Showing papers on "Absorption (logic) published in 1988"

Quantitative analysis of the effect of disorder-induced mode coupling on infrared absorption in silica.

Abstract: The infrared-absorption spectrum of a-${\mathrm{SiO}}_{2}$ is analyzed in terms of its transverse-optic (TO) and longitudinal-optic (LO) vibrational modes. It is shown that the independent-oscillator model for the a-${\mathrm{SiO}}_{2}$ dielectric function fails to yield a consistent value of mode strength for the optically active oxygen asymmetric stretch (${\mathrm{AS}}_{1}$) TO mode at 1076 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (in-phase motion of adjacent oxygen atoms) when different but equivalent methods of measurement and analysis are used. This inconsistency is resolved by introducing disorder-induced mechanical coupling between the ${\mathrm{AS}}_{1}$ mode and the relatively optically inactive oxygen asymmetric stretch (${\mathrm{AS}}_{2}$) mode (out-of-phase motion of adjacent oxygen atoms) into the oscillator model. Coupled ${\mathrm{AS}}_{1}$- and ${\mathrm{AS}}_{2}$-mode LO-TO frequency pairs are experimentally observed as peaks at approximately 1256--1076 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 1160--1200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively, in oblique-incidence p-polarized absorption spectra of thin a-${\mathrm{SiO}}_{2}$ films grown thermally on c-Si wafers.Additionally, two other LO-TO-mode pairs are observed in these spectra as absorption peaks at approximately 820--810 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 507--457 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The simplest form of the coupled-mode model consistent with experiment is found to be one in which the ${\mathrm{AS}}_{1}$-mode LO-TO frequency splitting is due to the ${\mathrm{AS}}_{1}$ transverse effective charge and the ${\mathrm{AS}}_{2}$-mode LO-TO splitting is due to the mechanical coupling between these two modes and not to the ${\mathrm{AS}}_{2}$ transverse effective charge, which is negligibly small. The ${\mathrm{AS}}_{2}$ TO and the ${\mathrm{AS}}_{1}$ LO modes found at approximately 1200 and 1256 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively, are shown to be consistent with experimental mode strengths and with the analytic requirements that all LO and TO modes be interspersed and that, as a result of lying between the ${\mathrm{AS}}_{1}$-mode LO-TO pair frequencies, the ${\mathrm{AS}}_{2}$-mode LO-TO frequency splitting be inverted. Comparison of these experimental LO-TO-mode pair frequencies with the vibrational density of states (VDOS) spectrum of a-${\mathrm{SiO}}_{2}$ shows that the TO absorption peaks correspond quite well with the VDOS spectral peaks, whereas the LO absorption peaks do not appear to exhibit any such correspondence.

Structure and physical properties of single crystals of the 84-K superconductor Bi 2.2 Sr 2 Ca 0.8 Cu 2 O 8+δ

Abstract: Single crystal sof the 84-K superconductor ${\mathrm{Bi}}_{2.2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{0.8}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ were characterized by x-ray diffraction, dc magnetic susceptibility, electrical resistivity, and microwave absorption. The structure has ${[\mathrm{Cu}{\mathrm{O}}_{2}]}_{\ensuremath{\infty}}$ planes separated by calcium atoms, edge-shared bismuth oxide double layers, and an incommensurate superlattice along $b$ with a period of 4.76. The in-plane resistivity above ${T}_{c}$ is linear in $T$, with ${\ensuremath{\rho}}_{\mathrm{RT}}=130$ \ensuremath{\mu}\ensuremath{\Omega} cm. Initial results on Pb substitution yielding ${T}_{c}'\mathrm{s}$ of 107 K are reported.

Straggling in thin silicon detectors

Abstract: Experimental and theoretical data for dielectric functions, x-ray absorption coefficients, and generalized oscillator strengths needed for a description of the energy-loss spectrum of fast charged particles in solid silicon are given. Theories used to calculate spectra of total energy loss ("straggling spectra") are described. The convolution method is used to calculate straggling functions for thin silicon absorbers. They are compared with those obtained from other theories (Landau). For relativistic particles ($\ensuremath{\gamma}g100$), the Vavilov-Shulek theories give incorrect functions for absorbers of thicknesses $tl1$ mm. The conversion of energy-loss spectra into ionization spectra is discussed, and the latter are compared with experimental functions. Good agreement is found between calculated and observed functions for electrons, mesons, protons, and their antiparticles and for $\ensuremath{\alpha}$ particles. From this agreement, the error ($1\ensuremath{\sigma}$) of the theoretical values of the most probable energy loss ${\ensuremath{\Delta}}_{p}$ and the full width at half maximum, $w$, is estimated to be less than \ifmmode\pm\else\textpm\fi{}1%.

