Absorption (logic)

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

An Investigation of Trapped Holes and Trapped Excitons in Alkaline Earth Fluorides

Abstract: The e.p.r. and optical absorption spectra of the self-trapped hole (V$\_{K}$) centres in CaF$\_{2}$, SrF$\_{2}$ and BaF$\_{2}$ have been measured and it is shown that the molecular axes of these centres are alined along $\langle $100$\rangle $. Preferential alinement of the V$\_{K}$ centres along a particular cube edge is readily produced by bleaching into the optical absorption bands of the V$\_{K}$ centre at 77 K with linearly polarized light. The anisotropy created in the crystals by preferential alinement is detected in both the e.p.r. and optical spectra. The thermally induced decay of the optically alined V$\_{K}$ centres is monitored by e.p.r. methods and it is shown that, in CaF$\_{2}$, linear (0 degrees) motion of the V$\_{K}$ centres predominates over the entire decay range (85 to 138 K). In SrF$\_{2}$ linear motion predominates in the early stages of decay (ca. 110 K) but random (90 degrees) motion sets in at higher temperatures (ca. 120 K). In BaF$\_{2}$ only random motion is observed. The kinetics of reorientation and decay of the V$\_{K}$ centre are investigated. A band observed in the X-ray fluorescence of CaF$\_{2}$ (at 279 nm), SrF$\_{2}$ (at 300 nm) and BaF$\_{2}$ (at 310 nm) is assigned to emission of the self-trapped exciton by comparison with a similar band in the recombination luminescence of the V$\_{K}$ centre (due to e$^{-}$ + V$\_{K}$). The temperature dependence of the X-ray fluorescence intensity is described. The polarization properties of the recombination luminescence are investigated on crystals containing optically alined V$\_{K}$ centres. The e.p.r. and optical absorption spectra of trapped hole centres involving other lattice defects are also described. One such centre, with its molecular axis close to $\langle $111$\rangle $, involves an interstitial fluorine ion and is named the V$\_{H}$ centre. Other centres with molecular axes close to $\langle $100$\rangle $ appear to be stabilized by impurity ions and are labelled V$\_{KA}$ and V$\_{KA^{\prime}}$ centres. The V$\_{H}$ centre occurs in CaF$\_{2}$, SrF$\_{2}$ and BaF$\_{2}$, the V$\_{KA}$ centre in SrF$\_{2}$ and BaF$\_{2}$, and the V$\_{KA^{\prime}}$, centre only in BaF$\_{2}$. The kinetics of reorientation and decay of the V$_{KA}$ centre have been investigated. We give a general discussion of the properties of hole centres, including the H centre, in alkaline earth fluorides.

Complete Hydrogen and Helium Particle Spectra from 30- to 60-MeV Proton Bombardment of Nuclei with A = 12 to 209 and Comparison with the Intranuclear Cascade Model

Abstract: Differential cross sections for the production of proton, deuteron, triton, helium-3, and $\ensuremath{\alpha}$ particles from as many as 10 targets ($A=12\ensuremath{-}209$) were measured using 29-, 39-, and 62-MeV incident protons. The particles were detected, with $\ensuremath{\approx}0.2$-MeV [full width at half maximum (FWHM)] energy resolution for protons, over a secondary energy range of $\ensuremath{\approx}2\ensuremath{-}6 \mathrm{to} 62$ MeV in a total absorption telescope composed of three solid-state detectors. Representative results are shown for cross sections differential in energy and angle, as well as for angle-and energy-integrated cross sections. For incident 60-MeV protons the integral magnitude of the nonevaporation charged-particle production is found to be $\ensuremath{\sim}{10}^{2}{A}^{\frac{1}{3}}$ mb. Fewer protons but more complex particles were measured for carbon and oxygen targets than expected from an ${A}^{\frac{1}{3}}$ dependence for either component alone. The continuum cross sections for $z=1$ particles at a given angle (mb ${\mathrm{sr}}^{\ensuremath{-}1}$ Me${\mathrm{V}}^{\ensuremath{-}1}$) are nearly independent of incident energy when measured with incident protons in the 30- to 60- MeV energy range. Nonevaporation production of complex particles ($A\ensuremath{\ge}2$) is 25-40% of that for protons. The proton spectra have been compared with predictions from the intranuclear cascade model. Differential spectral predictions compare well with the measured spectra for angles in the range \ensuremath{\sim}25-60\ifmmode^\circ\else\textdegree\fi{}, and relatively poor predictions for small and large angles are more favorable when reflection and refraction by the potential well are included. Evidence is given that predictions for backward angles are greatly improved by allowing proton scattering from nucleon pairs within the model nucleus, but the $A$-dependent underprediction at extreme forward angles is not understood at all. The calculated angle-integrated spectra reproduce the measured spectral shape but consistently predict $\ensuremath{\approx}30%$ too few nonevaporation protons for targets with $A\ensuremath{\ge}27$.[NUCLEAR REACTIONS $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{27}\mathrm{Al}$, $^{54}\mathrm{Fe}$, $^{56}\mathrm{Fe}$, $^{60}\mathrm{Ni}$, $^{89}\mathrm{Y}$, $^{120}\mathrm{Sn}$, $^{197}\mathrm{Au}$, $^{209}\mathrm{Bi}$, ($p,{p}^{\ensuremath{'}}X$), ($p,dX$), ($p,tX$), ($p,^{3}\mathrm{He}X$), ($p,\ensuremath{\alpha}X$), $E=62,39,29$ MeV; semi; measured $\ensuremath{\sigma}(E;{E}_{{p}^{\ensuremath{'}}},{E}_{d},{E}_{{3}_{\mathrm{He}}},{E}_{\ensuremath{\alpha}},\ensuremath{\theta})$; deduced $\ensuremath{\sigma}(E)$. $2\ensuremath{\lesssim}{E}_{{p}^{\ensuremath{'}}},{E}_{d},{E}_{t},{E}_{{3}_{\mathrm{He}}},{E}_{\ensuremath{\alpha}}\ensuremath{\lesssim}70$ MeV. Comparisons with intranuclear cascade model.]

