Showing papers on "Absorption (logic) published in 1987"

Electrooptical effects in silicon

Abstract: A numerical Kramers-Kronig analysis is used to predict the refractive-index perturbations produced in crystalline silicon by applied electric fields or by charge carriers. Results are obtained over the 1.0-2.0 \mu m optical wavelength range. The analysis makes use of experimental electroabsorption spectra and impurity-doping spectra taken from the literature. For electrorefraction at the indirect gap, we find \Delta n = 1.3 \times 10^{5} at \lambda = 1.07 \mu m when E = 10^{5} V/cm, while the Kerr effect gives \Delta n = 10^{-6} at that field strength. The charge-carrier effects are larger, and a depletion or injection of 1018carriers/cm3produces an index change of \pm1.5 \times 10^{-3} at \lambda = 1.3 \mu m.

Absorption of circularly polarized x rays in iron.

Abstract: The transmission of synchrotron radiation through magnetized iron at energies above the K-absorption edge shows relative differences for right and left circular polarization of several times ${10}^{\mathrm{\ensuremath{-}}4}$. The observed spin dependence of the near-edge photoabsorption is proportional to the difference of the spin densities of the unoccupied bands. In the extended absorption region up to 200 eV above the Fermi level a small spin-dependent absorption is observed and thus is expected to give information on the magnetic neighborhood of the absorbing atom.

Not-so-resonant, resonant absorption.

Abstract: When an intense electromagnetic wave is incident obliquely on a sharply bounded overdense plasma, strong energy absorption can be accounted for by the electrons that are dragged into the vacuum and sent back into the plasma with velocities v\ensuremath{\simeq}${v}_{\mathrm{osc}}$. This mechanism is more efficient than usual resonant absorption for ${v}_{\mathrm{osc}/\mathrm{\ensuremath{\omega}}g\mathrm{L}}$, with L being the density gradient length. In the very high-intensity ${\mathrm{CO}}_{2}$-laser--target interaction, this mechanism may account for most of the energy absorption.

Effect of grain boundaries on the Raman spectra, optical absorption, and elastic light scattering in nanometer-sized crystalline silicon

Abstract: The intensity of the Raman-active ${\ensuremath{\Gamma}}_{25\mathcal{'}}$ mode of nanometer-sized crystalline silicon, nc-Si, normalized to that of calcium fluoride, ${\mathrm{CaF}}_{2}$, at 322 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was measured for samples deposited under controllably varied conditions. Changes of the intensity by a factor of up to approximately 6.7 were found. These are correlated with the lattice expansion and with the compressive stress in thin films of the material. It is suggested that the enhancement of the scattering cross section, which scales with the observed optical-absorption coefficient and diffuse elastic light scattering, is due to enhanced coupling of the electromagnetic field of the incident light to the charge-density fluctuations at the grain boundaries of the quasi-isolated crystallites.

Model dielectric constants of GaP, GaAs, GaSb, InP, InAs, and InSb.

Abstract: A new method is described for calculation of the real and imaginary parts of the dielectric function of semiconductors at energies below and above the lowest band gaps, in which the model is based on the Kramers-Kronig transformation and strongly connected with the electronic energy-band structures of the medium. This model reveals distinct structures at energies of the ${E}_{0}$, ${E}_{0}$+${\ensuremath{\Delta}}_{0}$, ${E}_{1}$, ${E}_{1}$+${\ensuremath{\Delta}}_{1}$, and ${E}_{2}$ critical points. Analyses are presented for GaP, GaAs, GaSb, InP, InAs, and InSb, and results are in satisfactory agreement with the experimental information over the entire range of energies. The model is able to properly give the optical constants, such as the refractive indices and the absorption coefficients, which are important for a variety of optoelectronic device applications.

Intersubband relaxation in GaAs- Al x Ga 1 − x As quantum well structures observed directly by an infrared bleaching technique

Abstract: Direct intersubband excitations in narrow, modulation doped GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum well structures are studied by picosecond infrared spectroscopy. The bleaching of the intersubband absorption induced by a high-intensity picosecond pump pulse, is studied by a delayed probe pulse. Typical intersubband relaxation times are of the order of 10 ps at 300 K for a subband splitting of about 150 meV. Polar LO-phonon scattering is discussed as the most relevant relaxation mechanism.

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.

