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Showing papers on "Absorption (logic) published in 1980"


Journal ArticleDOI
TL;DR: In this paper, inverse bremsstrahlung results in a non-Maxwellian velocity distribution for which the absorption is reduced by up to a factor of 2 compared with the Maxwellian distribution usually assumed.
Abstract: When $\frac{Z{{v}_{0}}^{2}}{{{v}_{e}}^{2}}\ensuremath{\gtrsim}1$, inverse bremsstrahlung results in a non-Maxwellian velocity distribution for which the absorption is reduced by up to a factor of 2 compared with the Maxwellian distribution usually assumed. Transport and atomic processes are also altered. Especially in materials with $Z\ensuremath{\gg}1$, this is significant at lower intensities than for the well-known nonlinearity for which the measure is $\frac{{{v}_{0}}^{2}}{{{v}_{e}}^{2}}$.

288 citations


Journal ArticleDOI
B. Lengeler1, P. Eisenberger1
TL;DR: In this article, a procedure for determining interatomic distances, coordination numbers, and mean relative displacements from extended x-ray absorption fine structure (EXAFS) data on disordered metallic systems is described.
Abstract: A procedure is described for determining interatomic distances $r$, coordination numbers $N$, and mean relative displacements $\ensuremath{\sigma}$ from extended x-ray absorption fine structure (EXAFS) data on disordered metallic systems. This procedure includes the case when the spectrometer has finite energy resolution. It is shown how the absolute value of the EXAFS can be determined. A way has been developed to determine the inelastic loss factors of the photoelectrons from model compounds of known structure. The procedure has been checked on four metallic systems (Cu, $\ensuremath{\alpha}\ensuremath{-}\mathrm{Al}\mathrm{Cu}$-AlCu, ${\ensuremath{\theta}}^{\ensuremath{'}}$ and $\ensuremath{\theta}$ AlCu). The accuracy of the analysis is 1% for the interatomic distances, 15% for the coordination numbers, and 20% for $\ensuremath{\sigma}$. The reliability of the data analysis, which depends strongly on the number of different atoms in a shell, on the magnitude of their mean relative displacement and on the difference in their backscattering power will be discussed at length.

285 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined the discrete transitions corresponding to the atomic 3d, 4s, 4p, 5s, \mathrm{and} 5p$ transitions and the corresponding shakeup processes for Cu atom and for a Cu(ii) model system.
Abstract: As a first step in the study of the structure at x-ray absorption edges, we have examined the discrete transitions corresponding to the atomic $1s\ensuremath{\rightarrow}3d, 4s, 4p, 5s, \mathrm{and} 5p$ transitions and the corresponding shakeup processes for Cu atom and for a Cu(ii) model system, Cu${\mathrm{Cl}}_{2}$. For Cu${\mathrm{Cl}}_{2}$ the lowest strong transitions have the character $1s\ensuremath{\rightarrow}4p$ ($f=0.00133$). About 7.5 eV lower is a group of transitions involving $1s\ensuremath{\rightarrow}4p$ simultaneous with ligand-to-metal shakedown. About 18.7 eV below the main peak is a weak (65 times weaker) quadrupole-allowed transition corresponding to $1s\ensuremath{\rightarrow}3d$ (i.e., $1{s}^{2}3{d}^{9}\ensuremath{\rightarrow}1{s}^{1}3{d}^{10}$). These results are in agreement with typical assignments of x-ray absorption spectra of Cu(ii) systems except that the middle transition is usually assigned as $1s\ensuremath{\rightarrow}4s$, whereas we find this transition to be $1s\ensuremath{\rightarrow}4p$ plus shakedown. (Transitions of the character $1s\ensuremath{\rightarrow}4s$ are calculated but have intensities far too low to have been observed.)

207 citations


Journal ArticleDOI
TL;DR: In this paper, a modified version of the EXAFS equation with many-body effects was proposed to obtain more accurate structure determination when standards used for comparison do not closely approximate the unknown.
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.