Many-body effects on extended x-ray absorption fine structure amplitudes

Abstract: Careful extended x-ray absorption fine structure (EXAFS) measurements at various temperatures have been made on Pt, Cu, Mn${\mathrm{Cl}}_{2}$, Fe${\mathrm{Cl}}_{2}$, Co${\mathrm{Cl}}_{2}$, Ge, GaAs, ZnSe, and CuBr. Care was taken to minimize experimental errors in the measurements. The thermal variation in the disorder about the average shell distance was determined and found to agree well with calculation. Analysis of EXAFS data showed explicitly that the standard independent particle model formulation of the EXAFS equation is in error. The measurement can be accommodated by modifying the equation and adding many-body effects. The modifications consisted of adding the overlap factor ${S}_{0}^{2}$ which introduces a dependence on the type of center atom and subtracting from the path length of the photoelectron a term $\ensuremath{\Delta}$ in the exponential containing the mean free path $\ensuremath{\lambda}$. It is found that the atomic value of ${S}_{0}^{2}$ enters into EXAFS, that $\ensuremath{\lambda}$ varies significantly from material to material, and that $\ensuremath{\Delta}\ensuremath{\approx}{r}_{1}$, the distance to the first coordination shell of atoms. The new EXAFS equation containing many-body effects permits more accurate EXAFS structure determination when standards used for comparison do not closely approximate the unknown.

X-ray nonlinear optical processes using a self-amplified spontaneous emission free-electron laser

Abstract: In contrast to the long-wavelength regime, x-ray nonlinear optical processes are characterized in general by sequential single-photon single-electron interactions. Despite this fact, the sequential absorption of multiple x-ray photons depends on the statistical properties of the radiation field. Treating the x rays generated by a self-amplified spontaneous emission free-electron laser as fully chaotic, a quantum-mechanical analysis of inner-shell two-photon absorption is performed. It is demonstrated that double-core-hole formation via x-ray two-photon absorption is enhanced by chaotic photon statistics. Numerical calculations using rate equations illustrate the impact of field chaoticity on x-ray nonlinear ionization of helium and neon for photon energies near $1\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. In the case of neon, processes are discussed that involve up to seven photons. Assuming an x-ray coherence time of $2.6\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, double-core-hole formation in neon is found to be statistically enhanced by about 30% at an intensity of ${10}^{16}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$.

Recoilless optical absorption and Doppler sidebands of a single trapped ion

Abstract: Spectroscopic measurements of the electric-quadrupole-allowed 5${d}^{10}$6s ${\mathrm{}}^{2}$${S}_{1/2}$ to 5${d}^{9}$6${s}^{2}$ ${\mathrm{}}^{2}$${D}_{5/2}$ transition near 282 nm on a single, laser-cooled ${\mathrm{Hg}}^{+}$ ion give a recoil-free absorption line (carrier) and well-resolved motional sidebands. From the intensity ratio of the sidebands to the carrier, the effective temperature of the ${\mathrm{Hg}}^{+}$ ion was determined to be near the theoretical minimum of 1.7 mK. A fractional resolution of better than 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ for this ultraviolet transition is achieved.