Excitonic absorption and superconductivity in YBa2Cu

Abstract: Optical and neutron-diffraction studies of superconducting and nonsuperconducting samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{y}}$ are described. Superconducting samples, with y\ensuremath{\approxeq}0.1, showed two strong electronic transitions, at 3000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/0.37 eV and at 20 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/2.5 eV, which are interpreted as charge-transfer bands (i.e., excitons). Nonsuperconducting samples, with y\ensuremath{\approxeq}0.8 (prepared by heating of superconducting samples in vacuum to deplete the oxygen content), did not show these features. Our measurements support an excitonic mechanism for the superconductivity in high-${T}_{c}$ copper oxide compounds.

Electronic states of BaPb1-xBixO3 in the semiconducting phase investigated by optical measurements.

Abstract: The semiconducting ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (0.35lx\ensuremath{\le}1) is investigated in detail by optical measurements. The optical spectrum shows that ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ has a clear optical gap over the whole semiconducting compositional range. It is clearly shown by resonant Raman scattering measurements that the origin of this gap is a charge-density wave (CDW) accompanied by the breathing-mode distortion. No apparent effect of Pb substitution can be observed in the reflectivity spectra for the sample with xg0.7. For x\ensuremath{\le}0.7, the CDW gap decreases with x and simultaneously the absorption tail extends into the gap. This spectral behavior can be well interpreted in terms of the recently proposed theoretical model with emphasis on a local CDW and/or a substantial energy difference between Pb and Bi sites arising from the strong electron-phonon interaction. Thus it can be concluded that the strong electron-phonon interaction plays an essential role in the semiconducting phase of this material as well as in the metallic phase.

Theory of photoabsorption in modulation-doped semiconductor quantum wells.

Abstract: A systematic study of the electronic and optical properties of modulation-doped GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells is undertaken. We consider cases in which the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As barriers are doped either n or p type and the GaAs wells are filled with a gas of free carriers. The electronic band structure, exciton binding energy, exciton oscillator strength, and interband absorption are studied as functions of well width and doping concentration. A multiband effective-mass method is used which takes coupling between heavy- and light-hole states into account. In our model for excitons we include effects of valence-subband nonparabolicity, free-carrier screening, and the k dependence of optical matrix elements. The interband optical absorption is obtained using Fermi's golden rule with a correction made for the screened final-state interaction. Our theoretical results are compared with available experimental data.

X-ray absorption studies of CeO 2 , PrO 2 , and TbO 2 . I. Manifestation of localized and extended f states in the 3 d absorption spectra

Abstract: The 3d x-ray absorption spectra of ${\mathrm{CeO}}_{2}$, ${\mathrm{PrO}}_{2}$, and ${\mathrm{TbO}}_{2}$ show some interesting features. The main lines in these spectra are unusually broad. They are identified by comparison with the 3d-4f atomiclike multiplets observed in Z-1 elements. In these oxides, the positions and the similar form of a weaker structure which is systematically observed at about 3--5 eV above each main line indicate that it is due to the transitions to the extended f type of states. These observations demonstrate a specific behavior of the outermost f electron which differentiates these oxides from other intermediate-valence materials and from the Ce intermetallic compounds.

Energy-level correlation function and ac conductivity of a finite disordered system