Two types of oxygen-deficient centers in synthetic silica glass

Abstract: The effects of ArF excimer laser irradiation on dehydrated high-purity silica glass were investigated on both the optical-absorption bands due to oxygen-deficient centers (ODC) and the formation of ${E}^{\ensuremath{'}}$ centers. With an intense uv flux from an excimer laser, an ${E}^{\ensuremath{'}}$-center density of the order of ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was created. The 7.6-eV absorption band remains at the original level, while the 5.0-eV absorption band having the 4.3-eV emission band decreased. Both bands were reduced by heat treatment in an ${\mathrm{O}}_{2}$ atmosphere. These results suggest that there exist two types of ODC: ODC(I), which is responsible for the 7.6-eV band, and ODC(II), for the 5.0-eV band. The concentrations of ODC(I) and ODC(II) were evaluated to be 1 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through an analysis of the gas treatment data and of the order of ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through the growth curve of the ${E}^{\ensuremath{'}}$ centers, respectively. The structural origin of ODC(I) has been attributed to the Si-Si bond, judging from the fact that the peak energy and cross section of the absorption were in close agreement with those of the ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ molecule. Theoretical calculations on defect energy levels by O'Relly and Robertson supported the structural model for IDC(I) and suggested an unrelaxed oxygen vacancy for ODC(II). These assignments were also consistent with the results of a quenching experiment in which the fictive temperature of samples was changed; the equilibrium between concentrations of ODC(I) and ODC(II) shifted in a reasonable manner.

X-ray absorption study of the O 2p hole concentration dependence on O stoichiometry in YBa2Cu3Ox.

Abstract: A detailed x-ray absorption study of the oxygen $K$ edge of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ is presented. A preedge peak is observed for all samples with $x\ensuremath{\ge}6.4$ which we argue to be due to holes in the $\mathrm{O} 2p$ band. By comparison to ${\mathrm{Li}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ the $x$ dependence of the number $\mathrm{O} 2p$ holes in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ is determined.

Photoexcitation and doping studies of poly(3-hexylthienylene)

Abstract: Comprehensive (0.05--3-eV) spectroscopic measurements are reported for poly(3-hexylthienylene), P3HT, during photoexcitation (i.e., photoinduced absorption) and after doping with ${\mathrm{NOPF}}_{6}$. A one-to-one correspondence between the photoinduced and doping-induced spectral changes is observed, typical of conducting polymers. The results demonstrate that charges are predominantly stored in bipolarons in P3HT. We find two subgap electronic absorptions and a series of infrared-active vibrational (IRAV) modes. The four localized IRAV modes (1088, 1161, 1200, 1354 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) associated with bipolaron distortions of the P3HT chains and made infrared active through coupling to the uniform translation of the bipolaron were observed in both the doping-induced and photoinduced absorption spectra, as in polythiophene and poly(3-methylthienylene). An additional absorption (1396 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) was identified as arising from an ir-active localized mode associated with the nonuniform translation (shape oscillation) of the bipolaron. Comparison of the energies of the photoinduced and doping-induced electronic transitions yields an estimate of the change in Coulomb energy of the bipolaron on photoexcitation, ${U}_{B}$\ensuremath{\approxeq}0.25 eV; this relatively small value of ${U}_{B}$ is consistent with bipolaron formation in P3HT.

Photoinduced self-localized structural distortions in YBa2Cu3O7- delta.

Abstract: Photoinduced absorption measurements of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ($\ensuremath{\delta}=0.75$) from 150 to 4000 ${\mathrm{cm}}^{\ensuremath{-}1}$ are reported. We observed photoinduced infrared-active vibrational modes, associated phonon bleachings, and an electronic transition from a self-localized gap state. The results indicate a local tetragonal to orthorhombic distortion around the charge carriers. The dynamic mass associated with the distortion is smaller in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ than in ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$. The photoinduced structural distortion and the associated self-localized gap state demonstrate that the photoexcitations are polarons or bipolarons. Since these photoinduced absorption features are not observed in the isostructural compound ${\mathrm{La}}_{2}$Ni${\mathrm{O}}_{4}$ we conclude that polaron (or bipolaron) formation may play an important role in the high-temperature superconductivity.

