Absorption (logic)

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

Evidence of quantum size effect in nanocrystalline silicon by optical absorption

Abstract: The optical absorption spectrum in nanocrystalline silicon $(n\ensuremath{-}\mathrm{Si})$ was determined from both light transmittance and reflectance measurements. We observed that $n\ensuremath{-}\mathrm{Si}$ has a phonon structure in the optical absorption spectrum. This structure originates from momentum-conserving TO phonon absorption and emission, and provides direct evidence that $n\ensuremath{-}\mathrm{Si}$ is an indirect-band-gap semiconductor with quantum size effects. By using small-angle x-ray scattering to measure the nanocrystal size distribution, we found that the band-gap widening varies as ${(1/L)}^{1.6}$ with decreasing nanocrystal diameter L.

Optical potential from first principles

Abstract: We develop a method to construct a microscopic optical potential from chiral interactions for nucleon-nucleus scattering. The optical potential is constructed by combining the Green's function approach with the coupled-cluster method. To deal with the poles of the Green's function along the real energy axis we employ a Berggren basis in the complex energy plane combined with the Lanczos method. Using this approach, we perform a proof-of-principle calculation of the optical potential for the elastic neutron scattering on $^{16}\mathrm{O}$. For the computation of the ground state of $^{16}\mathrm{O}$, we use the coupled-cluster method in the singles-and-doubles approximation, while for the $A\ifmmode\pm\else\textpm\fi{}1$ nuclei we use particle-attached/removed equation-of-motion method truncated at two-particle--one-hole and one-particle--two-hole excitations, respectively. We verify the convergence of the optical potential and scattering phase shifts with respect to the model-space size and the number of discretized complex continuum states. We also investigate the absorptive component of the optical potential (which reflects the opening of inelastic channels) by computing its imaginary volume integral and find an almost negligible absorptive component at low energies. To shed light on this result, we computed excited states of $^{16}\mathrm{O}$ using the equation-of-motion coupled-cluster method with singles-and-doubles excitations and we found no low-lying excited states below 10 MeV. Furthermore, most excited states have a dominant two-particle--two-hole component, making higher-order particle-hole excitations necessary to achieve a precise description of these core-excited states. We conclude that the reduced absorption at low energies can be attributed to the lack of correlations coming from the low-order cluster truncation in the employed coupled-cluster method.

Excited-state relaxations and Franck-Condon shift in Si quantum dots

Abstract: Excited-state relaxations in molecules are responsible for a redshift of the absorption peak with respect to the emission peak (Franck-Condon shift). The magnitude of this shift in semiconductor quantum dots is still unknown. Here we report first-principle calculations of excited-state relaxations in small (diameter $l~2.2\mathrm{nm})$ Si nanocrystals, showing that the Franck-Condon shift is surprisingly large $(\ensuremath{\sim}60\mathrm{meV}$ for a 2.2-nm-diameter nanocrystal). The physical mechanism responsible for the Stokes shift changes abruptly around $\ensuremath{\sim}1$ nanometer in size, providing a clear demarcation between ``molecules'' and ``nanocrystals.''

Variation with Temperature of the Continuous Absorption Spectrum of Diatomic Molecules: Part I. Experimental, The Absorption Spectrum of Chlorine

Abstract: A photographic method was used to investigate the continuous absorption spectrum of chlorine at 18\ifmmode^\circ\else\textdegree\fi{}, 168\ifmmode^\circ\else\textdegree\fi{}, 281\ifmmode^\circ\else\textdegree\fi{}, 426\ifmmode^\circ\else\textdegree\fi{}, 580\ifmmode^\circ\else\textdegree\fi{} and 765\ifmmode^\circ\else\textdegree\fi{}C. The effect of increasing the temperature is to decrease the absorption coefficient at the maximum, and to broaden the region of continuous absorption. The results were analyzed to obtain the contributions of individual vibrational levels to the total absorption. The absorption from the level ${v}^{\ensuremath{'}\ensuremath{'}}=0$ has a single maximum, that from the level ${v}^{\ensuremath{'}\ensuremath{'}}=1$ has two maxima, and it appears probable that the absorption from the level ${v}^{\ensuremath{'}\ensuremath{'}}=2$ has three maxima.

Color Centers in Additively Colored Alkali Halide Crystals Containing Alkaline Earth Ions

Abstract: The measurements of Pick on the absorption bands produced in additively colored crystals of KCl and NaCl containing additions of divalent ions are reviewed and an interpretation of the origin of these bands is presented. Specific models are given for the ${Z}_{1}$, ${Z}_{2}$, and ${Z}_{3}$ centers responsible for the absorption bands observed by Pick. These resemble models proposed by Pick, but differ in several essential points. It is concluded that the divalent ions remain atomically dispersed in KCl when the crystal is quenched to room temperature from elevated temperatures, whereas some of the divalent ions and their associated positive-ion vacancies migrate to $F$-centers during the quenching process in NaCl. The observations also suggest that the divalent ions are not very mobile in KCl until a temperature near 200\ifmmode^\circ\else\textdegree\fi{}C is attained, although the pair consisting of a positive and a negative ion vacancy becomes significantly mobile in the vicinity of 100\ifmmode^\circ\else\textdegree\fi{}C. A number of experiments are proposed which may provide a more stringent test of the models suggested.