Abstract: The space- and energy-difference- (\ensuremath{\omega}) dependent Wigner-type correlation function for the energy levels is studied for the Anderson disordered tight-binding model, including some of the effects of localization. For small metallic particles it is confirmed that these correlations, in the absence of spin and magnetic effects, are similar to those of the Gaussian orthogonal ensemble. This is in agreement with the conjecture made by Gor'kov and Eliashberg and the analytical results obtained by Efetov. The results are used to obtain the effective conductivity ${\ensuremath{\sigma}}_{\mathrm{eff}(\mathrm{\ensuremath{\omega}}}$), relevant for the absorption of low-frequency electromagnetic radiation, in all regimes, including the effects of localization and screening. It is found that in the orthogonal case, ${\ensuremath{\sigma}}_{\mathrm{eff}(\mathrm{\ensuremath{\omega}}}$) does not change at low \ensuremath{\omega} when level correlations become important. The conditions to observe changes in the absorption in this regime due to changes in the ensemble symmetry are formulated. In all cases except for the microscopic (critical) one, ${\ensuremath{\sigma}}_{\mathrm{eff}(\mathrm{\ensuremath{\omega}})\mathrm{\ensuremath{\propto}}{\ensuremath{\omega}}^{2}}$, and in the latter region, ${\ensuremath{\sigma}}_{\mathrm{eff}(\mathrm{\ensuremath{\omega}})\mathrm{\ensuremath{\propto}}{\ensuremath{\omega}}^{5/3}}$. For systems much larger than the localization length, the level correlations decay in space with a length \ensuremath{\xi} ln(${\ensuremath{\Delta}}_{\ensuremath{\xi}}$/\ensuremath{\omega}), as \ensuremath{\omega}\ensuremath{\rightarrow}0 (${\ensuremath{\Delta}}_{\ensuremath{\xi}}$ is the average spacing between levels within a localization volume). The connection with Mott's calculation of the ac conductivity in the insulating phase is made. It is shown that these ideas explain the ``level attraction'' found in one dimension by Gor'kov, Dorokhov, and Prigara. These considerations are generalized to the evaluation of the space-dependent level correlation function in an arbitrary dimension.

The one-dimensional nonlinear heat equation with absorption: regularity of solutions and interfaces

Abstract: We consider the equation $u_t = (u^m )_{xx} - \lambda u^n $ with $m > 1$, $\lambda > 0$, $n \geqq m$ as a model for heat diffusion with absorption. Hence we assume that $u \geqq 0$ for $x \in \mathbb{R}$, $t \geqq 0$. We study the regularity of the solution to the Cauchy problem for this degenerate parabolic equation. When the initial datum $u_0 (x)$ is positive only in a part of the space $\mathbb{R}$, we also study the regularity of the free boundaries that appear. The asymptotic behavior of solutions and free boundaries is also discussed.

Electronic structure, defect states, and optical absorption of amorphous Si 1 − x N x [0≤x/(1-x)≤2]

Abstract: We present an extensive study of the electronic properties of amorphous silicon nitride for a wide range of N concentrations. Local densities of states on Si and N for the ``ideal'' random network structure, defect states introduced into the semiconducting gap by both Si and N dangling bonds, the effect of wrong bonds (N\char22{}N bonds below stoichometry and Si\char22{}Si bonds above stoichiometry) on the local density of states, and, the energy dependence (from 0 to 10 eV) of the imaginary part of the dielectric function have been obtained as a function of N content. The main purpose has been the development of a theoretical model for amorphous compound that were both internally consistent and in overall agreement with experiment. We stress the following main results: (i) The semiconducting gap opens continuously from pure a-Si to the stoichiometric compound a-${\mathrm{Si}}_{3/7}$${\mathrm{N}}_{4/7}$. For low N content the gap widening is linear in x/(1-x) whereas, just before stoichiometry is reached, the gap opens very sharply to its maximum value. (ii)

Transition probabilities in neutral barium.

Abstract: We present emission measurements on a hollow-cathode discharge containing barium that were taken with the aid of a Fourier-transform spectrometer (FTS). We have determined the branching fractions for ten upper levels of the neutral barium atom. In the case of the resonance level 6s6p ${}^{1}$P\ifmmode^\circ\else\textdegree\fi{}${}_{1}$, we had to augment our FTS measures with literature data that had recently been obtained by use of different laser-excitation techniques. Using four published lifetimes and also through combining our emission measurements with relative absorption data from the literature, we could convert the branching fractions of eight upper levels into transition probabilities. The resulting A values (with accuracies ranging from 1% to 60%) give support to the correction of earlier literature data, recommended by Jahreiss and Huber [Phys. Rev. A 31, 692 (1985)].

Anomalous small-angle x-ray scattering determination of the partial structure factors and kinetic study of unmixed Cu-Ni-Fe alloys

Abstract: Different unmixed states of a Cu-Ni-Fe alloy have been studied by anomalous small-angle x-ray scattering (ASAXS). The data were obtained at different x-ray energies near the Fe and Ni absorption edges: ASAXS intensities increase by a factor of 3, varying with the atomic contrast (with respect to Cu). Various methods for the determination of partial structure factors (PSF) have been tested and results are found reliable if we use the intensity data in the proximity of at least two edges. The homoatomic PSF curves have similar shapes with a maximum ${I}_{m}$ which increases and shifts towards lower q values as the power laws ${I}_{m}$\ensuremath{\sim}${t}^{0.80}$ and ${q}_{m}$\ensuremath{\sim}${t}^{\mathrm{\ensuremath{-}}0.26}$, and the scaling behavior is obeyed. These features are very similar to the ones observed during late stages of aging of concentrated binary systems. On the other hand, there is evidence that the phase separation of Cu-Ni-${\mathrm{Fe}}_{0.15}$ at 500?deC cannot be modeled by a two-phase system, and different possible partitioning of Cu, Ni, and Fe are discussed.