Evidence for Fe4+ in YBa2 (Cu1-xMx)3O7-y (M=57Fe, 57Co) by absorption and emission Mössbauer spectroscopy.

Abstract: $^{57}\mathrm{Fe}$ absorption and $^{57}\mathrm{Co}$ emission M\"ossbauer spectroscopy was applied to iron- and cobalt-doped ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${0}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ high-temperature superconductors (M${=}^{57}$Fe, $^{57}\mathrm{Co}$; 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$\ensuremath{\le}x\ensuremath{\le}0.1, y=0 and y=0.8) between 4.2 and 295 K with and without external magnetic field. four iron species A, B, C, and D, are observed with different dominance at different x and y. Aftereffects of the electron capture are ruled out. At 4.2 K spontaneous antiferromagnetic order is shown by contributions A, B, and C in spectra of samples with ${x}_{\mathrm{Fe}\ensuremath{\ge}0.06}$. A model-independent separation of the magnetically split subspectra shows an asymmetry parameter of the electric field gradient above 0.8 for A and B. Temperature and external magnetic field dependence of the magnetic splittings is indicative of superparamagnetic relaxation of small clusters. The species A, B, and C are magnetically interrelated. We suggest a preferential Cu(1) substitution by Fe and Co. Three of the four iron species, viz. A, B, and D have been assigned to high-spin ${\mathrm{Fe}}^{4+}$; C is high spin ${\mathrm{Fe}}^{3+}$. We provide evidence that Co is mostly in the four-valent state, too. Four-valent Fe and Co substitution in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{y}}$ is in reasonable agreement with the dopant concentration dependence of ${T}_{c}$. Possible crystallographic assignments of all four iron (cobalt) sites are discussed.

Photoconductivity in trans -polyacetylene: Transport and recombination of photogenerated charged excitations

Abstract: Photoconductivity in unoriented Shirakawa-type polyacetylene as well as in highly oriented Durham-Graz-type polyacetylene has been measured. Both cw and transient experiments on different time scales have been carried out. The existence and the behavior of a fast and a slow component of the photoconductivity have been investigated in detail and the relationship between the two components has been elucidated. For the fast component we have observed a relaxation time of ${\ensuremath{\tau}}_{1/2}$\ensuremath{\approxeq}100 ps, a migration distance of the carriers up to 400 A\r{}, and a mobility of \ensuremath{\mu}\ensuremath{\approxeq}2 ${\mathrm{cm}}^{2}$/V s. The mobility of the slow component is 2 orders of magnitude lower. In the oriented samples two kinds of anisotropy of the photocurrent have been found, one with respect to the direction of the applied electric field with a value of ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{\perp}}}$=50, and the other with respect to the polarization of the incident light. From a comparison of these data with those of photoinduced absorption measurements, we attribute the fast component mainly to polarons and the slow component to a hopping transport of charged solitonlike defects.

Influence of electron-hole scattering on subpicosecond carrier relaxation in Al x Ga 1 − x As GaAs quantum wells

Abstract: We have simulated subpicosecond-time-scale pump-and-probe absorption experiments in $\frac{{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}}{\mathrm{GaAs}}$ quantum wells using an ensemble Monte Carlo calculation of the photoexcited electron-hole system. For excess carrier energies less than the optical-phonon energy, our results show that the apparent 200-fs relaxation observed in differential transmission experiments is directly attributable to relaxation of electrons through electron-electron scattering with inelastic electron-hole scattering playing a smaller role. Photoexcited holes are found to thermalize on a 50-fs time scale which is much shorter than the relaxation time of the electrons. However, a non-thermal hole distribution develops after the pulse due to optical-phonon absorption at 300 K which should give rise to absorption features at higher photon energies.

Pulsed-beam neutron-lifetime measurement

Abstract: We have measured the neutron lifetime using a superconducting electron spectrometer and a pulsed beam of cold neutrons. Spatially defined neutron bunches are completely contained within the spectrometer's active volume while the \ensuremath{\beta} decay rate is measured. The flux is determined from the radioactivity of nearly totally absorbing thick samples of cobalt and gold exposed to the neutron beam. We obtain ${\ensuremath{\tau}}_{n}$=876\ifmmode\pm\else\textpm\fi{}21 sec.

