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Showing papers on "Absorption (logic) published in 1981"


Journal ArticleDOI
C. K. N. Patel1, Andrew C. Tam2
TL;DR: In this paper, the authors discuss the theory and experiments dealing with the pulsed optoacoustic effect (i.e., generation of a transient acoustic wave by absorption of an optical pulse) in condensed matter.
Abstract: The authors discuss the theory and experiments dealing with the pulsed optoacoustic effect (i.e., generation of a transient acoustic wave by absorption of an optical pulse) in condensed matter. Their primary interest lies in the measurement of small absorption coefficients (\ensuremath{\ll}${10}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$). At present an experimental capability of measuring absorption coefficients as small as ${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ has been demonstrated, and further improvement is foreseen. The pulsed optoacoustic absorption measurement technique has been applied to the following linear spectroscopic studies: (1) precise measurements of the optical absorption spectra of ${\mathrm{H}}_{2}$O and ${\mathrm{D}}_{2}$O; (b) accurate determination of absorption strengths and profiles of high harmonics ($n=6, 7, \mathrm{and} 8$) of vibrational modes in transparent organic liquids (e.g., benzene); (c) quantitative absorption spectra of thin (\ensuremath{\sim} 1-10 \ensuremath{\mu}m) liquid films; and (d) quantitative absorption spectra of solids and finely powdered crystals. The usefulness of the pulsed optoacoustic technique to nonlinear spectroscopy has been demonstrated in the following studies: (a) quantitative two-photon absorption spectroscopy of the weak two-photon ($^{1}B_{2\ensuremath{\mu}}\ensuremath{\leftarrow}^{1}A_{1g}$) transition in benzene; and (b) optoacoustic Raman-gain spectra for a variety of liquids where an ability to measure Raman gains as small as ${10}^{\ensuremath{-}5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ has been demonstrated. In addition to reviewing the above studies the authors discuss future possible applications and compare the pulsed optoacoustic spectroscopy technique with other optoacoustic absorption measurement techniques.

568 citations


Journal ArticleDOI
TL;DR: In this article, the integrated near-field intensity efficiency of metallic spheres immersed in water has been calculated for dipolar and multipolar surface-plasmon modes of Ag, Cu, and Au spheres of varying radii.
Abstract: Enhancement of the Raman scattering and fluorescence emission on noble metals (Ag, Cu, and Au) is believed to be caused in part by large local fields at the incident wavelength on the surface of metallic microstructures, such as colloidal suspensions and surface roughness on electrodes and thin films. For metallic spheres immersed in water, calculations are made of the integrated near-field intensity efficiency (${Q}_{\mathrm{NF}}$) and that part associated only with the radial field component as a function of incident wavelength (200- 1200 nm) and sphere radius (0- 300 nm) which exceeds the usual Rayleigh limit and extends well into the Lorenz-Mie region. The calculated wavelength and radius dependencies of ${\mathrm{Q}}_{\mathrm{NF}}$ are compared with those for the better-known efficiencies: extinction (${Q}_{\mathrm{E}}$), far-field scattering (${Q}_{\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{sca}}$), and absorption (${Q}_{\mathrm{abs}}$). The peak values of these efficiencies have been evaluated when the incident wavelength is in resonance with dipolar and multipolar surface-plasmon modes of Ag, Cu, and Au spheres of varying radii immersed in water.

360 citations


Journal ArticleDOI
TL;DR: In this paper, the relation between ionic coordinations and the distribution of electric field gradients (EFG) is investigated, and an analytic approximation for the distribution function is derived for amorphous Gd-Ni alloys.
Abstract: The observation of nuclear quadrupole interactions in amorphous solids provides a unique possibility of obtaining information about the angular distribution of local ionic coordinations, complementary to the information about radial distributions deduced from x-ray and neutron diffraction and from extended x-ray absorption fine structure measurements. In the present paper the relation between ionic coordinations and the distribution of electric field gradients (EFG) is investigated. It is shown that the distribution function $P({V}_{\mathrm{zz}},\ensuremath{\eta})$ of the splitting parameters ${V}_{\mathrm{zz}}$ (the electric field gradient) and $\ensuremath{\eta}$ (the asymmetry parameter) in general yields zero probability both for ${V}_{\mathrm{zz}}=0$ and for $\ensuremath{\eta}=0$. For solids which are isotropic on the average, the distribution function of the components ${V}_{\mathrm{ik}}$ of the EFG tensor depends only on two variables, the invariant functions of the tensor components [$\mathrm{Det}({V}_{\mathrm{ik}}) \mathrm{and} \ensuremath{\Sigma}{V}_{\mathrm{ik}}^{2}$]. Expressions for these quantities in terms of the radial coordinates of the ions causing the EFG and of the bond angles between pairs of ions are given. For amorphous solids with random ionic coordination an analytic approximation for the distribution function $P({V}_{\mathrm{zz}},\ensuremath{\eta})$ is derived. This function is strongly dominated by the distribution of ions in the first coordination shell. The results are applied to the analysis of M\"ossbauer spectra of $^{155}\mathrm{Gd}$ in amorphous Gd-Ni alloys.

