Absorption (logic)

About: Absorption (logic) is a research topic. Over the lifetime, 5733 publications have been published within this topic receiving 236302 citations.

Color Centers in Cesium Halide Single Crystals

Abstract: Color centers have been investigated in the CsCl-type alkali halides. Cesium chloride, bromide, and iodide single crystals were grown from the melt and CsCl crystals also from solution. Coloration was produced by 130-kv x rays, 3.0-Mev electrons, and by electrolysis. In CsI coloration resulted from electrolysis only. The absorption of uncolored and colored crystals has been measured from 0.175 to 3.5 \ensuremath{\mu} at 25\ifmmode^\circ\else\textdegree\fi{}, -78\ifmmode^\circ\else\textdegree\fi{}, and -190\ifmmode^\circ\else\textdegree\fi{}C. After coloration all three crystals show one strong band in the visible (near infrared for CsI) and several weaker bands at shorter and longer wavelengths, which shift with temperature change. The spectral positions in m\ensuremath{\mu} for a number of the bands at -190\ifmmode^\circ\else\textdegree\fi{}C are: The strongest band (${\ensuremath{\lambda}}_{6}$)behaves similarly to the $F$ band in the NaCl-type alkali halides. The half-width of the band (0.20 to 0.23 ev at -190\ifmmode^\circ\else\textdegree\fi{}C) and its increase with temperature, the shifting of the band maximum toward longer wavelengths upon warming to room temperature (by \ensuremath{\sim}0.08 ev), and the conversion by bleaching with light into other bands support the assignment of this band as the $F$ band. The spectral position of the band maximum approximately follows the Mollwo relation (${\ensuremath{\lambda}}_{max}=\mathrm{const}{d}^{n}$, where $d=\mathrm{interionic}\mathrm{distance}$ and $n=2.5$).Bleaching experiments suggest the assignment of the band ${\ensuremath{\lambda}}_{10}$ as the $M$ band and the bands between the $F$ and $M$ bands as $R$ bands. The origin of the ultraviolet bands is still uncertain.

Direct calculation of inverse-bremsstrahlung absorption in strongly coupled, nonlinearly driven laser plasmas

Abstract: Inverse-bremsstrahlung absorption in strongly coupled plasmas produced by high-intensity lasers is studied numerically. The simultaneous presence of high density and intensity makes it difficult to treat this problem with standard methods. A technique for modeling collisional plasmas is demonstrated which uses a hierarchical tree code---an accelerated molecular dynamics algorithm with an $N$ log $N$ computation time---adapted to model periodic, non-equilibrium two-component plasmas. Good agreement is found with standard theoretical results for classical, weakly coupled plasmas. In a series of further simulations, the dependence of the inverse-bremsstrahlung absorption coefficient on plasma coupling parameter, laser frequency, and the ratio of quiver to thermal velocity ${v}_{0}{/v}_{\mathrm{te}}$ is computed. An important outcome of this study is that the Langdon effect---a change of the velocity distribution function due to an imbalance of heating and equilibration rates---is verified in a direct microscopic particle simulation.

ASCA PV observations of the Seyfert 1 galaxy MCG – 6 – 30 – 15: rapid variability of the warm absorber

Abstract: We present a detailed re-analysis of the two {\it ASCA} Performance Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15. Confirming the results of Fabian et al. (1994), we find definite evidence for the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm absorber and a doubling of the warm absorber column density within the 3 weeks separating the two observations. No intra-day {\it flux-correlated} variability of the warm absorber is found. However, we report the discovery of an `event' in which the warm absorber parameters temporarily change for \sim10\,000\thinspace s before returning to their original values. Possible interpretations are discussed but a contradiction remains: the constancy of the ionization state of the warm absorber argues that it lies at large distances from the central source whereas the short term change in column density argues for small distances. Fluorescent iron emission is examined. As found by Fabian et al. (1994), the iron line is broad and strong (equivalent width \sim300\thinspace eV). The line profile is also suggestive of it being skewed. Such a line would be expected from a relativistic accretion disk. We also find very rapid primary X-ray variability. Assuming relativistic beaming to be unimportant, the derived efficiency is comparable to the maximum obtainable from accretion onto a Schwarzschild black hole. Correlated variability outside of the energy range of {\it ASCA} might exceed this maximum, thus requiring efficient accretion onto a Kerr hole.