Optical-absorption coefficient of In 1 − x Ga x As/InP

Abstract: Optical-absorption coefficient data are presented over the 0.7--1.5-eV spectral range (825--1750 nm) for ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$As/InP; both at the lattice-matched condition x=0.47 and for the 0.45lxl0.51 composition range. Absorption data for such epilayers are compared at 10, 77, and 300 K. This comparison involves numerous epilayers grown by organometallic vapor-phase epitaxy, a lattice-matched layer grown by molecular-beam epitaxy, and published data for ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$As grown by liquid-phase epitaxy. It is demonstrated that these growth techniques yield material with equivalent absorption characteristics. For all compositions, the absorption coefficient \ensuremath{\alpha}(h\ensuremath{ u}) rises abruptly to near 6000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at the band gap, and increases more gradually to 30 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at 1.5 eV. For energies above 1.3 eV, all the ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$As epilayers studied here show essentially the same absorption characteristics, regardless of composition or temperature.

Orientation-dependent x-ray-absorption near-edge studies of high-T c superconductors

Abstract: Orientation-dependent x-ray-absorption near-edge measurements have been made on the Cu and Y $K$ edges in oriented ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$ and $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ superconductors. The orientation dependence of the Cu edges is quite similar to that for other ${\mathrm{Cu}}^{2+}$ planar compounds, and allows a clear identification of a number of near-edge features. The results also demonstrate the strong similarity of the Cu-O planes in the two materials, and clearly establish that ${\mathrm{Cu}}^{1+}$ is being formed on the linear-chain sites for $x=0.77$.

Observation of a monolayer phase transition on the meniscus in a Langmuir-Blodgett transfer configuration.

Abstract: Using a novel fluorescence microscope configuration on a lipid monolayer, we demonstrate for the first time that $\ensuremath{\rho}\mathrm{H}$-dependent liquid-to-solid phase transitions can occur locally on the meniscus in a typical Langmuir-Blodgett transfer, at surface pressure substantially below the phase transition pressure on the bulk. A $\ensuremath{\rho}\mathrm{H}$ gradient is proposed, originating from acidification induced by absorption of ambient ${\mathrm{CO}}_{2}$ and the restricted hydroxide-ion exchange between the bulk and meniscus subphases.

Picosecond infrared spectroscopy of hot carriers in a modulation-doped Ga 0.47 In 0.53 As multiple-quantum-well structure

Abstract: Intersubband scattering and cooling of hot carriers is investigated in an $n$-type modulation-doped ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$ ${\mathrm{In}}_{0.52}$As multiple-quantum-well structure. The nonlinear absorption is monitored in several picosecond pump-and-probe experiments after selective excitation of the electrons to the $n=2$ subband. In addition, the relaxation of hot electron-hole pairs created by interband absorption is studied. An upper limit of 3 ps for the lifetime of the electronic $n=2$ subband is estimated. The cooling is dominated by LO-phonon scattering; the energy loss rate is reduced by a factor of up to 130 compared to the value obtained from theoretical calculations.

Excited-state absorption spectra of V 2 + in KMgF 3 and MgF 2

Abstract: We have measured the excited-state absorption spectra of ${\mathrm{V}}^{2+}$ in ${\mathrm{KMgF}}_{3}$ and ${\mathrm{MgF}}_{2}$. The observed absorption bands can be identified as being due to the $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$a and $^{4}\mathrm{T}_{1}$b transitions. The position and shape of the $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$b transition are determined primarily by the displacement in the ${a}_{1g}$ mode while the coupling of the orbital triplet states to ${e}_{g}$ distortions is found to have a substantial influence on the nature of the $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$a transition. In fact, this transition is observed as a broad band rather than as a sharp line, as is predicted solely on the basis of the ${a}_{1g}$ coordinate. The $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$a transition is shifted up in energy relative to its predicted position and significantly overlaps the emission spectrum, thereby providing an explanation for the poor lasing efficiency reported for ${\mathrm{V}}^{2+}$ lasers.

