Absorption (logic)

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

Anomalous Stokes shift in CdSe nanocrystals

Abstract: We investigate the temperature dependence of exciton fluorescence, absorption, and Stokes shift for five sizes of high quality $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ nanocrystals from the perspective of the fine structure model and exciton-acoustic phonon coupling. The Stokes shift is found to have a weak temperature dependence for all sizes. Within the fine structure model, we use the temperature dependence of the Stokes shift to infer the upper level energy and oscillator strength. We also interpret our ensemble measurements using an exciton-acoustic phonon scattering model. We find that neither the fine structure nor exciton-acoustic phonon scattering is able to adequately explain our data in isolation and conclude that a comprehensive theory which includes the physics of both models on an equal footing is required.

Experimental Measurements of the C 12 + C 12 Nuclear Reactions at Low Energies

Abstract: The nuclear reactions $^{12}\mathrm{C}(^{12}\mathrm{C}, \ensuremath{\alpha})^{20}\mathrm{Ne}$ and $^{12}\mathrm{C}(^{12}\mathrm{C}, p)^{23}\mathrm{Na}$ have been observed from 5-MeV down to 2.45-MeV center-of-mass energy. Angular distributions and energy distributions of the protons and $\ensuremath{\alpha}$ particles were analyzed to obtain the total cross sections and other nuclear information. The Coulomb and angular momentum barrier penetrability was factored out to elucidate the intermediate resonance structure in the nuclear factor $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{S}$. The observed rise in the nuclear factor at the lowest energies may be interpreted as "absorption under the barrier" as proposed by Michaud and Vogt. The importance of these reactions for the carbon burning era of nucleosynthesis and energy generation in the later evolution of stars is mentioned and reaction rates are estimated for various burning temperatures.

Evidence of local structural inhomogeneity in FeSe 1 − x Te x from extended x-ray absorption fine structure

Abstract: Local structure of ${\text{FeSe}}_{1\ensuremath{-}x}{\text{Te}}_{x}$ has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and $\text{Fe}\text{ }K$ edge EXAFS has permitted to quantify the local interatomic distances and their mean-square relative displacements. The Fe-Se and Fe-Te bond lengths in the ternary system are found to be very different from the average crystallographic Fe-Se/Te distance, and almost identical to the Fe-Se and Fe-Te distances for the binary FeSe and FeTe systems, indicating distinct site occupation by the Se and Te atoms. The results provide a clear evidence of local inhomogeneities and coexisting electronic components in the ${\text{FeSe}}_{1\ensuremath{-}x}{\text{Te}}_{x}$, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors.

Nature of the broadband luminescence center in MgO: Cr 3 +

Abstract: The luminescence spectrum of MgO:${\mathrm{Cr}}^{3+}$ is characterized by a series of sharp features at around 7000 \AA{} and a broad band which stretches from about 7000 \AA{} to 1 \ensuremath{\mu}m. The nature of the centers responsible for this broadband luminescence is examined by a combination of phase-sensitive detection methods and polarized absorption and luminescence methods. The phase-sensitive detection method allows us to separate clearly the broadband luminescence from the remainder of the spectrum and a sharp no-phonon line associated with the broadband luminescence is uncovered. The centers responsible for the broadband luminescence are found to have axes of symmetry along [011]-type directions, supporting the conclusion that this luminescence originates on ${\mathrm{Cr}}^{3+}$ ions in rhombic centers. At high temperatures broad emission is also observed from cubic site ${\mathrm{Cr}}^{3+}$ ions. The cubic site luminescence is shown to behave in accordance with theoretical expectations.

Density functional theory studies of the structural, electronic, and phonon properties of Li 2 O and Li 2 CO 3 : Application to CO 2 capture reaction

Abstract: The structural, electronic, and phonon properties of ${\text{Li}}_{2}\text{O}$ and ${\text{Li}}_{2}{\text{CO}}_{3}$ solids are investigated using density functional theory (DFT) and their thermodynamic properties for ${\text{CO}}_{2}$ absorption and desorption reactions are analyzed. The calculated bulk properties for both the ambient- and the high-pressure phases of ${\text{Li}}_{2}\text{O}$ and ${\text{Li}}_{2}{\text{CO}}_{3}$ are in good agreement with available experimental measurements. The calculated band gap of the high-pressure phase of ${\text{Li}}_{2}\text{O}$ (8.37 eV, indirect) is about 3 eV larger than the one corresponding to the ambient ${\text{Li}}_{2}\text{O}$ phase (5.39 eV, direct), whereas the calculated band gap for the high-pressure phase of ${\text{Li}}_{2}{\text{CO}}_{3}$ (3.55 eV, indirect) is about 1.6 eV smaller than that for the ambient phase of ${\text{Li}}_{2}{\text{CO}}_{3}$ (5.10 eV, direct). The oxygen atoms in the ambient phase of the ${\text{Li}}_{2}{\text{CO}}_{3}$ crystal are not equivalent as reflected by two different sets of C-O bond lengths (1.28 and $1.31\text{ }\text{\AA{}}$) and they form two different groups. When ${\text{Li}}_{2}{\text{CO}}_{3}$ dissociates, one group of O forms ${\text{Li}}_{2}\text{O}$, while the other group of O forms ${\text{CO}}_{2}$. The calculated phonon dispersion and density of states for the ambient phases of ${\text{Li}}_{2}\text{O}$ and ${\text{Li}}_{2}{\text{CO}}_{3}$ are in good agreement with experimental measurements and other available theoretical results. ${\text{Li}}_{2}\text{O}(s)+{\text{CO}}_{2}(g)\ensuremath{\leftrightarrow}{\text{Li}}_{2}{\text{CO}}_{3}(s)$ is the key reaction of lithium salt sorbents (such as lithium silicates and lithium zircornates) for ${\text{CO}}_{2}$ capture. The energy change and the chemical potential of this reaction have been calculated by combining DFT with lattice dynamics. Our results indicate that although pure ${\text{Li}}_{2}\text{O}$ can absorb ${\text{CO}}_{2}$ efficiently, it is not a good solid sorbent for ${\text{CO}}_{2}$ capture because the reverse reaction, corresponding to ${\text{Li}}_{2}{\text{CO}}_{3}$ releasing ${\text{CO}}_{2}$, can only occur at very low ${\text{CO}}_{2}$ pressure and/or at very high temperature when ${\text{Li}}_{2}{\text{CO}}_{3}$ is in liquid phase.