332 citations


Journal ArticleDOI
TL;DR: In this article, the multiple-trapping mechanism was used to explain photo-induced optical absorption transients in a unified way with the multiple trapping mechanism, and transient photoconductivity provided a spectroscopy of the density of localized states.
Abstract: Measurements of transient photoinduced optical absorption in $a$-${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ show that carriers thermalize within a distribution of localized states in the band gap. Photocurrent as well as photoinduced optical absorption transients are explained in a unified way with the multiple-trapping mechanism. Under such circumstances, transient photoconductivity provides a spectroscopy of the density of localized states.

332 citations


Journal ArticleDOI
P. E. Schmid1
TL;DR: In this paper, optical absorption measurements at 300 and 4 K on a series of heavily doped Si:As and Si:B samples are reported and the interband contribution is isolated and confronted with the predictions of an electron-gas calculation.
Abstract: Optical-absorption measurements at 300 and 4 K on a series of heavily doped Si:As and Si:B samples are reported. The interband contribution is isolated and confronted with the predictions of an electron-gas calculation. Disorder effects are observed and impurity-derived states are found to play a significant role, invalidating the electron-gas model at concentrations lower than ${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The gap shrinkage follows a critical behavior, going to zero at the insulator-metal transition and varying approximately linearly with concentration at high doping. The discrepancy between device-based and optical determinations of the gap shrinkage is discussed.

251 citations


Journal ArticleDOI
TL;DR: In this article, a theory for electron-avalanche breakdown in solids is explained by a theory that agrees with experimental results for the magnitude of the breakdown field and its temperature dependence, pulse-duration dependence, material-to-material variation, and wavelength dependence.
Abstract: Electron-avalanche breakdown in solids is explained by a theory that agrees with experimental results for the magnitude of the breakdown field and its temperature dependence, pulse-duration dependence, material-to-material variation, and wavelength dependence for $\ensuremath{\lambda}\ensuremath{\ge}1$ \ensuremath{\mu} m. The good agreement between experiment and theory with no parameters adjusted is obtained by using improved magnitudes and energy dependences of the electron-phonon relaxation frequencies. The contributions of both optical and acoustical phonons to electron loss and energy-space diffusion must be included. The breakdown field ${E}_{B}$ is calculated by solving an eigenvalue equation obtained from the diffusion-transport equation. Simple models and interpretations of the diffusion equation afford physical insight into breakdown and render the breakdown conditions predictable. Preliminary results indicate that the diffusion approximation fails for wavelengths considerably shorter than 1 \ensuremath{\mu} m, where multiphoton absorption must also be considered.

203 citations


Journal ArticleDOI
TL;DR: In this article, the results of experimental studies of photoluminescence and photoconductivity in cis- and trans-${(\mathrm{CH})}_{x}$ are presented.
Abstract: The results of experimental studies of photoluminescence and photoconductivity in cis- and trans-${(\mathrm{CH})}_{x}$ are presented. For cis-${(\mathrm{CH})}_{x}$, we find recombination luminescence in the scattered light spectrum at 1.9 eV, near the interband absorption edge. The luminescence turns on sharply for excitation energies greater than 2.05 eV, implying a Stokes shift of 0.15 eV. Studies of the temperature dependence ($Tg~7$ K) show no loss of luminescence intensity even at temperatures as high as 300 K. Isomerization of the same sample quenches the luminescence; we find no indication of luminescence near the interband absorption edge of trans-${(\mathrm{CH})}_{x}$ even at temperatures as low as 7 K. These results are discussed in the context of parallel phototransport studies. The quenching of the luminenscence upon cis-trans isomerization is concurrent with the appearance of a large photoconductive response. The photoconductivity in trans-${(\mathrm{CH})}_{x}$ has a threshold at 1.0 eV, well below the interband absorption edge at 1.5 eV, implying the presence of states deep inside the gap. The observation of luminescence in cis-${(\mathrm{CH})}_{x}$, but not in trans-${(\mathrm{CH})}_{x}$, and the observation of photoconductivity in trans-${(\mathrm{CH})}_{x}$, but not in cis-${(\mathrm{CH})}_{x}$ provide confirmation of the proposal that solitons are the photogenerated carriers. In trans-${(\mathrm{CH})}_{x}$, the degenerate ground state leads to free soliton excitations, absence of band-edge luminescence, and photoconductivity. In cis-${(\mathrm{CH})}_{x}$ the nondegenerate ground state leads to confinement of the photogenerated carriers, absence of photoconductivity, and to the observed recombination luminescence.