Excitonic absorption in modulation-doped GaAs/AlxGa

Abstract: We have performed transmission measurements in a range of undoped through heavily-modulation-doped GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As multiple-quantum-well structures. The observed absorption spectra demonstrate quenching of the excitonic oscillations with increasing quasi-two-dimensional electron gas. The electron density corresponding to the total bleaching of the lowest excitonic oscillation is greater than or equal to 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ for a quantum well size of 200 A\r{}. Theoretical calculations of the absorption spectra which include the effect of carrier screening have been made. The results show that both long-range and short-range many-body effects should be included to explain the experimentally observed spectra. In the modulation-doped case, we conclude that the phase-space filling and exchange of the electron gas are the dominant effects on excitonic absorption. From the observation that the linewidth increases with the electron density, we demonstrate that the exciton lifetime reduces due to the interaction between the electrons and the excitons.

Demonstration of quasiresonant x-ray photoexcitation in a laser-created plasma.

Abstract: Resonant photoexcitation in a two-component plasma is demonstrated for the first time in the x-ray range The 1s-3p resonance line of Al xiii at 6053 A\r{} is used to pump the 2${\mathit{p}}^{\mathrm{*}}$${(}^{1}$${\mathit{S}}_{0}$)-2${\mathit{p}}^{5}$3d ${(}^{3}$${\mathit{D}}_{1}$) line of Sr xxix at 6059 \AA{} Absorption of the aluminum pump line by the strontium plasma is shown unambiguously The resulting increase in the x-ray fluorescence intensity of the pumped level is measured in a time-resolved experiment

Temperature-dependent far-infrared reflectance of La-Sr-Cu-O and La-Ca-Cu-O: Bardeen-Cooper-Schrieffer electrodynamics but uncertain energy gap.

Abstract: The available far-infrared data for polycrystalline La-Sr-Cu-O and La-Ca-Cu-O show a reflectance edge with energy near 2.5${k}_{B}$${T}_{c}$. This edge has been variously interpreted as the onset of absorption due to an energy gap, and as a low-frequency plasma edge caused by strong far-infrared resonances. Our measured temperature dependence of the reflectance edge closely fits the temperature dependence of the order parameter in a mean-field theory, and hence is consistent with the energy-gap hypothesis. In this paper, we construct a model dielectric function for ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ which is consistent with mean-field theory and the hypothesis of a plasma edge. We find that the temperature dependence of the plasma frequency in this model also closely fits the measured temperature dependence of the reflectance edge. Furthermore, both hypotheses accurately predict the experimentally observed temperature dependence of the absorption at frequencies much less than the reflectance edge. This observation has significant implications for the construction of fast low-loss superconducting devices. We conclude that the electrodynamics of the superconducting transition in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ are well described by a Bardeen-Cooper-Schrieffer-like mean-field theory. However, given the identical predictions of the energy-gap and plasma-edge hypotheses, it is premature to deduce a precise value for the magnitude of the energy gap from the infrared data.

Indirect optical absorption and radiative recombination in silver bromoiodide.

Abstract: The low-temperature (4.2 K) and room-temperature absorption spectra of several ${\mathrm{AgBr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{I}}_{\mathrm{x}}$ (x=0\char21{}0.25) samples have been obtained. Conventional analysis of these spectra yields the band-gap energy (${E}_{g}$), which decreases with iodide content. The band-gap energies at 4.2 K range from 2.684 eV for pure AgBr to 2.448 eV for ${\mathrm{AgBr}}_{0.75}$${\mathrm{I}}_{0.25}$. The more accurate band-gap information is used to estimate the trap depth of the acceptor states observed in the low-temperature emission. Data indicate that both donors and acceptors are shallow traps in ${\mathrm{AgBr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{I}}_{\mathrm{x}}$ systems.

Prebreakdown energy absorption from intense laser pulses at 532 nm in NaCl.

Abstract: Details of the photoacoustic measurement and model calculations of prebreakdown energy deposition in NaCl at 532 nm [Jones, Shen, Braunlich, Kelly, and Epifanov, Phys. Rev. B 35, 894 (1987)] are provided. The primary absorption process is shown to be four-photon excitation of valence electrons to the conduction band. Calibration of the acoustic system by two-photon absorption of 266-nm laser light indicates focal point temperature increases over 300 K are attained for nondamaging pulses at 532 nm. Interpretation of the experimental data considering secondary energy absorption by free carriers and transient crystal defects yields the result that the generalized four-photon cross section lies in the range 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}114}$ ${\mathrm{cm}}^{8}$${\mathrm{sec}}^{3}$\ensuremath{\le}${\mathrm{\ensuremath{\sigma}}}^{(4)}$\ensuremath{\le}20\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}114}$ ${\mathrm{cm}}^{8}$ ${\mathrm{sec}}^{3}$. It is further shown that avalanche generation of free carriers is not evident even for these large temperature increases. The calibrated results were obtained from reactive-atmosphere-processed ultrapure NaCl crystals; the importance of sample purity is discussed.