Laser Monitoring in the Atmosphere

Abstract: Remote measurements of trace constituents and physical parameters in the Earth’s upper atmosphere using an active technique like the lidar have been made possible by the rapid development of powerful tunable laser sources which have opened a new field in atmospheric spectroscopy by providing sources which can be tuned to characteristic spectral features of atmospheric constituents. The interaction of a laser beam with the atmosphere is in principle dominated at all wavelengths by elastic processes i.e. Rayleigh scattering from atmospheric gas molecules and Mie scattering due to particles of different natures and shapes. By considering the simplified lidar equation written as $$ {n_r} = {n_0} \cdot \frac{{{A_0}}}{{{R^2}}}({\beta _R} + {\beta _M} + {\beta _i}) \cdot \Delta R \cdot \exp ( - 2({\tau _R} + {\tau _M} + {\tau _i})) $$ (where no, nr are the number of photons emitted and received per pulse from range R; Ao is the receiver area; β R, β M and β i are the volume backscattering coefficients by Rayleigh scattering, Mie scattering and resonant scattering by a given species i; τ R, τ M and τ i being the corresponding optical thickness for the path length R; ΔR is the range resolution), the determination of a given constituent number density Ni is then possible by two means: either β i is much larger than β R, + β M allowing the measurement of Ni based on a scattering process; or τ i is much larger or of the same order of magnitude than τ R + τ M allowing the measurement of Ni based on an absorption process.

Density effects on high-n molecular Rydberg states: CH3I in He, Ne, Ar, and Kr.

Abstract: Absorption studies of the high-n Rydberg states of ${\mathrm{CH}}_{3}$I perturbed by varying number densities of He, Ne, Ar, and Kr (up to 23.0, 24.0, 11.3, and 6.6\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, respectively) are reported. Energy shifts, which increase with increasing perturber number density, are observed and analyzed for both discrete [nd${(}^{2}$${E}_{3/2}$)] and autoionizing [nd'${(}^{2}$${E}_{1/2}$)] states. These shifts vary linearly with the perturber number density for principal quantum numbers n\ensuremath{\ge}10. Moreover, depending upon the nature of the perturber, these shifts are to the blue region (He), slightly to the blue region (or nearly zero) (Ne), or the red region (Ar and Kr). We explain these results quantitatively on the basis of the electron scattering length in the various rare gases, as well as on the polarization of the medium by ${\mathrm{CH}}_{3}$${\mathrm{I}}^{+}$. For low perturber number densities, it is well known that the Fermi model of perturber effects on high-n Rydberg states is invalid. The present experimental results show, for the first time, that this model also fails at high number densities. On the other hand, the energy shifts can be reproduced quantitatively by extending a model developed by Alekseev and Sobel'man to high number densities.

Atomic hydrogen and solar Lyman α flux deduced from STP 78‐1 UV observations

Abstract: Simultaneous observations of the Lyman {alpha} and 1,026 {angstrom} airglow by a spectrometer on the STP 78-1 satellite have been analyzed. A spherical model of the multiple scattering of solar Lyman {alpha} and Lyman {beta} radiation was utilized which includes the effect of pure absorption by O{sub 2} and a nonisothermal temperature profile. Analysis of data near 0{degrees} latitude yields an exobase temperature T{sub c} = 1,300 {+-} 200 K, exobase density N{sub c} = 2.6 {+-} 0.2 {times} 10{sup 4} cm{sup {minus}3}, and escape flux F{sub c} = 2{+-} 0.5 {times} 10{sup 8} cm{sup {minus}2} s{sup {minus}1}. The solar Lyman {alpha} line center flux determined form analysis of the Lyman {alpha} airglow was 6.0 {+-} 1.2 {times} 10{sup 11} photons cm{sup {minus}2} s{sup {minus}1} {angstrom}{sup {minus}1}. Analysis of the 1,026 {angstrom} airglow showed that this emission is dominated by multiple scattering of solar Lyman {beta} photons by atomic oxygen and that the Lyman {beta} airglow contributes less than 20% to the observed emission.