176 citations


Journal ArticleDOI
TL;DR: Li et al. as discussed by the authors measured the true absorption cross section of pions in nuclei at 85, 125, 165, 205, 245, and 315 MeV for positive pions and at 125 and 165 MEV for negative pions.
Abstract: The cross section for true absorption of pions in nuclei was obtained from experiments at 85, 125, 165, 205, 245, and 315 MeV for positive pions and at 125 and 165 MeV for negative pions. The results are compared with theoretical calculations. The inclusive pion scattering angular distribution was also measured, and the results indicate that quasifree scattering plays an important role for backward scattering. The total pion-nucleus cross section is decomposed into its major channels: elastic scattering, inclusive inelastic scattering, true absorption, and single charge exchange, as a function of pion energy and charge. For light nuclei, the true absorption cross section has a strong energy dependence, reflecting the shape of the (3,3) resonance. The $A$ dependence of the true absorption cross section is much stronger than that of the inclusive inelastic scattering.NUCLEAR REACTIONS Li, C, Al, Fe, Nb, Bi (${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},{\ensuremath{\pi}}^{\ensuremath{'}}$); measured $\ensuremath{\sigma}({\ensuremath{\theta}}_{{\ensuremath{\pi}}^{\ensuremath{'}}})$, ${E}_{{\ensuremath{\pi}}^{+}}=85, 125, 165, 205, 245, \mathrm{and} 315$ MeV, ${E}_{{\ensuremath{\pi}}^{\ensuremath{-}}}=125, 165$ MeV; deduced ${\ensuremath{\sigma}}_{\mathrm{inelastic}}$, ${\ensuremath{\sigma}}_{\mathrm{absorption}}$; decomposition of ${\ensuremath{\sigma}}_{\mathrm{tot}}$.

136 citations


Journal ArticleDOI
TL;DR: In this article, the authors extend the formal theory of the interaction of many discrete states with many continua and present three model calculations which illustrate the significant aspects of resonant photoemission.
Abstract: We extend to photoemission the formal theory of the interaction of many discrete states with many continua and present three model calculations which illustrate the significant aspects of resonant photoemission. The first two models treat $3p$ core level to $3d$ band absorption, followed by super-Coster-Kronig decay (${3p}^{5}{3d}^{n+1}\ensuremath{\rightarrow}{3p}^{6}{3d}^{n\ensuremath{-}1}\ensuremath{\epsilon}f$), which interferes with the direct excitation of the $3d$ valence band. The first calculation is for a simple band model which applies approximately to Cr. No $d\ensuremath{-}d$ interactions or atomic effects are included, yet interference characteristic of Fano resonances is clearly evident. Specifically, a strong dip in the valence-band photoemission occurs near threshold. The second model contains hole-hole interactions and exhibits a resonant two-bound-hole satellite in photoemission. The dependence of the photoemission intensity on photon energy shows a larger Fano $q$ parameter for the satellite than for absorption, in agreement with an experiment on Ni. Further, the satellite shows strong enhancement at resonance, whereas the main line (valence-band emission) shows primarily an interference dip, as observed. The third model is for metals with filled $3d$ bands, such as Cu, Zn,.... The absorption is from $3p$ to the $4s\ensuremath{-}4p$ band. The resulting super-Coster-Kronig decay of the $3p$ hole gives rise to the ${M}_{2,3}{M}_{4,5}{M}_{4,5}$ Auger peak (fixed kinetic energy) as well as a resonant satellite at fixed binding energy. The latter is due to a singularity [$N(hv,{E}^{B})\ensuremath{\sim}{({E}^{B}\ensuremath{-}{E}_{0}^{B})}^{\ensuremath{-}\ensuremath{\lambda}}$] in the photoemission intensity caused by the strong interaction of the $4s\ensuremath{-}4p$ conduction electrons and the ${3d}^{8}$ configuration in the final state.

126 citations


Journal ArticleDOI
TL;DR: In this article, a diagrammatic valence-bond (VB) method was used to model the low-energy charge transfer (CT) excitation characteristic of both molecular conductors and complex-ion-radical salts.
Abstract: The low-energy charge transfer (CT) excitation characteristic of both $\ensuremath{\pi}$-molecular conductors and complex-ion-radical salts is interpreted as a nearest-neighbor Coulomb interaction $V$ that is comparable to the bandwidth, $4|t|$. Partly filled segregated regular stacks in organic conductors are represented by extended Hubbard models, whose exact CT energies and intensities are obtained by diagrammatic valence-bond (VB) methods for four electrons on finite rings and chains, together with an approximate treatment of $V$ in partly filled infinite stacks for infinite on-site correlations $U$. Finite $V\ensuremath{\sim}4|t|$ yields an intense low-lying CT band, containing $V$ and $U\ensuremath{-}2V$ excitations, that depends weakly on the band filling. Finite $V$ also splits the usual CT absorption around $U$ for half filled bands into strong absorptions around $U\ensuremath{-}V$, weak ones around $U$, and much weaker bands around $U+V$ and $U+2V$. The CT spectra of mixed-valence tetrathiofulvalene (TTF) salts are modeled with $V\ensuremath{\sim}0.4$ eV, $U\ensuremath{\sim}1.4$ eV, and $|t|\ensuremath{\sim}0.10\ensuremath{-}0.13$ eV. Similar CT transitions in complex tetracyanoquinodimethane (TCNQ) salts are consistent with the insensitivity of the $V$ peak's position to the filling or the structure. Restricting the basis to one valence state per site produces several general consequences for dipole-allowed optical transitions.

66 citations


Journal ArticleDOI
TL;DR: In this article, the wavelength dependence of the following laser-plasma coupling processes were performed: (1) laser-light absorption, (2) energy transport, and (3) ablation and target acceleration behavior.
Abstract: Experimental studies of the wavelength dependence of the following laser-plasma coupling processes were performed: (1) laser-light absorption, (2) energy transport, and (3) ablation and target-acceleration behavior. At a fixed intensity around ${10}^{14}$ W/${\mathrm{cm}}^{2}$, the ablation process was dominated by heat transport by cold electrons for 0.53-and 1.06-\ensuremath{\mu} m lasers and by hot electrons for a 10.6-\ensuremath{\mu} m laser.