153 citations


Journal ArticleDOI
TL;DR: In this paper, a quasi-Ge model analysis with exciton corrections was applied to the interband magneto-optical absorption in the Faraday and Voigt configurations.
Abstract: Interband magneto-optical absorption in the Faraday and the Voigt configurations has been studied near liquid-He temperature in ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As grown by liquid-phase epitaxy on InP. Using a quasi-Ge model analysis with exciton corrections, the interband data has yielded ${E}_{g}=0.813\ifmmode\pm\else\textpm\fi{}0.001$ eV, ${m}_{0}(\frac{1}{{m}_{c}}+\frac{1}{{m}_{\mathrm{hh}}})=26.5\ifmmode\pm\else\textpm\fi{}0.5$, ${m}_{0}(\frac{1}{{m}_{c}}+\frac{1}{{m}_{\mathrm{lh}}})=44.0\ifmmode\pm\else\textpm\fi{}1.0$, and ${\ensuremath{\gamma}}_{3}\ensuremath{-}{\ensuremath{\gamma}}_{2}=0.7\ifmmode\pm\else\textpm\fi{}0.2$. Using ${E}_{p}=25.3$ eV and $\ensuremath{\Delta}=0.36$ eV (from linear interpolation between InAs and GaAs) and $\frac{{m}_{c}}{{m}_{0}}=0.041$ (from earlier intraband measurements), the present interband data has yielded a set of parameters for the quasi-Ge model; in particular, ${m}_{\mathrm{hh}}\ensuremath{\simeq}0.47{m}_{0}$ and ${m}_{\mathrm{lh}}\ensuremath{\simeq}0.050{m}_{0}$. A precise determination of ${E}_{p}$ requires a direct measurement of $\ensuremath{\Delta}$ and ${g}_{c}$.

82 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured the relaxation of photoinduced optical absorption following pulsed laser excitation and found that the diffusion coefficient of the excess carriers is time dependent, in agreement with the drift mobility of photocarriers and the predictions of the continuous-time random-walk theory of dispersive transport in disordered materials.
Abstract: Relaxation of photoinduced optical absorption following pulsed laser excitation was measured between 0.5 \ensuremath{\mu}s and 10 ms in doped and undoped $a$-Si: H as a function of temperature. The recombination was found to be bimolecular diffusion limited. The diffusion coefficient of the excess carriers is time dependent ($\ensuremath{\sim}{t}^{\ensuremath{-}0.3}$) in agreement with the drift mobility of photocarriers and the predictions of the continuous-time random-walk theory of dispersive transport in disordered materials.

79 citations


Journal ArticleDOI
TL;DR: In this paper, the van der Waals force between two different semi-infinite dielectric media, separated by a region of vacuum of nominal thickness, was calculated when the surface of one of the two media is rough.
Abstract: We have calculated the van der Waals force between two different, semi-infinite dielectric media, separated by a region of vacuum of nominal thickness $l$, when the surface of one of the two media is rough. The calculation is carried out by the method of van Kampen, Nijboer, and Schram [Phys. Lett. 26A, 307 (1968)] to yield the force at the absolute zero of temperature in the regime $l\ensuremath{\ll}\ensuremath{\lambda}$, where $\ensuremath{\lambda}$ is the smaller of the principal absorption wavelengths of the two dielectric media. The result obtained has the form $f(l)=\ensuremath{-}\frac{{C}_{3}}{{(\frac{l}{a})}^{3}}\ensuremath{-}\frac{{\ensuremath{\delta}}^{2}}{{a}^{2}}\left(\frac{{C}_{4}}{{(\frac{l}{a})}^{4}}+\frac{{C}_{5}}{{(\frac{l}{a})}^{5}}+\ensuremath{\cdots}\right)+O\left(\frac{{\ensuremath{\delta}}^{4}}{{a}^{4}}\right)$ in the limit $\frac{l}{a}$ is large. Here $a$ is the transverse correlation length, the mean distance between consecutive peaks and valleys on the rough surface, while $\ensuremath{\delta}$ is the root-mean-square departure of the surface from flatness. Explicit expressions have been obtained for the coefficients ${C}_{4}$ and ${C}_{5}$, and numerical estimates of the magnitude of the roughness-induced contribution to the van der Waals force are obtained in the case that the two media are the same, and their common dielectric constant is given by $\ensuremath{\epsilon}(\ensuremath{\omega})=1\ensuremath{-}(\frac{{{\ensuremath{\omega}}_{p}}^{2}}{{\ensuremath{\omega}}^{2}})$, where ${\ensuremath{\omega}}_{p}$ is a plasma frequency. It is found that surface roughness increases the magnitude of the van der Waals force over its value when the surfaces of both media are flat.