Excitation of the 1s5 and 1s4 levels of neon by low-energy electrons

Abstract: Excitation coefficients for production of 1${s}_{5}$ and 1${s}_{4}$ levels (Paschen notation) of neon in collisions with low-energy electrons have been measured using a drift-tube technique combined with laser absorption and laser-induced fluorescence techniques. The absorption and fluorescence signals have been analyzed using coupled rate equations which include the effects of population mixing among these levels in collisions with neon atoms. The excitation coefficients for the metastable and resonance levels ${\ensuremath{\alpha}}_{M}$/N and ${\ensuremath{\alpha}}_{R}$/N have been obtained as functions of the electric field to gas density ratio E/N. The measured values of ${\ensuremath{\alpha}}_{M}$/N vary from 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$ ${\mathrm{m}}^{2}$ at E/N=1.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ V ${\mathrm{m}}^{2}$ to 6.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ ${\mathrm{m}}^{2}$ at 3.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ V ${\mathrm{m}}^{2}$. The values of ${\ensuremath{\alpha}}_{R}$/N are comparable in the experimental E/N range. The experimental excitation coefficients are in agreement with values calculated from a Boltzmann analysis using a recommended set of electron excitation cross sections derived from published experimental electron beam data.

"Coulomb logarithm" for inverse-bremsstrahlung laser absorption.

Abstract: The ``Coulomb logarithm'' for inverse-bremsstrahlung laser absorption is examined for plasmas of different ionic charge, spanning the classical and quantum-mechanical limits. Previously, this term has not been calculated exactly for the conditions of interest in laser fusion experiments; it has only been estimated from physical considerations. For short-wavelength irradiation (e.g., 0.35 \ensuremath{\mu}m), uncertainties in the ``logarithmic'' factor can produce variations of 20\char21{}50 % in the laser absorption coefficient. A more exact treatment of this term is presented here. For low-Z plasmas, a modified Born approximation is used that reproduces previous results for long-range interactions that cannot be described by a single electron-ion collision, and it simultaneously treats the short-range electron-ion encounters. For high-Z plasmas, the Coulomb logarithm is calculated in terms of the classical, nonlinear electron trajectory in a self-consistent electrostatic potential; strong ion-ion correlations are treated by the nonlinear Debye-H\"uckel model. There are no indeterminate quantities in the calculations.

Analysis of models reducible to a class of diffusion processes in the positive quarter plane

Abstract: This paper is concerned with the two-dimensional generalization of the single server queue “diffusion approximation”: this model consists of a stochastic process in $\mathbb{R}^ + \times \mathbb{R}^ + $ being a diffusion in the open quarter plane with absorption on the boundaries $\{ 0 \} \times \mathbb{R}^ + \cup \mathbb{R}^ + \times \{ 0 \}$ and jumps back into this open domain. On the boundary this process is a linear diffusion with absorption on $( {0,0} )$ and jumps back into the x or y axis. This model describes within the same framework such phenomena as coupling of processors or simultaneous arrivals of jobs which do not preserve the classical “product form” solutions. First we exhibit a pathwise construction of the process which leads to the existence, the uniqueness and the Markov property. From this, we derive a functional equation for the Laplace transform of the steady state distribution. The solution of this functional equation is obtained by reduction to a boundary value problem on the righ...

High-resolution infrared spectroscopy of hydrogen impurities in strontium titanate.

Abstract: A high-resolution infrared spectroscopic investigation of structure related to the O-H dipole stretching mode (around 3500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) was carried out in ${\mathrm{SrTiO}}_{3}$. We discuss various models for the position of the hydrogen impurity in the matrix and justify the position that we have suggested in a previous paper. With this new assignment, we discuss the 16 highest intensity peaks observed at low temperature. The high-energy satellites are interpreted as arising from the absorption of hydrogen impurities located on inequivalent sites. These sites are made inequivalent by a combination of the antiferrodistortive transition at 105 K and the ferroelectric fluctuations that become important when the temperature decreases.