Journal ArticleDOI
TL;DR: In this paper, a high-resolution photoluminescence and absorption study with the application of external stress and magnetic fields on the 0.97-eV ($G$) line was conducted.
Abstract: We report on a high-resolution photoluminescence and absorption study with the application of external stress and magnetic fields on the 0.97-eV ($G$) line. We determine the symmetry of the related center to be monoclinic $I$. It is shown that satellite lines $E$, ${E}^{\ensuremath{'}}$, and ${E}^{*}$, which are observed in luminescence or absorption, are local modes of the $G$-line center. The low-energy local modes $E$ and ${E}^{\ensuremath{'}}$ both exhibit isotope effects upon $^{13}\mathrm{C}$ implantation of the Si samples. The doublet structure due to $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ gives evidence that one substitutional carbon atom is incorporated in the complex. We observe a nonthermalizing and stress-independent triplet structure of the $G$ line in luminescence as well as in absorption with intensities almost identical to the natural abundances of $^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$. This establishes our basic model of substitutional carbon atom interacting with a single silicon atom, ${\mathrm{Si}}^{*}$, in a prominent position. Relaxation of the complex leads to a symmetry lowering from a trigonal $〈111〉$ defect configuration to a monoclinic $I({C}_{1h})$ symmetry with a (110) mirror plane. Two particular models are discussed in detail, one being a ${\mathrm{C}}_{S}\ensuremath{-}{\mathrm{Si}}_{I}$ dumbbell having relaxed off a bond-centered $〈111〉$ position, the other being a carbon-vacancy complex similar to the electron-paramagnetic-resonance $\mathrm{S}\mathrm{i}\ensuremath{-}E$ center.

Journal ArticleDOI
TL;DR: In this article, the frequency dependence and magnitude of the far-infrared absorption of a small-metal-particle composite material have been measured over the range of 4-100 cm.
Abstract: The frequency dependence and magnitude of the far-infrared absorption of a small-metal-particle composite material has been measured over the range of 4-100 ${\mathrm{cm}}^{\ensuremath{-}1}$. The composite samples consisted of spherical metal particles, with radii varying from 100-1000 \AA{}, randomly dispersed in an insulating host. In some samples, a nonlinear dependence of the absorption on the metal-particle concentration was observed, indicating that interparticle interactions can play an important role. It is shown that the magnitude of the absorption cannot be explained by classical electromagnetic theory.

Journal ArticleDOI
TL;DR: The first measurements of high-energy x-ray emission by high-Z$ microballoon targets irradiated at $5\ifmmode\times\else\texttimes\fi{}{10}^{14}l\ensuremath{\phi}l2\IFmmode \times \else \texttimes \fi{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16}$ W/${\mathrm{cm}}^{2}$ by 10.6-
Abstract: The first measurements of high-energy x-ray emission ($h\ensuremath{ u}\ensuremath{\sim}30\ensuremath{-}300$ keV) by high-$Z$ microballoon targets irradiated at $5\ifmmode\times\else\texttimes\fi{}{10}^{14}l\ensuremath{\phi}l2\ifmmode\times\else\texttimes\fi{}{10}^{16}$ W/${\mathrm{cm}}^{2}$ by 10.6-\ensuremath{\mu}m laser light are reported. An exponential spectrum with a slope $k{T}_{H}\ensuremath{\sim}250$ keV provides the best fit to spectrometer data at ${\ensuremath{\phi}}_{l}={10}^{16}$ W/${\mathrm{cm}}^{2}$. The hard-x-ray yield indicates that a substantial fraction, probably between 10% and 100%, of the absorbed laser energy is converted to hot electrons. The slope $k{T}_{H}$ is proportional to the fastest ion energy.

Journal ArticleDOI
TL;DR: In this paper, the degenerate four-wave mixing (DFWM) in a resonant homogeneously broadened gas was studied and peak reflectivities of 38% were readily obtained.
Abstract: Detailed measurements have been made of degenerate four-wave mixing (DFWM) in a resonant homogeneously broadened gas. The measurements were performed in S${\mathrm{F}}_{6}$ using a pulsed C${\mathrm{O}}_{2}$ laser operated on the 10.4-\ensuremath{\mu} m branch. A pulse width of 1.2 nsec was used to reduce effects of collisional relaxation. Data are also presented for pulse widths of 140 nsec. At high-energy fluences (\ensuremath{\sim}0.1 J/${\mathrm{cm}}^{2}$) pulse breakup is observed and is believed to arise from multiple-photon effects. The data obtained with the short pulse width are compared to a four-level saturable-absorber model where excellent agreement is obtained when pump absorption is included. The qualitative behavior of DFWM due to dispersion is also demonstrated in the wings of the S${\mathrm{F}}_{6}$ absorption spectrum. Peak reflectivities of 38% were readily obtained. In addition contributions due to a coherent three-level nonlinearity are presented showing a contribution equal to the measured saturable contribution.