55 citations


Journal ArticleDOI
TL;DR: In this article, absorption and extended-fine-structure studies on the carbon $K$ edge in micro-crystalline, poly-crystaline, and oriented crystalline (monochromator) graphite were performed.
Abstract: We report absorption and extended-fine-structure studies on the carbon $K$ edge in microcrystalline, polycrystalline, and oriented crystalline (monochromator) graphite. On the basis of a density-of-states analysis, strong excitonic effects about the core hole are inferred. The fine structure at the absorption edge is found to differ for the three materials. This is explained by the orientational dependence of the polarization selection rules for electric dipole transitions from an $s$ core to $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ conduction-band states. The extended x-ray fine structure (EXAFS) above the edge exhibits similar oscillations for the bulk crystalline samples (polycrystalline and monochromator graphite), but considerably weaker structure is observed for the microcrystalline sample. A Fourier analysis of the oscillations yields the nearest-neighbor shell separations. Finally, it is shown that the intensity modulations caused by carbon contamination on optical Au and Pt mirror surfaces that are exposed to high-intensity synchrotron radiation closely resemble those above the C $K$ edge in bulk crystalline graphite. This indicates the formation of graphitic overlayers on such mirrors even under ultrahigh-vacuum (1 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ Torr) conditions. These findings furthermore demonstrate the feasibility of EXAFS studies of adsorbates on surfaces by monitoring the (totally) reflected radiation off the substrate.

54 citations


Journal ArticleDOI
TL;DR: In this article, the appearance-potential fine structure was analyzed above the threshold for electron-bombardment excitation of the O $1s$ core, and it was shown that the outgoing electron has angular momentum $l = 0, allowing simple Fourier inversion of the fine structure.
Abstract: To measure O-Al separation at Al(100) surfaces disordered (low-energy electron-diffraction beams extinguished) by reaction with oxygen, the extended appearance-potential fine structure was analyzed above the threshold for electron-bombardment excitation of the O $1s$ core. Calculation shows that the outgoing electron has angular momentum $l=0$, allowing simple Fourier inversion of the fine structure. The separation, 1.98\ifmmode\pm\else\textpm\fi{}0.05 \AA{}, suggests that oxygen lies under the top layer, a result undetectable in extended-x-ray-absorption fine-structure measurements on thicker films.

53 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of velocity-changing collisions on two-photon and stepwise absorption line shapes were investigated using the phenomenological Keilson-Storer or the classical hardsphere collision kernel.
Abstract: We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited $3{P}_{\frac{1}{2}}$ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-St\"orer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the $3{P}_{\frac{1}{2}}$ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium $3{P}_{\frac{1}{2}}$ atoms and helium, neon, and krypton perturbers.

39 citations


Journal ArticleDOI
K. Nakai1, T. Kobayashi1, T. Numao1, Tatsuo Shibata1, J. Chiba1, K. Masutani1 
TL;DR: In this paper, cross sections for true absorption by nuclei (Al, Ti, Cu, Sn, Au) in the energy range from 20 to 280 MeV were measured by a new method of detecting nuclear rays following the reaction.
Abstract: Cross sections for true absorption by nuclei (Al, Ti, Cu, Sn, Au) of pions (${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) in the energy range from 20 to 280 MeV were measured by a new method of detecting nuclear $\ensuremath{\gamma}$ rays following the reaction The incident-energy dependence of the cross sections in the low-energy region (${T}_{\ensuremath{\pi}}l50$ MeV) was well reproduced by an optical-model calculation, while the higher-energy part seems to indicate complex mechanisms of pion absorption

Journal ArticleDOI
TL;DR: In this article, the direct and indirect exciton gap energies in GaSe 1-x S x mixed crystals were analyzed using optical absorption and luminescence excitation spectra.
Abstract: We have investigated the quantitative relationship between the polytypes and the direct and indirect exciton gap energies in GaSe 1- x S x mixed crystals. The X-ray diffraction analysis shows that GaSe 1- x S x is the e -type for 0(GaSe)\({\leq}x{\lesssim}0.01\), the γ-type for \(0.05{\lesssim}x{\lesssim}0.4\), the β-type for \(0.5{\lesssim}x{\leq}1\)(GaS), and a mixture of the e - and γ-types for \(0.01{\lesssim}x{\lesssim}0.03\). The direct and indirect exciton gap energies are determined from optical absorption and luminescence excitation spectra. The plots of the direct and indirect exciton gap energies ν s the compositional parameter x show kinks in the neighborhood of x =0.4. Furthermore, a jump in the indirect exciton gap energy is found at around x =0.02. These anomalies at x =0.02 and 0.4 are attributed to the e -γ and γ-β polytype transformations, respectively.

Journal ArticleDOI
TL;DR: In this article, the propagation problem associated with the forward four-wave degenerate mixing induced by the third-order non-linear susceptibility was discussed, and exact analytical solutions of two-photon resonances were presented.
Abstract: We discuss the propagation problem associated with the forward four-wave degenerate mixing induced by the third-order non-linear susceptibility \chi_{(-\omega; \omega, \omega, -\omega)}^{(3)} = \chi_{1}^{(3)} + i\chi_{2}^{(3)} . Exact analytical solutions are presented of two-photon resonances, evidencing the influence of the Kerr effect on the efficiency of the process. Around a two-photon resonance, we solve the problem of the nearly parallel propagation of two beams undergoing auto and induced two-photon absorption (TPA). The resonant interaction is studied in detail and analytical solutions are provided with operational accuracy for current experimental conditions. We show by the way that in the two-beam two-photon experiments, when the phase matching condition is fulfilled, although TPA is governed by \chi_{2}^{(3)} , the real part of \chi^{(3)} must be considered in a substantial way. All the above solutions are put in a form simple enough that the numerical computations can be performed on a programmable calculator.