Determination of the PIn antisite structure in InP by optically detected electron-nuclear double resonance.

Abstract: Optically detected electron-nuclear double resonance (ODENDOR) is reported for the ${\mathrm{P}}_{\mathrm{In}}$ antisite in p-type InP. In both electron-irradiated and as-grown samples, ENDOR signals from the four nearest P neighbors and the next In shell were detected by monitoring radio-frequency-induced changes in magnetic circular dichroism associated with absorption near the band edge of InP while maintaining microwave resonance of the ${\mathrm{P}}_{\mathrm{In}}$ antisite. The electronic wave function of the ${\mathrm{P}}_{\mathrm{In}{}^{+}}$ state is highly localized, essentially all of it accounted for within these first two neighbor shells. The optical-absorption band associated with the antisite is centered above the InP band gap, and only its tail is observed.

Evolution of intermediate excitons in fluid argon and krypton.

Abstract: Reflectivity spectra of fluid argon and krypton are reported for number densities ranging from \ensuremath{\rho}\ensuremath{\sim}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ to that of the triple-point liquid, complemented by spectra of solid samples near the triple point. Dispersion analysis by a decomposition into Lorentzians of the complex dielectric constant spectra (yielding reflectivities that fit the experimental values) revealed the existence of ``extra'' absorption bands in the immediate vicinity of the $^{3}\mathrm{P}_{1}$ and $^{1}\mathrm{P}_{1}$ atomic resonance lines, growing very rapidly with density; these bands were identified as corresponding to the n=1 \ensuremath{\Gamma}((3/2)) and n'=1 \ensuremath{\Gamma}((1/2)) intermediate excitons in the solid. Comparing the evolution of the n=1 exciton in fluid krypton with density fluctuations leads to the conclusion that for the creation of this exciton a momen- tary minimum local density of ${10}^{22}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ is needed within a certain small volume (1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ ${\mathrm{cm}}^{3}$). The results are also discussed in terms of a recent theory on localization in topologically disordered systems. Results on the changes in characteristic parameters, like line separations, widths, and oscillator strengths are presented.

Excitations of a diamagnetic hole state in the copper-oxide superconductors.

Abstract: The strong infrared (ir) absorption features observed in the copper oxide superconductors are interpreted to be internal electronic and vibrational excitations of a spin-zero charge-transfer complex (${P}^{+}$) formed between a hole and an essentially single paramagnetic square planar Cu${\mathrm{O}}_{4}$ unit (${P}^{0}$). The large magnitude and bimodal frequency dependence of the electronic polarizability implied by the ir data for ${P}^{+}$ favors a combined excitonic and "charged phonon" mechanism for superconductivity. It is possible that the carriers of the superconductivity are the highly correlated electrons belonging to the ${P}^{0}'\mathrm{s}$, while the ${P}^{+}'\mathrm{s}$ play the role of highly polarizable impurities. The ${P}^{+}'\mathrm{s}$ can delocalize as band polarons.

Four-wave-mixing measurements of energy migration and radiationless relaxation processes in alexandrite crystals

Abstract: Four-wave-mixing techniques were used to establish and probe population gratings of ${\mathrm{Cr}}^{3+}$ ions in both mirror and inversion sites in ${\mathrm{BeAl}}_{2}$${\mathrm{O}}_{4}$:${\mathrm{Cr}}^{3+}$ crystals as a function of temperature between about 6 and 300 K. The four-wave-mixing signal intensity and decay rate were monitored as a function of the crossing angle of the laser ``write'' beams. The variation of the signal intensity with crossing angle is explained theoretically with use of a model based on the interaction between the laser and a two-level atomic system. Theoretical fits to the results provide information concerning the relative importance of the absorption and dispersion contributions to the signal, and the dephasing time of the atomic system. The latter is found to be 2.2 psec for the mirror site ions and 80 psec for the inversion site ions. The signal decay rate of the inversion site ions was found to be independent of crossing angle at all temperatures, while the signal decay rate for ions in the mirror site increased with increasing crossing angle at temperatures below about 150 K. This indicates the presence of long-range energy migration, and the diffusion coefficient describing this process was found to increase to about 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ ${\mathrm{cm}}^{2}$?${\mathrm{c}}^{\mathrm{\ensuremath{-}}1}$ at 6 K.