Journal ArticleDOI
TL;DR: In this article, the composition-dependent electronic structure of fcc (1.9l~xl~2.9) was studied using photoelectron spectroscopy with synchrotron radiation.
Abstract: The composition-dependent electronic structure of fcc $\mathrm{La}{\mathrm{H}}_{x}$, $1.9l~xl~2.9$, has been studied using photoelectron spectroscopy with synchrotron radiation ($10l~\mathrm{hv}l~50 \mathrm{eV}$). Complementary optical reflectance measurements ($0.16l~\mathrm{hv}l~4 \mathrm{eV}$) have been performed for fcc $\mathrm{La}{\mathrm{H}}_{x}$, $1.9l~xl~2.9$, and $\mathrm{Nd}{\mathrm{H}}_{x}$, $2.01l~xl~2.27$. For $\mathrm{La}{\mathrm{H}}_{x}$, $x\ensuremath{\simeq}2$, the occupied $d$ bands are \ensuremath{\sim}1.5 eV wide and the hydrogen-induced bonding band is centered \ensuremath{\sim}5 eV below the Fermi level, ${E}_{F}$ (full width \ensuremath{\sim}6 eV). Hydrogen occupation of both octahedral and tetrahedral sites is revealed for $xl~2$, analogous to what has been observed for other metal dihydrides. With increasing hydrogen concentration, emission from the $d$ bands near ${E}_{F}$ decreases and the bonding band shifts to higher binding energy; the optical spectra show a red shift of interband absorption features, increased octahedral site occupation, and increased screening of a low-energy plasmon. For $\mathrm{La}{\mathrm{H}}_{x}$ samples at the upper end of the composition range, $x\ensuremath{\sim}2.9$, the photoemission spectra show very weak valence-band emission, and the optical spectra suggest semiconducting behavior. The binding energies of the ${5p}_{\frac{1}{2},\frac{3}{2}}$ cores, ${E}_{B}$, measured relative to ${E}_{F}$ are shown to increase with $x$ (the total shift is \ensuremath{\sim}0.8 eV for La \ensuremath{\rightarrow} La${\mathrm{H}}_{2}$ and 0.9 eV for La${\mathrm{H}}_{2}$ \ensuremath{\rightarrow} La${\mathrm{H}}_{3}$). Our results are compared to band calculations by Gupta and Burger and by Misemer and Harmon and to results of NMR, specific heat, and resistivity studies.

Journal ArticleDOI
TL;DR: In this article, the number of open channels available to direct reactions for the even-even C + C, C + O, and O + O systems is calculated in terms of the surface transparent absorption needed to observe resonant structure.
Abstract: Resonance phenomena observed experimentally in some lighter-heavy-ion collisions at energies well above the Coulomb barrier are discussed in terms of the number of open channels available to direct reactions for the even-even C + C, C + O, and O + O systems. The calculated number of open channels shows a characteristic energy dependence which reflects the surface transparent absorption needed to observe resonant structure. The weak absorption is particularly pronounced in reactions involving $^{12}\mathrm{C}$, $^{14}\mathrm{C}$, and $^{16}\mathrm{O}$ nuclei.

Journal ArticleDOI
TL;DR: In this article, the dynamics of collisionless infrared multiphoton absorption in S${\mathrm{F}}_{6}$ was studied using picosecond C${O}}_{2}$ laser pulses, and it was found that at very high laser intensities, the absorption in the quasicontinuum has a considerable intensity dependence.
Abstract: The dynamics of collisionless infrared multiphoton absorption in S${\mathrm{F}}_{6}$ was studied using picosecond C${\mathrm{O}}_{2}$ laser pulses. It was found that at this new regime of light-matter interaction at very high laser intensities, the absorption in the quasicontinuum has a considerable intensity dependence. The deviation from an energy fluence scaling law was found to begin at about 200 MW/${\mathrm{cm}}^{2}$. Above this intensity, the multiphoton interaction between the laser and the molecule should be described by the full coherent Schr\"odinger equation rather than by the usual master rate equations. A novel method of preheating the S${\mathrm{F}}_{6}$ molecules is also described and the saturation behavior of these preheated molecules is measured with the picosecond pulses.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the absorption of far-infrared radiation by Pd small particles randomly embedded in KCl and found that the frequency dependence of the absorption coefficient was typically quadratic below 30.
Abstract: Measurements have been made of the absorption of far-infrared radiation by Pd small particles randomly embedded in KCl. The small particle specimens had Pd volume fractions between 0.001 and 0.1. At volume fractions of 0.03 and below, the absorption coefficient was proportional to the Pd concentration. The frequency dependence of the absorption coefficient was typically quadratic below 30 ${\mathrm{cm}}^{\ensuremath{-}1}$ with a slower dependence above 30 ${\mathrm{cm}}^{\ensuremath{-}1}$. The absorption coefficients were about a factor of 10 larger than those calculated from a theory based on electric and magnetic dipole absorption.