Journal ArticleDOI
TL;DR: In this paper, a theoretical calculation using molecular orbitals has been made on the charge-transfer-type transition in a charge-rightarrow complex to clarify the origin of the $D$ band in KCl.
Abstract: It has been shown that the $D$ band in alkali halides containing ${s}^{2}$ ion (${\mathrm{Tl}}^{+}$, ${\mathrm{In}}^{+}$, ${\mathrm{Ga}}^{+}$, ${\mathrm{Sn}}^{2+}$) is composed of three bands (named ${D}_{1}$, ${D}_{2}$, ${D}_{3}$ in the order of increasing energy). Of the three bands, the ${D}_{1}$ band is the weakest whereas the ${D}_{2}$ band is the strongest. In KCl: ${\mathrm{Tl}}^{+}$, the ${D}_{1}$ band is observed at about 6.72 eV whereas the ${D}_{2}$ band is observed at about 7.23 eV. A theoretical calculation using molecular orbitals has been made on the charge-transfer-type transition ${({e}_{g})}^{4}\ensuremath{\rightarrow}{({e}_{g})}^{3}({t}_{1u}^{*})$ in a ${(\mathrm{T}\mathrm{l}{\mathrm{Cl}}_{6})}^{5\ensuremath{-}}$ complex to clarify the origin of the $D$ band in KCl: ${\mathrm{Tl}}^{+}$. The configuration interaction with $({a}_{1g}^{*})({t}_{1u}^{*})$ configuration responsible for the $A$, $B$, and $C$ bands is taken into account in the calculation. Three absorption bands are theoretically derived; one is located at 7.74 eV, the others at 7.28 and 6.71 eV. It is shown that the latter two bands correspond well to the observed ${D}_{2}$ and ${D}_{1}$ bands, respectively. It is also demonstrated that the theoretically derived 7.74-eV band is not observed experimentally being hidden under the much stronger exciton band. The present calculation also gives a satisfactory agreement with experiment for the $A$ and $C$ band positions.

Journal ArticleDOI
TL;DR: In this paper, exact analytic solutions for the mean field equation connecting the input and the output fields are presented, and the authors demonstrate that the mean-field equation follows naturally from these solutions.
Abstract: The problem of dispersive optical bistability has so far been treated only in the mean-field approximation. A rigorous justification of the mean-field theory can only be obtained from exact solutions of the steadystate Maxwell-Bloch equations which retain the spatial dependence of the field. In this paper exact analytic solutions are presented for these equations. The authors demonstrate that the mean-field equation connecting the input and the output fields follows naturally from these solutions in the limits $T\ensuremath{\rightarrow}0$, ${\ensuremath{\delta}}_{F}\ensuremath{\rightarrow}0$, and $\ensuremath{\alpha}L\ensuremath{\rightarrow}0$ for the mirror transmission coefficient, the detuning of the field from the cavity resonance, and the linear absorption, respectively, with $\frac{\ensuremath{\alpha}L}{2T}$ and $\frac{{\ensuremath{\delta}}_{F}L}{2cT}$ remaining finite. The results are illustrated with the help of graphs showing the output versus input intensity for different values of the relevant parameters. The effect of these parameters on the phase shift of the output field is also displayed.