Optically Active Phonon Processes in CdS and ZnS

Abstract: Transmission spectra of thin (10 to 30 \ensuremath{\mu}) CdS single-crystal samples have been recorded in the hitherto uninvestigated region of 250 to 400 ${\mathrm{cm}}^{\ensuremath{-}1}$. A number of new absorption maxima are located which necessitated a new phonon assignment with considerably lower values of the acoustic modes. Some 30 absorption peaks are attributed to multiphonon combinations of the following Brillouin zone-edge frequencies: LO=295 ${\mathrm{cm}}^{\ensuremath{-}1}$, T${\mathrm{O}}_{1}$=261 ${\mathrm{cm}}^{\ensuremath{-}1}$, T${\mathrm{O}}_{2}$=238 ${\mathrm{cm}}^{\ensuremath{-}1}$, LA=149 ${\mathrm{cm}}^{\ensuremath{-}1}$, T${\mathrm{A}}_{1}$=79 ${\mathrm{cm}}^{\ensuremath{-}1}$, and T${\mathrm{A}}_{2}$=70 ${\mathrm{cm}}^{\ensuremath{-}1}$. The transmission spectrum of hexagonal ZnS is obtained for the first time in the region 300 to 750 ${\mathrm{cm}}^{\ensuremath{-}1}$. Again some 27 absorption maxima are assigned to multiphonon combinations of LO=346 ${\mathrm{cm}}^{\ensuremath{-}1}$, T${\mathrm{O}}_{1}$=318 ${\mathrm{cm}}^{\ensuremath{-}1}$, T${\mathrm{O}}_{2}$=297 ${\mathrm{cm}}^{\ensuremath{-}1}$, LA=181 ${\mathrm{cm}}^{\ensuremath{-}1}$, T${\mathrm{A}}_{1}$=92 ${\mathrm{cm}}^{\ensuremath{-}1}$, and T${\mathrm{A}}_{2}$=73 ${\mathrm{cm}}^{\ensuremath{-}1}$. The phonon assignment for the optical lattice absorption in cubic ZnS due to Deutsch is shown to be inconsistent with the requirements of the Brout sum rule and other regularities expected for the zincblende-type semiconductors. Two new assignments (LO=339 ${\mathrm{cm}}^{\ensuremath{-}1}$, TO=298 ${\mathrm{cm}}^{\ensuremath{-}1}$, LA=155 ${\mathrm{cm}}^{\ensuremath{-}1}$, and TA=93 ${\mathrm{cm}}^{\ensuremath{-}1}$, and LO=339 ${\mathrm{cm}}^{\ensuremath{-}1}$, TO=298 ${\mathrm{cm}}^{\ensuremath{-}1}$, LA=190 ${\mathrm{cm}}^{\ensuremath{-}1}$, and TA=115 ${\mathrm{cm}}^{\ensuremath{-}1}$) only slightly different from each other are proposed. The characteristic phonon frequencies are discussed in terms of structure and effective ionic charge.

Orbital angular momentum and interpretation of core-absorption magnetic circular dichroism on the band picture in Co-based Heusler alloys Co 2 Y Sn ( Y = Ti , Zr , and Nb )

Abstract: The magnetic circular dichroism (MCD) of core-level absorption [x-ray absorption spectroscopy (XAS)] spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloys ${\mathrm{Co}}_{2}\mathrm{TiSn},$ ${\mathrm{Co}}_{2}\mathrm{ZrSn},$ and ${\mathrm{Co}}_{2}\mathrm{NbSn}.$ The many-peak structures which are clearly observed in the Co $2\stackrel{\ensuremath{\rightarrow}}{p}3d$ XAS-MCD spectra cannot be well explained by atomic multiplet calculations but are better explained by the Co $3d$ unoccupied states predicted by linear muffin-tin orbital atomic sphere approximation band structure calculations. The MCD spectra show an obvious contribution of the orbital angular momentum component to the magnetic moment in these alloys. The ratio between the orbital angular momentum and spin derived from the MCD spectra varies by more than a factor of 2 within these alloys. The mechanism of this variation is discussed in comparison with the band structure calculations.