Journal ArticleDOI
TL;DR: In this article, the relative cross sections for two-nucleon coincidence measurements of the $2N+\ensuremath{\pi{+}-, $2p$) and the (${ esuremath{-pi}}^{\ensure math{-}}-, $\mathrm{pn}$) reactions were determined where the initial two nucleons are in a relative $s$ state, and coupled either to isospin zero or to one.
Abstract: The relative cross sections for the $2N+\ensuremath{\pi}\ensuremath{\rightarrow}2N$ reactions were determined where the initial two nucleons are in a relative $s$ state, and coupled either to isospin zero or to one. This information is extracted from two-nucleon coincidence measurements of the (${\ensuremath{\pi}}^{+}$, $2p$) and the (${\ensuremath{\pi}}^{\ensuremath{-}}$, $\mathrm{pn}$) reactions on targets of $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$.

Journal ArticleDOI
TL;DR: In this paper, room-temperature polarized reflectance measurements have been made on cesium tetracyanoquinodimethanide, over the frequency range between the far infrared and the near ultraviolet.
Abstract: Room-temperature polarized reflectance measurements have been made on cesium tetracyanoquinodimethanide, ${\mathrm{Cs}}_{2}$${(\mathrm{TCNQ})}_{3}$, over the frequency range between the far infrared and the near ultraviolet. The optical properties of the compound were obtained by Kramers-Kronig analysis. These properties are dominated by vibrational features at low frequencies and by electronic excitations at high frequencies. The observed vibrational features include ordinary intramolecular modes and "anomalous" infrared activity of the totally symmetric vibrations. This latter absorption results from the interaction of these vibrations with the unpaired electron on the TCN${\mathrm{Q}}^{\ensuremath{-}}$ ion. Two electronic excitations are observed for the electric field polarized along the TCNQ chains. The excitation higher in frequency is attributed to an intermolecular charge transfer from one TCN${\mathrm{Q}}^{\ensuremath{-}}$ ion to an adjacent ion, while the lower-frequency excitation results from a charge-transfer transition to an adjacent neutral molecule. Estimates for the effective on-site Coulomb repulsion energy $U$ of 1.14 eV, and for the transfer matrix element $t$ of 0.17 eV, are obtained from these data. Experimental values for the electron-molecular-vibration coupling constants are also obtained.

Journal ArticleDOI
TL;DR: In this paper, the angular dependence of soft-x-ray absorption near the boron $K$ threshold of hexagonal BN display a prominent peak at 192.0 eV.
Abstract: High-resolution measurements of soft-x-ray absorption near the boron $K$ threshold of hexagonal BN display a prominent peak at 192.0 eV. The angular dependence of this absorption for $p$-polarization geometry is consistent with a final state constructed mostly of $\ensuremath{\pi}$-like wave functions. Although higher-energy parts of the spectrum can be approximated by an effective-mass exciton theory, the 192.0-eV peak requires a substantial central-cell correction. This core exciton is thus highly localized in real space and delocalized in $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ space. It seems likely that a mixture of $\ensuremath{\sigma}$- and $\ensuremath{\pi}$-like wave functions is necessary to describe the final state; this could account for the observed line strength at normal incidence which is not zero.

Journal ArticleDOI
TL;DR: In this article, a selective resonant Raman enhancement is observed in solid films of the blue complex formed between starch and iodine, and a theoretical explanation of this phenomenon is given in terms of the theory of resonant secondary emission, and explicit analytical expressions for the Raman cross sections as a function of the exciting wavelength are derived for the case of resonance with broad and structureless absorption bands.
Abstract: A selective resonant Raman enhancement is observed in solid films of the blue complex formed between starch and iodine. The intensity of the 162-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon peak is strongly enhanced when the laser frequency is resonant with the main electronic absorption band at \ensuremath{\simeq} 590 nm, whereas the 112-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is strongly enhanced when the laser frequency is tuned in the blue region in resonance with the shoulder at \ensuremath{\simeq} 455 nm. Similar behavior has been observed in a number of compounds containing polyiodide chains such as iodinedoped polyvinyl alcohol, sulfur polynitride, and polyacetylene. A theoretical explanation of this phenomenon is given in terms of the theory of resonant secondary emission, and explicit analytical expressions for the Raman cross sections as a function of the exciting wavelength are derived for the case of resonance with broad and structureless absorption bands (i.e., a rapidly relaxing system with strong electron-vibration interactions). A structural model for the chromophore of these compounds is also proposed.

Journal ArticleDOI
TL;DR: In this article, the elastic core effect of Al-Mg and Al-7 was measured on the magnesium $K$ edge using the soft-x-ray beam delivered by the ACO (Anneau de Collision d'Orsay) storage ring, as well as the extended x-ray absorption fine structure (EXAFS) spectra of elemental aluminium (1556 eV) and magnesium.
Abstract: The extended x-ray absorption fine structure (EXAFS) spectra of Al-3 at.% Mg and Al-7 at.% Mg have been measured on the magnesium $K$ edge (1303 eV) using the soft-x-ray beam delivered by the ACO (Anneau de Collision d'Orsay) storage ring, as well as the EXAFS of elemental aluminium (1556 eV) and magnesium. With the use of quasiexperimental phase shifts determined from aluminum and magnesium data, the first-shell radius around a magnesium impurity in an Al-Mg alloy has been determined within \ifmmode\pm\else\textpm\fi{}0.015 \AA{}, yielding a direct measurement of the elastic core effect which turns out to be large \ensuremath{\sim}0.08 \AA{}. It is discussed in the framework of the elastic theory of continuous media and of the lattice static method, both of which fail to give the correct magnitude of the local relaxation.