Journal ArticleDOI
TL;DR: In this paper, the effects of the periodic crystal potential are included by second-order perturbation theory within the self-consistent field approximation for covalent semiconductors.
Abstract: The macroscopic dielectric response function ${\ensuremath{\epsilon}}_{M}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}, \ensuremath{\omega})$ is calculated analytically for covalent semiconductors in the high-frequency limit. The effects of the periodic crystal potential are included by second-order perturbation theory within the self-consistent-field approximation. A study of the plasmon line shape, i.e., of the energy-loss function $\mathrm{Im}[\ensuremath{-}\frac{1}{{\ensuremath{\epsilon}}_{M}}(0, \ensuremath{\omega})]$ and of the optical absorption $\mathrm{Im}{\ensuremath{\epsilon}}_{M}(0, \ensuremath{\omega})$ demonstrates the importance of local-field corrections in covalent semiconductors. In the long-wavelength limit, the theory predicts that the unknown effective pseudopotential form factors ${U}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{G}}}$ for reciprocal-lattice vectors with $|\stackrel{\ensuremath{\rightarrow}}{\mathrm{G}}|\ensuremath{\ge}|{\stackrel{\ensuremath{\rightarrow}}{\mathrm{G}}}_{400}|$ may be obtained successively for increasing $|\stackrel{\ensuremath{\rightarrow}}{\mathrm{G}}|$ and increasing excitation energies $\ensuremath{\hbar}\ensuremath{\omega}$ from energy-loss experiments or optical measurements. Available experimental data in two cases (Si and Ge) show absorption edges near the predicted energies associated with particular ${U}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{G}}}$. For the first time, the anisotropic plasmon dispersion has been calculated for semiconductors and is compared with recent loss experiments on single crystals of Si, GaAs, and InSb. Owing to the similarity of the crystal potential in these compounds, the theory predicts the same trends for all three materials, namely: ${\ensuremath{\omega}}_{p}^{[100]}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})g{\ensuremath{\omega}}_{p}^{[110]}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})g{\ensuremath{\omega}}_{p}^{[111]}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$. For Si and GaAs this is in agreement with experiment whereas unexpectedly the experiment shows that ${\ensuremath{\omega}}_{p}^{[110]}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})g{\ensuremath{\omega}}_{p}^{[100]}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})g{\ensuremath{\omega}}_{p}^{[111]}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$ for InSb.

Journal ArticleDOI
Tai-Chang Chiang1, D. E. Eastman1, F. J. Himpsel1, G. Kaindl1, Masakazu Aono1 
TL;DR: In this article, the post-collision interaction (PCI) in the Auger decay of the $4d$ hole was studied and a large PCI Auger shift of 1 eV was observed for collapsed $f$-wave functions.
Abstract: The nature of the final $f$-wave functions, i.e., spatially collapsed versus uncollapsed, involved in $4d\ensuremath{\rightarrow}4f$, $\ensuremath{\epsilon}f$ giant absorption resonances in ${\mathrm{I}}^{\ensuremath{-}}$, Xe, and ${\mathrm{Cs}}^{+}$ is revealed by studying the post-collision interaction (PCI) in the ${N}_{4,5}{O}_{2,3}{O}_{2,3}$ Auger decay of the $4d$ hole. While small PCI Auger shifts \ensuremath{\lesssim}0.2 eV are observed for ${\mathrm{I}}^{\ensuremath{-}}$ and Xe with uncollapsed $f$-wave functions, a giant PCI Auger shift of \ensuremath{\sim} 1 eV is observed for ${\mathrm{Cs}}^{+}$ with collapsed $f$-wave functions.

Journal ArticleDOI
TL;DR: In this paper, optical absorption measurements on very pure CuCl single crystals, prepared by a novel process, show the exponential drop of the direct optical absorption edge to continue below an absorption coefficient.
Abstract: Optical absorption measurements on very pure CuCl single crystals, prepared by a novel process, show the exponential drop of the direct optical absorption edge to continue below an absorption coefficient $K\ensuremath{\sim}{10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. This rules out unambiguously the existence of an indirect-gap absorption below 3 eV, which is not only consistent with band-structure calculations but also excludes pure CuCl from being a suitable candidate for intrinsic high-${T}_{c}$ superconductivity (on the grounds of existing theories).

Journal ArticleDOI
TL;DR: In this paper, the authors measured self-absorption and photon scattering measurements to determine the radiative width of nine energy levels in the BN and elemental B. The influence of the energy levels on the deduced values of the radioregressive widths of BN was discussed.
Abstract: The widths of nine energy levels in $^{11}\mathrm{B}$ at 2.125, 4.445, 5.021, 6.743, 6.793, 7.286, 7.978, 8.559, and 8.920 MeV were determined by carrying out self-absorption and photon scattering measurements. The temperature variation of the scattering cross section was measured at $T=298$ K and $T=730$ K to study in some detail the effective temperature ${T}_{e}$ of $^{11}\mathrm{B}$ in ${\mathrm{B}}_{4}$C, BN, and elemental B. The influence of ${T}_{e}$ on the deduced values of the radiative widths of $^{11}\mathrm{B}$ is discussed.

Journal ArticleDOI
TL;DR: In this paper, the onset of strongly inhibited electron thermal conduction during the rise of the laser intensity was found to increase with the target atomic number (Z) of the x-ray pulses.
Abstract: Temporally resolved sub-kiloelectronvolt x-ray emission pulses have been obtained from disk targets illuminated with 1.06-\ensuremath{\mu}m wavelength laser light at 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$. Striking variations in the x-ray pulse shape from Be, Ti, Sn, Au, and U targets indicate the onset of strongly inhibited electron thermal conduction during the rise of the laser intensity. The laser intensity threshold for this strong inhibition is found to increase with target atomic number $Z$.

