Absorption (logic)

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

Phonon-Assisted Recombination of Free Excitons in Compound Semiconductors

Abstract: In this paper we study the band-edge optical transitions in "direct" semiconductors involving the interacting exciton-phonon system, especially the longitudinal-optical (LO)-phonon-assisted recombination of free excitons. By introducing an extended Green's-function approach to include propagation in and between the various bands in the exciton spectrum, we obtain rather directly the transition rate to lowest order in the exciton-photon coupling which contains the effects of the interactions between the exciton and lattice vibrations to all orders, and thus includes line broadening, shift, asymmetry, and all renormalizations. Under the appropriate conditions (sufficiently separated bands), this result is equivalent to an expression previously derived by Toyozawa, but has the virtue that all terms appearing in it (in particular, the asymmetry term) are given in precise and general expressions. The relation of the general result to that obtained by conventional perturbation theory is discussed. The radiative decay of free excitons by the one- LO- and two-LO-phonon-assisted processes are formulated in the framework of perturbation theory. The limitation of this approach and its relationship to that in which the exciton-photon interaction is treated more accurately are briefly discussed. It is also shown that the second-order perturbation-theory result for the one-phonon-assisted processes (both for absorption and emission), including the contributions from the full intermediate state (hydrogenic) spectrum, can be evaluated exactly in closed form. The one- and two-LO-phonon-assisted emission spectra for CdS are calculated for several temperatures up to 77\ifmmode^\circ\else\textdegree\fi{}K using only experimentally determined parameters and taking the anisotropy of the valence band into account. Except for the one-LO peak at $T=77\ifmmode^\circ\else\textdegree\fi{}$K, where the polariton effects are important, the calculated line shapes and widths, as well as the intensity ratio of peaks, are found to be in good accord with the observed spectra. The corresponding calculations for ZnO at 77\ifmmode^\circ\else\textdegree\fi{}K are in good agreement with experimental spectrum. The zero-LO exciton peak due to the one-acoustic-phonon-assisted process is calculated in weak exciton-photon coupling approximations for both the deformation potential and piezoelectric couplings. The widths of the calculated lines are orders of magnitude smaller than the observed widths, and we conclude that the difficulty lies in the use of the weak exciton-photon coupling approach.

Brillouin-Scattering Measurements on Silicon and Germanium

Abstract: This is a report of the first Brillouin scattering measurements on silicon and germanium. The measurements were made at $\ensuremath{\lambda}=6328 \mathrm{and} 4880$ \AA{} at which frequencies the optical absorption lies in the range ${10}^{4}$ to 6\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. As a result of the high absorption, the light sees only a few wavelengths' extent of the phonon, and so from the uncertainty principle the Brillouin peaks become very broad. The measured linewidths have been analyzed to give the real and imaginary part of the refractive indices. Good agreement is obtained with the values reported in the literature.

Energy Levels of Divalent Thulium in Ca F 2

Abstract: From the fluorescence and absorption spectra of Ca${\mathrm{F}}_{2}$: ${\mathrm{Tm}}^{2+}$ an energy-level scheme for the system is proposed. The cubic-crystal-field parameters are determined, and evidence is presented that the $f\ensuremath{-}f$ transitions are of magnetic-dipole origin. A fluorescent band is observed that appears to originate in closely coupled ${\mathrm{Tm}}^{2+}$---${\mathrm{Tm}}^{3+}$ pairs. The large crystal-field splitting in Ca${\mathrm{F}}_{2}$: ${\mathrm{Tm}}^{2+}$ is confirmed by the absorption spectra of its isoelectronic systems of Ca${\mathrm{F}}_{2}$: ${\mathrm{Yb}}^{3+}$ and Ca${\mathrm{F}}_{2}$: ${\mathrm{Ce}}^{3+}$.

Optical Absorption and Fluorescence of Oxygen in Alkali Halide Crystals

Abstract: Single crystals of NaCl, KCl, and KBr were grown from the melt in an oxygen atmosphere, and their optical absorption, fluorescence excitation, and fluorescence emission spectra were measured at 300\ifmmode^\circ\else\textdegree\fi{}, 77\ifmmode^\circ\else\textdegree\fi{}, and 4.2\ifmmode^\circ\else\textdegree\fi{}K. The weak absorption band caused by oxygen was the same in all three crystals, and did not vary with temperature. The band had maximum absorption at 5.0 ev and a half-width of 1.0 ev. All fluorescence excitation spectra contained a component identical to this absorption band. The fluorescence emission spectra consisted of a series of peaks in the wavelength range 4000-10 000 A, with an approximately equal energy separation of 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$. At 300\ifmmode^\circ\else\textdegree\fi{}K, 12 to 15 peaks were resolved, and at 4.2\ifmmode^\circ\else\textdegree\fi{}K each of these peaks split into 4 to 6 components. From these optical results and from paramagnetic resonance experiments, it is concluded that ${\mathrm{O}}_{2}^{\ensuremath{-}}$ molecule-ions located in anion sites in the crystal are responsible for the absorption and fluorescence.

Growth of intact water ice on Ru(0001) between 140 and 160 K: experiment and density-functional theory calculations

Abstract: We report low-energy electron diffraction (LEED) and reflection absorption IR spectroscopy (RAIRS) results for water adsorption on $\mathrm{Ru}(0001)$ at temperatures between 140 and $160\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where water forms intact hydrogen bonded structures on the surface. We find that ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ adsorption show identical behavior, with no evidence for a structural isotope effect. At low coverage LEED shows a diffuse $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ pattern, which becomes sharp and intense only as the coverage reaches 0.6 to 0.67 monolayer. The LEED pattern becomes broadened and diffuse as the surface saturates with a coverage of 0.76 monolayer. In RAIRS the low-frequency bands associated with the out of plane libration of hydrogen bonded water appear at low coverage, with the water stretch and scissors bands appearing as broad bands only as the coverage is increased. Water adsorbs flat on $\mathrm{Ru}(0001)$ at low coverage, forming small clusters which buckle to create an extended, hydrogen bonded structure only as the adlayer is completed. The free $\mathrm{OH}(\mathrm{OD})$ stretch band appears only as the monolayer approaches completion, indicating flat or H-down adsorption up to 0.67 monolayer with H-up water appearing as the $\sqrt{3}$ structure compresses. Density functional calculations at low coverage find that water forms stable clusters with water adsorbed flat on the surface. Calculations for a complete 0.67 monolayer structure find water adsorbed near the Ru atop site in a hydrogen bonded honeycomb network, containing chains of ``flat'' lying water, linked by upright chains bonded ``H down'' in the hexagonal, hydrogen bonded superstructure. This structure is $\ensuremath{\sim}20%$ more stable than the conventional ice bilayer structure and is expected to wet the $\mathrm{Ru}(0001)$ surface. We propose a model in which disordered, short chains of ``flat'' and H-down water are imbedded in a honeycomb network of hydrogen bonded water, which imposes long-range order on the adlayer but allows substantial local disorder, and discuss the agreement with existing experimental results.