Journal ArticleDOI
TL;DR: In this article, the photon-stimulated desorption (PSD) of ions from the high-coverage phase on Mo(100) has been studied using synchrotron radiation in the spectral range 17-570 eV.
Abstract: The photon-stimulated desorption (PSD) of ${\mathrm{O}}^{+}$ ions from the $\ensuremath{\alpha}$-oxygen (high-coverage) phase on Mo(100) has been studied using synchrotron radiation in the spectral range 17-570 eV. The ions were detected and mass analyzed by means of time-of-flight spectroscopy. The ${\mathrm{O}}^{+}$ yield shows resonances at all Mo core levels and the $1s$ level of oxygen. The results are discussed in terms of inter- and intra-atomic Auger transitions following the primary core excitation. The applicability of PSD as a probe for surface extended x-ray absorption fine-structure measurements is discussed.

Journal ArticleDOI
TL;DR: In this article, a semimicroscopic single-folding potential model was proposed to describe the angular distribution of a single particle in a 1.5-MeV domain for a given center-of-mass angle.
Abstract: The differential cross section angular distributions for 99-MeV $^{6}\mathrm{Li}$ elastic scattering from $^{12}\mathrm{C}$, $^{28}\mathrm{Si}$, $^{40}\mathrm{Ca}$, $^{58}\mathrm{Ni}$, $^{90}\mathrm{Zr}$, and $^{208}\mathrm{Pb}$ have been measured for center-of-mass angles up to ${\ensuremath{\theta}}_{max}=43\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}69\ifmmode^\circ\else\textdegree\fi{}$. The angular distributions show progressively less structure with increasing target mass which is characteristic of strongly absorbed particles. Phenomenological optical-model analyses with Woods-Saxon form factors exhibit both discrete and continuous ambiguities in the parameters which yield a good fit to the data. The $A$ dependence of several real central potential families and the effects of a surface absorption term have been investigated. A semimicroscopic single-folding $^{6}\mathrm{Li}$ potential based on an ($\ensuremath{\alpha}+d$)- cluster model for $^{6}\mathrm{Li}$ gave a good description of the data only after renormalization of the real potential by a factor of \ensuremath{\sim}0.5.

Journal ArticleDOI
TL;DR: In this paper, the authors showed that the Raman process is responsible for the transition from the excited spin-orbit triplets to the ground-state level of the ground level at a temperature between 6 and 13 K.
Abstract: In dilute ZnS:$^{57}\mathrm{Fe}$ absorbers the isolated ${\mathrm{Fe}}^{2+}$ ions give a single M\"ossbauer absorption line at all temperatures ($1.4l~Tl~295 \mathrm{K}$), but a relaxation broadening is observed near 8 K. A convenient relaxation line-shape theory has enabled the transition rate ${W}_{{\ensuremath{\Gamma}}_{4}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}}$ from the first excited spin-orbit triplet ${\ensuremath{\Gamma}}_{4}$ to the ground-state level ${\ensuremath{\Gamma}}_{1}$ to be measured between 6 and 13 K. Two additional ${\mathrm{Fe}}^{2+}$ quadrupole doublets appear for $T\ensuremath{\lesssim}5$ K in the ZnS:$^{57}\mathrm{Co}$ emission spectra. These are due to the slow relaxation contributions of the excited spin-orbit triplets ${\ensuremath{\Gamma}}_{4}$ and ${\ensuremath{\Gamma}}_{5}$ of ${\mathrm{Fe}}^{2+}$ which are then observed out of the thermal equilibrium following the decay of the radioactive parent $^{57}\mathrm{Co}$. The two transition rates ${W}_{{\ensuremath{\Gamma}}_{4}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}}$ and ${W}_{{\ensuremath{\Gamma}}_{5}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}}$ were obtained by following the intensities of the ${\ensuremath{\Gamma}}_{4}$ and ${\ensuremath{\Gamma}}_{5}$ lines in the emission spectra below 5 K when these rates are comparable to the inverse nuclear lifetime $\frac{1}{\ensuremath{\tau}}$. Both the absorber and source measurements of ${W}_{{\ensuremath{\Gamma}}_{4}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}}$ show approximately a ${T}^{5.0}$ mean thermal variation, whereas the source measurements of ${W}_{{\ensuremath{\Gamma}}_{5}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}}$ present a ${T}^{4.0}$ mean variation. These relaxation rates are attributed to Raman processes, which have been shown to follow the thermal dependence ${W}_{e\ensuremath{\rightarrow}g}\ensuremath{\propto}{(\ensuremath{\hbar}{\ensuremath{\omega}}_{0})}^{7}[6!{(\frac{{k}_{B}T}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}})}^{7}+(3)(5!){(\frac{{k}_{B}T}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}})}^{6}+(3)(4!){(\frac{{k}_{B}T}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}})}^{5}+3!{(\frac{{k}_{B}T}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}})}^{4}]$ instead of the usual ${T}^{7}$ dependence, when the temperature is smaller than the energy separation ${E}_{e}\ensuremath{-}{E}_{g}=\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$ between the excited level and the ground-state level.