Journal ArticleDOI
TL;DR: In this paper, the two-photon absorption in germanium using a Kane band model is reported. But it is essential that the split-off band be included as an intermediate state and that the nonparabolicity of the conduction band be treated correctly.
Abstract: Calculations are reported of the two-photon absorption in germanium using a Kane band model. All bands within 10 eV of the valence-band edge are considered. The matrix elements are evaluated using $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ theory to find the relevant wave functions. The spherical model of Baldereschi and Lipari is used to treat the distinction between the light-hole and heavy-hole bands properly. Detailed calculations are made to test various approximations that simplify the evaluation of these complicated nonlinear optical coefficients. For Ge it is essential that the split-off band be included as an intermediate state and that the nonparabolicity of the conduction band be treated correctly. The results of the most complete calculation are in good agreement with experimental results.

Journal ArticleDOI
N. Sclar1
TL;DR: In this article, the authors compared the performance of IR extrinsic and intrinsic photodetectors and showed that the intrinsic detectors achieve a closer approach to the theoretical limit than do the extrinsics.
Abstract: The temperature limitations for IR extrinsic and intrinsic photodetectors axe theoretically studied by comparing the detectivity limitations. This comparison indicates that the temperature superiority of the intrinsic detector is identified with the inequality B_{E}/\sigma_{E} > B_{I}/(\alpha_{I}/\sqrt{N_{C}N_{V}}) where B E , B I are the respective recombination coefficients of the extrinsic and intrinsic detectors and σ E and \alpha_{I}/\sqrt{N_{C}N_{V}}) are the respective optical absorption cross sections. The highest operational temperature for a given background radiation level for each species is established when the recombination coefficients are radiative limited. For the same spectral cutoff wavelength, refractive index and quantum efficiency, they will then have identical operational temperatures. Comparison with experiment shows that the intrinsic detectors achieve a closer approach (because of the inequality) to the theoretical limit than do the extrinsics. The prospects for achieving a closer approach to this limit with the extrinsics are discussed.

Journal ArticleDOI
TL;DR: In this article, the optical absorption of Ni-based dilute alloys was analyzed by a sensitive differential technique and the difference in the optical conductivity between the alloy and pure Ni was classified into two groups: sensitive to impurity species and not.
Abstract: A study of the optical absorption of Ni-based dilute alloys ($\mathrm{Ni}\mathrm{Co}$, $\mathrm{Ni}\mathrm{Fe}$, $\mathrm{Ni}\mathrm{V}$, $\mathrm{Ni}\mathrm{Ti}$) by a sensitive differential technique is presented. The experiment is analyzed for the difference in the optical conductivity between the alloy and pure Ni. The structures observed in the differential optical conductivity are classified into two groups; one is sensitive to impurity species and the other is not. The former is discussed in terms of the Friedel virtual-bound-state model of transition-metal-based dilute alloys. The latter is found to be described in terms of the electronic band structure of ferromagnetic nickel as arising from disorder-induced wave-vector-nonconserving optical-absorption processes. Based on these results we also present a reinterpretation of our previous $\mathrm{Ni}\mathrm{Cu}$ data. The data appear to be consistent with the coherent-potential-approximation calculations of the density of states for Ni-rich alloys. Peaks at $\ensuremath{\hbar}\ensuremath{\omega}=1$ eV ($\mathrm{Ni}\mathrm{Fe}$) and $\ensuremath{\hbar}\ensuremath{\omega}=3$ eV ($\mathrm{Ni}\mathrm{Cu}$) are interpreted in terms of impurity density-of-states peaks 1 eV above, and 3 eV below the Fermi level, respectively.

Journal ArticleDOI
TL;DR: The measured cross sections can be described using parameters extrapolated from the Sn region in a systematic way, except for a large increase required for the depth of the absorptive potential.
Abstract: The ($p$,$p$) and ($p$,$n$) cross sections were measured to accuracies of \ifmmode\pm\else\textpm\fi{}2% and \ifmmode\pm\else\textpm\fi{}3%, respectively, for 2.0- to 6.7-MeV protons on $^{107,109}\mathrm{Ag}$ and $^{115}\mathrm{In}$. Hauser-Feshbach calculations, which included $\ensuremath{\gamma}$-ray emission channels, were used to convert the ($p$,$n$) cross sections to proton absorption cross sections. Analysis of the ($p$,$p$) and deduced proton absorption cross sections were made simultaneously using a conventional optical-model potential. The measured cross sections can be described using parameters extrapolated from the Sn region in a systematic way, except for a large increase required for the depth of the absorptive potential.NUCLEAR REACTIONS $^{107,109}\mathrm{Ag}$ and $^{115}\mathrm{In}$ ($p$,$p$) and ($p$,$n$), $E=2.0 \mathrm{to} 6.7$ MeV, resolution 30 to 70 keV, enriched targets. Measured ($\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}$) ($p$,$p$) at 135\ifmmode^\circ\else\textdegree\fi{} and 165\ifmmode^\circ\else\textdegree\fi{}, and total $\ensuremath{\sigma}(p,n)$. Observed analog resonances. Statistical-model and optical-model analyses, deduced model parameters.