Optical absorption of ZnGa2Se

Abstract: Single crystals of ${\mathrm{ZnGa}}_{2}$${\mathrm{Se}}_{4}$:${\mathrm{Co}}^{2+}$ were grown by the chemical transport reaction method. The optical absorption of single crystals was investigated in the temperature range 20\char21{}300 K. The optical-absorption peaks due to the transition $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$${(}^{4}$P), $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$${(}^{4}$F), and $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{2}$${(}^{4}$F) of the ${\mathrm{Co}}^{2+}$ ion that has entered the substitutional tetrahedral sites appeared at 12 830, 6221, and 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at 289 K. The crystal-field parameter and the Racah parameter were given by Dq=400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and B=469 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The transition $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$${(}^{4}$P) split into a triplet state by first-order spin-orbit\char21{}coupling effects in the temperature range 55\char21{}289 K, and the ${\ensuremath{\Gamma}}_{8}$+${\ensuremath{\Gamma}}_{7}$ level split into ${\ensuremath{\Gamma}}_{8}$ and ${\ensuremath{\Gamma}}_{7}$ levels by second-order spin-orbit\char21{}coupling effects of ${\mathrm{Co}}^{2+}$ ions in the low-temperature range below 55 K. The spin-orbit\char21{}coupling parameter was found to be \ensuremath{\lambda}=-363 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Analytic solution for second-harmonic gyroresonant absorption and mode conversion.

Abstract: An entirely analytic treatment for second-harmonic mode conversion is presented, which uses the wave-phase-space $\mathbf{x}\ensuremath{-}\mathbf{k}$ space) method. Explicit expressions are derived for the coefficients of transmission, reflection, conversion (magnetosonic wave to ion-Bernstein wave), and absorption. A conservation law for wave energy flux in phase space is also presented.

EPR and optical absorption of a self-trapped hole in Rb 2 ZnCl 4 crystals

Abstract: A self-trapped hole (${V}_{k}$ center) has been identified by means of electron paramagnetic resonance in single crystals of ${\mathrm{Rb}}_{2}$${\mathrm{ZnCl}}_{4}$ x-irradiated at 77 K. This center presents a broad (\ensuremath{\sim}0.8 eV) absorption band at 385 nm which is anisotropic and \ensuremath{\sigma} polarized. The thermal and optical bleaching of the ${V}_{k}$ centers has been studied through EPR and optical experiments.

Measurement of the M2-E1 mixing in L x-ray transitions

Abstract: The ${\mathrm{La}}_{2}$ line of gold has been resolved using a Si (Li) detector combined with a zinc absorber. By analysis of the angular distribution of the 500-keV-proton-induced $\mathrm{Ll}$, ${\mathrm{La}}_{2}$, and ${\mathrm{La}}_{1,2}$ x-ray lines of gold, the mixing ratio for the magnetic quadrupole and electric dipole radiation has been determined. The mixing has been found to be a factor of 4 larger than previously calculated. The intensity of the $\mathrm{Ls}$ and $Lt$ lines of platinum also has been measured using this absorption technique. The results gave further evidence that the higher-order multipole terms in ${L}_{3}$ x-ray transitions of heavy elements were previously underestimated.

Infrared absorption and rotational dynamics of OH- and OD- defects in cesium halides.

Abstract: The rotational degrees of freedom of substitutional ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{OD}}^{\mathrm{\ensuremath{-}}}$ defects in CsCl, CsBr, and CsI have been studied between 2 and 300 K using infrared absorption in the range of the 2.7 and 3.7-\ensuremath{\mu}m stretching absorption. In CsCl the first direct evidence for quasi-free-rotational behavior of ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ in alkali-metal halides is found in the form of a vibration-rotation spectrum above 45 K, corresponding to a reorientational barrier of \ensuremath{\sim}30 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The small rotational constant of B=1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ without ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$-${\mathrm{OD}}^{\mathrm{\ensuremath{-}}}$ isotope shift, derived from the temperature dependence of the rotational P and R branches, indicates a considerably increased moment of inertia for ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ and possibly off-center motion of the molecule around the H ion. High-frequency librational sidebands observed below and above 45 K and their isotope shifts indicate radial and angular modes superimposed on the rotational motion and can be explained with a simple double-librator model. In CsBr and CsI doped with ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{OD}}^{\mathrm{\ensuremath{-}}}$, high-frequency sidebands and isotope shifts expected from a harmonic-librator model for center-of-mass libration in high barrier potentials are found. The observed second harmonic of the stretching absorption in CsCl and CsBr allows determination of the mechanical and electrical anharmonicity parameters.