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TL;DR: In this paper, a zero-phonon line was detected at the 14 175-degree line at the center of the Jahn-Teller interaction, which may be related to an antiresonance of the spin-orbit coupling.
Abstract: Zero-phonon structures of the ${\mathrm{Fe}}^{2+}$ $3{d}^{6}^{5}E(D)\ensuremath{\leftrightarrow}^{5}T_{2}(D)$ transition in absorption and emission include a coinciding line at 2376 ${\mathrm{cm}}^{\ensuremath{-}1}$. Polarized transmission spectra of the ${\mathrm{Co}}^{2+}$ $3{d}^{7}$ transition $^{4}A_{2}(F)\ensuremath{\rightarrow}^{4}T_{1}(P)$ display components split by second-order spin-orbit coupling and indicate an autoionization effect at the 14 175-${\mathrm{cm}}^{\ensuremath{-}1}$ line. Symmetry arguments support the interpretation of this antiresonance by a ${\mathrm{Co}}^{2+}\ensuremath{\rightarrow}{\mathrm{Co}}^{3+}$ charge-transfer process with a threshold at about 13 800 ${\mathrm{cm}}^{\ensuremath{-}1}$. The $^{4}T_{2}(F)\ensuremath{\rightarrow}^{4}A_{2}(F)$ luminescence transition of ${\mathrm{Co}}^{2+}$ gives rise to a zero-phonon line at 2867 ${\mathrm{cm}}^{\ensuremath{-}1}$. The fine structure of the $^{3}T_{1}(F)\ensuremath{\rightarrow}^{3}T_{1}(P)$ absorption of ${\mathrm{Ni}}^{2+}$ $3{d}^{8}$ suggests a strong reduction of spin-orbit coupling by the Jahn-Teller interaction. With ${\mathrm{Ni}}^{2+}$ ions, radiative recombination from the $^{3}T_{1}(P)$, $^{3}A_{2}(F)$, and $^{3}T_{2}(F)$ levels to the $^{3}T_{1}$ ground state is detected, and the first and the last transitions exhibit zero-phonon lines at 10 926 and 3607 ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively. An abundant novel emission band characterized by a zero-phonon transition at 10 170 ${\mathrm{cm}}^{\ensuremath{-}1}$ may be related to an $M$ center, i.e., a ${({V}_{\mathrm{Se}}^{\ensuremath{-}}{V}_{\mathrm{Se}}^{\ensuremath{-}})}^{2+}$ associated defect.

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TL;DR: In this paper, the Gamow-Teller strength for neutrino absorption in the angular range of 0, 30, √ √ ifmmode^ √circ\circ\else\text degree\fi{} was obtained.
Abstract: Neutron spectra from $^{37}\mathrm{Cl}$($p$,$n$)$^{37}\mathrm{Ar}$ at ${E}_{p}=120$ MeV are measured in the angular range 0\ifmmode^\circ\else\textdegree\fi{}-30\ifmmode^\circ\else\textdegree\fi{}. Neutron groups characterized with $L=0$ transfer are obtained up to approximately 12-MeV excitation energy in $^{37}\mathrm{Ar}$. From the observed 0\ifmmode^\circ\else\textdegree\fi{} cross section the Gamow-Teller strengths for $^{37}\mathrm{Cl}$\ensuremath{\rightarrow}$^{37}\mathrm{Ar}$ transitions are obtained which are directly related to neutrino absorption by $^{37}\mathrm{Cl}$. The cross section for $^{8}\mathrm{B}$ solar neutrinos incident on $^{37}\mathrm{Cl}$ is reported.

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TL;DR: The thermal recovery process after 80 K irradiation of NaCl:${\mathrm{Mn}}^{2+}$ have been investigated by using thermoluminescence (TL), optical absorption, and electron paramagnetic resonance (EPR) as mentioned in this paper.
Abstract: The thermal recovery processes after 80 K irradiation of NaCl:${\mathrm{Mn}}^{2+}$ have been investigated by using thermoluminescence (TL), optical absorption, and electron paramagnetic resonance (EPR). A main TL glow peak (II) is observed at about 170 K, both in pure and doped samples (the kinetics being, respectively, second and first order). It has been associated to the recombination of mobile ${V}_{k}$ centers with electron-trapped centers. For doped samples these electronic-centers have been identified as ${\mathrm{Mn}}^{0}$ $B$ centers; whereas for pure samples they are suggested to be ${F}^{\ensuremath{-}}$ centers. The kinetics of the peak as well as the change in the temperature of its maximum with manganese concentration can be accounted for, by using the theory of diffusion-limited reactions. The activation energy for the diffusion of ${V}_{k}$ centers has been found to be 0.37 eV. For doped samples, two other glow peaks at 108 K (I) and 222 K (III) are observed. Possible models for them have been advanced; in particular peak III seems to be associated to the thermal release of holes from ${V}_{F}$ centers. At variance with the data above room temperature, no correlation has been found between any of the TL peaks and an annihilation step in the $F$-center concentration.