Journal ArticleDOI
TL;DR: In this article, a theory for the nuclear-magnetic-resonance (NMR) absorption of chemically interacting species is derived which is valid when the correlation time for the molecular motion, which gives rise to NMR relaxation, is less than the chemical exchange time.
Abstract: A theory for the nuclear-magnetic-resonance (NMR) absorption of chemically interacting species is derived which is valid when the correlation time ${\ensuremath{\tau}}_{c}$ for the molecular motion\char22{}which gives rise to NMR relaxation\char22{}is less than ${\ensuremath{\tau}}_{e}$, the chemical exchange time. The previous theory is valid only if ${\ensuremath{\tau}}_{c}$ were much less than ${\ensuremath{\tau}}_{e}$. This latter theory assumed that the relaxation was defined using the usual Fermi Golden Rule form where the time integration is from zero to infinity. When ${\ensuremath{\tau}}_{c}$ begins to approach ${\ensuremath{\tau}}_{e}$, it becomes necessary to replace the $0\ensuremath{-}\ensuremath{\infty}$ time integration by a $0\ensuremath{-}t$ time integration where $t$ is the time of the next exchange collision. This modification of the previous theory leads to a correction to the previous theory for which the lowest-order term goes as $\frac{{\ensuremath{\tau}}_{c}}{{\ensuremath{\tau}}_{e}}$.

Journal ArticleDOI
TL;DR: Optical absorption and magnetic-circular-dichroism (MCD) spectra of off-centered and on-centered MCD spectra were investigated at temperatures from 80 to 300 K in this paper.
Abstract: Optical-absorption and magnetic-circular-dichroism (MCD) spectra of off-centered ${\mathrm{Cu}}^{+}$ in KI, KBr, and KC1 crystals and of on-centered ${\mathrm{Cu}}^{+}$ in NaC1 crystals are investigated at temperatures from 80 to 300 K In KI: ${\mathrm{Cu}}^{+}$ an absorption band due to the ${d}^{10}\ensuremath{\rightarrow}{d}^{9}p$ allowed transition is observed in addition to six absorption bands due to the ${d}^{10}\ensuremath{\rightarrow}{d}^{9}s$ forbidden transition Different MCD line shape and temperature dependence are observed between the off-centered ${\mathrm{Cu}}^{+}$ and on-centered ${\mathrm{Cu}}^{+}$ A discussion is given on the level assignment for the ${\mathrm{Cu}}^{+}$ absorption bands observed in KI

Journal ArticleDOI
TL;DR: In this article, the authors present an energy-level scheme and Fermi-level assignment for CVT-grown cinnabar which accounts for the photoluminescence (PL) features.
Abstract: In cinnabar ($\ensuremath{\alpha}$-HgS) grown by chemical vapor transport (CVT), four photoluminescence (PL) features, ${X}_{1}$, ${X}_{2}$, ${B}_{1}$, and ${B}_{2}$, are observed with below band-gap photoexcitation. Two of these PL features, ${X}_{1}$ at 1.873 eV and ${X}_{2}$ at 1.855 eV, are sharp, with half-widths of 3.7 and 6.6 meV, respectively, and two of them, ${B}_{1}$ at 2.19 eV and ${B}_{2}$ at 1.78 eV, are broad, with half-widths of \ensuremath{\sim} 100 meV. New photoluminescence, optical absorption, and PL excitation measurements are reported on this material. The details observed in these measurements, together with transport results, enable presentation of an energy-level scheme and Fermi-level assignment for CVT-grown cinnabar which accounts for the ${X}_{1}$, ${X}_{2}$, ${B}_{1}$, and ${B}_{2}$ PL features. In addition to the valence- and conduction-band continua, five energy states are postulated, three levels nearer to the conduction band and two levels nearer to the valence band. The binding energies of the former are 0.005, 0.402, and 0.420 eV, and those of the latter are 0.25 and 0.05 eV. Based upon PL excitation spectra and optical absorption, ${\ensuremath{\epsilon}}_{F}$, the equilibrium Fermi energy, is placed in the range $1.855\ensuremath{\lesssim}{\ensuremath{\epsilon}}_{F}\ensuremath{\lesssim}1.873$ eV. In order to explain the shape of the optical absorption and PL excitation spectra, matrix-element effects are examined for a very simple band-to-impurity absorption process. The two PL features ${X}_{1}$ and ${X}_{2}$ can be interpreted as bound-to-free transitions which are weakly coupled to lattice vibrations, whereas the ${B}_{1}$ and ${B}_{2}$ features can be interpreted as bound-to-bound transitions strongly coupled to lattice vibrations. The temperature dependence of the ${X}_{1}$ and ${B}_{2}$ peak intensities is discussed in terms of a configuration coordinate model. The linear broadening of the ${X}_{1}$ and ${X}_{2}$ peaks with temperature above \ensuremath{\sim} 20 K is attributed to interactions with phonons.

