordinated silicon atom) generation. At the large- enough light intensities near such a microcavity with a defect, the next exciton can be self- trapped. Then the next elementary defect can appear in the microcavity, and a chemical bond between it and the previously generated de…
Luminescence center excited state absorption in tungstates
2001
The excited state absorption of intrinsic luminescence center (self-trapped exciton) in tungstates (CaWO4, ZnWO4, PbWO4 and CdWO4) was studied. The transient absorption and luminescence spectra, decay kinetics and lifetime dependencies on temperature have been measured. The model of self-trapped exciton and nature of observed absorption bands were discussed.
Singularity formation in the Gross-Pitaevskii equation and collapse in Bose-Einstein condensates
2004
We study the mechanisms of collapse of the condensate wave function in the Gross-Pitaevskii theory with attractive interparticle interaction. We reformulate the Gross-Pitaevskii equation as Newton's equations for a flux of particles, and introduce the collapsing fraction of particles. We assume that this collapsing fraction is expelled from the condensate due to dissipation. Using this hypothesis we analyze the dependence of the collapse behavior on the initial conditions. We find that, for a properly chosen negative scattering length, the remnant fraction of atoms becomes larger when the initial aspect ratio of the condensate is increased.
Understanding Hawking Radiation from Simple Models of Atomic Bose-Einstein Condensates
2013
This chapter is an introduction to the Bogoliubov theory of dilute Bose condensates as applied to the study of the spontaneous emission of phonons in a stationary condensate flowing at supersonic speeds. This emission process is a condensed-matter analog of Hawking radiation from astrophysical black holes but is derived here from a microscopic quantum theory of the condensate without any use of the analogy with gravitational systems. To facilitate physical understanding of the basic concepts, a simple one-dimensional geometry with a stepwise homogenous flow is considered which allows for a fully analytical treatment.
Quantum effects in the dynamics of intensity-dependent two-mode two-photon models of radiation—matter interaction
1996
Abstract We study the two-photon interaction of a two-state localized system with two modes of a quantized electromagnetic or elastic field. Assuming the coupling strength and the atom-field detuning depending on the mode populations, we find that the quantum atomic dynamics manifests nonclassical features related to the specific nonlinear model investigated.
Formation of self-trapped excitons through stimulated recombination of radiation-induced primary defects in alkali halides
1998
Abstract A self-trapped exciton formation through photostimulated recombination of an F and an H center — the exciton-created primary defect pair, is proposed and experimentally examined in alkali halides at low temperatures.
Radiation Effects on Silica-Based Preforms and Optical Fibers - II: Coupling Ab Initio Simulations and Experiments
2008
International audience; Both experimental and theoretical approaches are combined to study the nature of precursor sites and radiation-induced point defects in pure and germanium-doped amorphous silica-based glasses.