X-ray absorption in atoms under intense laser fields.

Abstract: The absorption of x rays in atoms assisted by an intense laser field is considered. The general expression for the x-ray absorption cross section (${\ensuremath{\sigma}}_{x}$) as a function of the laser field is obtained in the intense-field regime using the Seely and Harris approximation [Phys. Rev. A 7, 1064 (1973)]. We find that in this regime, and when the two photon fields are polarized in the same direction, the cross section ${\ensuremath{\sigma}}_{x}$ decreases as ${\ensuremath{\varepsilon}}^{\mathrm{\ensuremath{-}}1}$, \ensuremath{\varepsilon} being a factor proportional to the intense-field amplitude. When the field is further increased to the limit of superintense fields, ${\ensuremath{\sigma}}_{x}$ varies as ${\ensuremath{\varepsilon}}^{\mathrm{\ensuremath{-}}3}$. A physical explanation for this behavior is provided.

Effective bond-stretching force constants in chalcopyrite CuGaSe 2 by temperature dependence of extended x-ray-absorption fine-structure spectra

Abstract: We report on a study of the temperature dependence of the extended x-ray-absorption fine-structure (EXAFS) Debye-Waller-like factor in the chalcopyrite ${\mathrm{CuGaSe}}_{2}$ at the Cu and Ga K absorption edges. The mean-square relative displacements are described by Einstein models which provide effective bond strengths between the cations and the nearest-neighbor chalcogen atoms. The results are discussed on the basis of the vibrational spectrum of ${\mathrm{CuGaSe}}_{2}$. This approach demonstrates that EXAFS T-dependent spectra may be used to obtain information about the nature of the bonds in complex materials.

Biexcitonic nonlinearity in GaAs/GaxAl1-xAs quantum wells and quantum-well wires.

Abstract: The recently reported high excitonic and biexcitonic binding energies in GaAs/${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells and quantum-well wires suggest the possibility of strong optical nonlinearities around the biexcitonic resonance. We calculate in a simple boson model both absorption and dispersion spectra in GaAs around this resonance for various intensities comparing bulk, quantum-well, and quantum-well-wire structures. An assessment of inhomogeneous line broadening due to a Gaussian distribution of quantum-well-wire radii is also presented.

Resonance Raman scattering and optical absorption studies of MnO - 4 in KClO 4 at high pressure

Abstract: We have measured the pressure dependence of both the optical absorption and the resonance Raman (RR) excitation profiles of the breathing mode, ${\ensuremath{ u}}_{1}$, and its first two overtones for the substitutional impurity ion ${\mathrm{MnO}}_{4}^{\mathrm{\ensuremath{-}}}$, in crystalline ${\mathrm{KClO}}_{4}$ We have used the powerful transform technique of the time-correlator theory of RR scattering to analyze our RR data by calculating the profile line shapes directly from the measured optical absorption This work constitutes the first successful application of the transform technique to the domain of high-pressure physics The optical absorption displays a blue shift with increasing pressure, and both the ground- and excited-electronic-state vibrational frequencies increase with pressure We calculate the renormalized Stokes loss parameter of ${\ensuremath{ u}}_{1}$ directly from ratios of the scaling factors used in fitting our calculated profile line shapes to the data This ``direct'' transform method presumes the broadening mechanisms to be homogeneous, and it gives a decrease in the renormalized Stokes loss parameter of less than 10% between one atmosphere and 106 GPa We have also used a previously developed extension of the time-correlator theory to incorporate inhomogeneous broadening in the transform calculations These calculations show that the effect of inhomogeneous broadening is not significant on the RR profile line shapes, but indicate that the decrease in the renormalized Stokes loss parameter with pressure could be as large as 15%