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TL;DR: In this article, the authors observed quantum-electromagnetic waves in pure bismuth samples and observed the quantum nature of these waves follows from the oscillating behavior of the microwave absorption versus magnetic field $H$ displays the extremal cross-sectional areas of the Fermi surface.
Abstract: We have observed quantum-electromagnetic waves in pure bismuth samples. The quantum nature of these waves follows from the following: (1) the oscillating behavior of the microwave absorption versus magnetic field $H$ displays $\ensuremath{\Delta}(\frac{1}{H})$ periods related to the extremal cross-sectional areas of the Fermi surface, (2) for $\stackrel{\ensuremath{\rightarrow}}{H}$ nearly orthogonal to the wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$, the amplitude of the oscillations diminishes and the periods are related to the nonextremal areas. The peculiar physical aspects of this phenomenon are that the cross-sectional areas are the largest ones in bismuth and that resonances are missing below a certain value of the Landau quantum number.

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TL;DR: In this paper, the authors analyzed pion elastic-scattering data for $40 and $48 µm using a covariant theory of the optical potential developed previously and found that the second-order potential exhibits a smooth dependence on energy and target mass number.
Abstract: We analyze pion elastic-scattering data for $^{40}\mathrm{Ca}$ and $^{48}\mathrm{Ca}$ using a covariant theory of the optical potential developed previously Combining these results with the information obtained from the analysis of pion scattering from $^{12}\mathrm{C}$ and $^{16}\mathrm{O}$ reported earlier, we are able to discuss the systematics of the pion-nucleus optical potential Our model contains a first-order optical potential which is obtained from a parameter-free calculation The parameters of a second-order potential are determined by requiring that the sum of the first-and second-order potentials provides a fit to the elastic-scattering data The parameters of the second-order potential exhibit a smooth dependence on energy and target mass number These parameters have a marked resonance behavior We find that the maximum value of the magnitude of the imaginary part of the second-order potential occurs at about 150 MeV while the first-order potential has a maximum for the magnitude of the imaginary part at about 240 MeV The imaginary part of the first-order potential has a width at half maximum of about 200 MeV This corresponds to a kinematic broadening of the (3,3) resonance due to the effects of the Fermi motion of the target nucleons (This increased width is unrelated to the effects of the true absorption process or of collision broadening) We also find a significant isospin violation in the second-order potential For example, the imaginary part of the second-order potential for ${\ensuremath{\pi}}^{\ensuremath{-}}$ scattering is found to be systematically larger than that for ${\ensuremath{\pi}}^{+}$ scattering for nuclei with $N=Z$ An explanation for this feature of the optical potential is presented

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TL;DR: In this article, an analysis of non-annihilation channels in four-pronged π-meson interactions at 5·68 GeV/c was presented, where the transverse momenta and azimuthal angles distributions of nucleons and π -mesons were compared with phase space.
Abstract: We present an analysis of nonannihilation channels in four-pronged\(\bar pp\) interactions at 5·68 GeV/c. The transverse momenta and azimuthal angles distributions of nucleons and π-mesons are compared with phase space. No essential influence of\(\Delta ^{ + + } /\overline {\Delta ^{ + + } }\) production on azimuthal correlation has been found. The seen difference of azimuthal angle distributions for like and unlike charged pions in annihilations appears to be more pronounced for small transverse momenta and for small transverse masses. For 10971 events of the reaction ¯pp→¯ppπ−π+ we present cross sections for\(\Delta ^{ + + } ,\overline {\Delta ^{ + + } } ,\Delta ^0 ,\overline {\Delta ^0 }\) production and differential cross sections. The reaction is dominated by the strong quasi-two-body channel\(\bar pp \to \overline {\Delta ^{ + + } } \Delta ^{ + + }\) with cross section (1·66 ±±0·10) mb. We study in detail the production mechanism of this channel in terms of the absorption model.