Electron correlations in the l-shell photoionization of heavy-elements

Abstract: The photoionization of the individual L subshells of $_{72}\mathrm{Hf}$, $_{74}\mathrm{W}$, $_{78}\mathrm{Pt}$, $_{79}\mathrm{Au}$, and $_{82}\mathrm{Pb}$ in the energy regime of the L edges has been studied in detail. Experimentally, the x-ray absorption spectra of thin sample foils were recorded using monochromatized synchrotron radiation. The energy dependence of the absorption is governed in its gross structure by the atomic photoionization with some superimposed oscillatory structure due to solid-state effects. The experimental data are compared to various theoretical predictions for atomic photoionization. Calculations in the framework of an independent-electron approach predict a smooth, power-law-like energy dependence. In contrast, the experimental data show small but significant deviations from such a behavior. The dispersionlike deviations are attributed to electron-correlation effects, as is confirmed by comparative calculations of photoionization with inclusion or omission of the correlations using the computer code of Liberman and Zangwill [Comput. Phys. Commun. 32, 75 (1984)]. The main influence of the correlation effects on the subshell ionization cross sections originates from the dielectric (anti-) screening of the external radiation field. Calculations in the local-density approximation for the response of the atom to the radiation field yield an even quantitative description of the experimental data in a large fraction of the investigated energy range.

Far-infrared rotational spectrum of HD: Line shape, dipole moment, and collisional interference

Abstract: The rotational transitions R(0) to R(3) in pure HD gas have been studied in absorption with 0.06-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ resolution at densities from 2 to 68 amagat at 295 K. The permanent dipole moment of HD was found to be 8.83(28), 7.94(2), 7.88(3), and 8.43(10) for R(0) to R(3), respectively, in units of ${10}^{\mathrm{\ensuremath{-}}4}$ D. These are in general agreement with ab initio calculations except for an unpredicted strong rotational state dependence. Collisional interference was observed and the interference parameter, proportional to the ratio of the average induced dipole moment to the allowed moment, is negative for R(0) and R(1), positive for R(2) and R(3), and of a magnitude consistent with calculation. Comparisons are made with earlier work, particularly that of McKellar [Can. J. Phys. 64, 227 (1986)].

Comparative optical study of the two-dimensional donor-type intercalation compounds graphite- KH x and their binary counterparts C 8 K and C 24 K

Abstract: We report the results of optical reflectivity studies of the stage-1 and -2 graphite-${\mathrm{KH}}_{\mathrm{x}}$ intercalation compounds prepared by direct reaction of highly ordered pyrolytic graphite and KH powder. The stage-1 and -2 binary graphite-K compounds are studied for comparison. The optical data are analyzed in terms of a model involving two-dimensional (2D) graphitic \ensuremath{\pi} electrons and three-dimensional (3D) nearly free K(4s) electrons. The model is used to interpret the observed values of the free-carrier unscreened plasma frequencies and the position of the interband absorption threshold to determine experimental values for the Fermi level (${E}_{F}$) in the carbon \ensuremath{\pi} band(s) and the fractional occupation of the K(4s) band. For the hydrides, we find quantitative evidence that the hydrogen states lie below ${E}_{F}$. Thus, hydrogen is present as ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$, acting as an acceptor, thereby compensating the electron donation to the \ensuremath{\pi} bands from the K(4s) states. This assumption and the optical data for the stage-1 and -2 hydrides results in a [H]/[K] ratio of 0.8, in excellent agreement with chemical analyses reported by Gu\'erard and co-workers, and leads to very small values for the fractional K(4s) band occupation ${f}_{\mathrm{K}}$0.03 electrons per K atom. Within the framework of a superimposed 2D (\ensuremath{\pi}) and 3D [K(4s)] rigid-band model, our experimental results support an empty K(4s) band (i.e., ${f}_{\mathrm{K}}$=0) in stage-2 ${\mathrm{C}}_{24}$K. In stage-1 ${\mathrm{C}}_{8}$K, the rigid-band model yields large values for ${f}_{\mathrm{K}}$ (${f}_{\mathrm{K}}$g0.5 electrons per K atom), unless the value of the optical mass of the electrons in the K(4s) states is larger than \ensuremath{\sim}2. The ${\mathrm{C}}_{8}$K results are also discussed in terms of more sophisticated energy-band calculations.