Showing papers on "Absorption (logic) published in 1978"

Optical properties of the F + center in crystalline Al 2 O 3

Abstract: Ultraviolet optical absorption, emission, and excitation spectra are presented for 14-MeV neutron-irradiated high-purity crystalline ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$. Low-temperature polarized excitation spectra for the 3.8-eV luminescence correlated well with polarized absorption at 4.8 eV (both polarizations) and 5.4 eV ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$). These optical properties compare favorably with those predicted using wave functions from an earlier pointion calculation for the ${\mathrm{F}}^{+}$ center by La, Bartram, and Cox. The 4.8- and 5.4-eV absorption bands are assigned to the $1A\ensuremath{\rightarrow}1B$ and $1A\ensuremath{\rightarrow}2A$ transitions, respectively, of the ${F}^{+}$ center and the 3.8-eV luminescence is assigned to the $1B\ensuremath{\rightarrow}1A$ transition. The $1A\ensuremath{\rightarrow}2B$ transition could not be conclusively identified, but preliminary evidence suggests it occurs near 6.3 eV. Similar absorption and emission bands are observed following energetic-proton and ${\mathrm{Al}}^{+}$ bombardment. Other uv optical properties are presented, including the quantum efficiency, lifetime, and temperature dependence of the emission.

Absorption and emission spectra of a Schwarzschild black hole

Abstract: The absorption spectrum of a Schwarzschild black hole is studied in detail. Accurate and useful computational methods based on the analytical resolution of the wave equation are developed. In this way phase shifts and absorption cross sections are obtained for a wide range of energy and angular momentum. Comparison with the explicit results valid for low and high frequencies is made. The total absorption cross section of the black hole is obtained as a function of the energy. It presents behavior characteristic of a diffraction pattern. The constant geometric-optics limit [$(\frac{27}{4})\ensuremath{\pi}r_{s}^{}{}_{}{}^{2}$] is approached in an oscillatory fashion. The physical interpretation of these results is given and a simple model which describes qualitatively the absorption of waves by the black hole is presented. From these absorption parameters, the Hawking emission rates are calculated and their properties discussed.

Solar neutrino experiments

Abstract: New results are presented for absorption cross sections of nine possible detectors of solar neutrinos ($^{7}\mathrm{Li}$, $^{37}\mathrm{Cl}$, $^{51}\mathrm{V}$, $^{55}\mathrm{Mn}$, $^{71}\mathrm{Ga}$, $^{81}\mathrm{Br}$, $^{87}\mathrm{Rb}$, $^{115}\mathrm{In}$, and $^{205}\mathrm{Tl}$). Special attention is given to nuclear physics uncertainties. The calculated cross sections are used (with the aid of illustrative solar models and ad hoc assumptions about neutrino propagation) to discuss what can be learned about the sun or weak interactions from each of the nine suggested solar neutrino experiments. An experimental program for neutrino spectroscopy of the solar interior is outlined. It is shown in addition that stellar collapses can be detected to typical distances of several kpc (kiloparsecs) by the proposed $^{7}\mathrm{Li}$, $^{37}\mathrm{Cl}$, and $^{115}\mathrm{In}$ solar neutrino detectors (provided that electron neutrinos do not decay or oscillate).

Electronic resonance enhancement of coherent anti-Stokes Raman scattering

Abstract: We analyze the enhancement of the coherent anti-Stokes Raman-scattering (CARS) susceptibility when the frequencies of the waves involved are tuned into resonance with discrete and continuum one-photon absorptions, and discuss the applications. We first derive the expression for the susceptibility by means of the usual iterative treatment of density-matrix perturbations. We then show that this derivation can be done in a straightforward manner by means of a time-ordered diagrammatic representation, which brings novel physical insight into CARS mechanisms. This representation can also be used to analyze the transient behavior of CARS as the pump fields are turned on and off. In addition, we discuss resonant CARS spectroscopy in the gas phase. The spectrum is composed of the expected enhanced Raman lines and also of double-electronic-resonance lines. All these lines occur as doublets. We derive their relative intensities based on detunings, collisional broadening, Franck-Condon overlap integrals, and rotational transition moments. The line contours are predicted by representing the susceptibility in the complex plane. The problems arising from saturation and the optical Stark effect are also considered; all should be small below pump densities of 100 MW/${\mathrm{cm}}^{2}$ in gas mixtures near STP. Fluorescence interference is negligible, except at power densities high enough for the Stark effect to be large. Beam absorption is also negligible at STP if the resonant species' concentration is less than 1000 ppm; phase matching is then satisfied. Finally, an experimental resonant CARS spectrum of ${\mathrm{I}}_{2}$ at 1 mb in air near STP is presented and interpreted; the susceptibility is about 400 times larger than that of ${\mathrm{O}}_{2}$ off resonance and under the same thermodynamic conditions.

Absolute two-photon absorption coefficients at 355 and 266 nm

Abstract: The absolute two-photon absorption coefficients of uv-transmitting materials have been measured using well-calibrated single picosecond pulses, at the third and fourth harmonic of a mode-locked YAlG:Nd laser system. Two-photon absorption coefficients of the order of ${10}^{\ensuremath{-}3}$ cm/MW were measured for alkali halides, and ${10}^{\ensuremath{-}4}$ cm/MW for harmonic-generating crystals. In materials with band gap greater than $2\ensuremath{\hbar}\ensuremath{\omega}$, no nonlinear absorption could be observed. Calculations based on the Keldysh theory with one adjustable parameter agree quite well with the experimentally observed dispersions in the two-photon absorption coefficients. The effect of multiphoton absorption on the conversion efficiency of harmonic generating crystals and on the breakdown of uv window materials is also discussed.

Core-hole screening in lanthanide metals

Abstract: The multicomponent structure of the $3d$ x-ray photoemission spectra of the light rare-earth metals is elucidated in terms of recent x-ray absorption and appearance-potential spectroscopy results for these materials. Both multiplet splitting due to $3d\ensuremath{-}4f$ exchange and satellite structure due to two competing screening mechanisms are identified. The main component of the $3d$ line arises when the core hole is screened by $5d$ conduction electrons; the satellite from a $4{f}^{n}\ensuremath{\rightarrow}4{f}^{n+1}$ process. The satellites in the metals occur at lower binding energies because the empty $4f$ level falls below the Fermi energy. This interpretation is supported by an "equivalent cores" argument.

Study of Density Fluctuations in the Absorption of Oxygen on Silicon

Abstract: We describe a new technique to measure the spatial distribution of plasma density fluctuations integrated over both frequency and wave vector. Measurements are reported of the frequencies, wave vectors, amplitudes, and also the spatial distributions of low-frequency density fluctuations present in the alcator tokamak. Two regimes can be identified. In high-density plasmas ($\overline{n}g{10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) the fluctuations are peaked at the limiter radius with a fluctuation amplitude of $\frac{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}{n}\ensuremath{\simeq}1$. At lower densities the fluctuation amplitudes peak in the interior of the plasma with $\frac{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}{n}\ensuremath{\simeq}0.07\ifmmode\pm\else\textpm\fi{}0.03$.

Relaxation of Tunneling Systems by Conduction Electrons in a Metallic Glass

Abstract: Acoustic experiments on the metallic glass ${\mathrm{Pd}}_{0.775}$${\mathrm{Si}}_{0.165}$${\mathrm{Cu}}_{0.06}$ are presented which show a tunneling-system decay rate at 0.01 K enhanced by a factor of ${10}^{4}$ over insulating glass. A theory is presented for decay based upon the coupling of tunneling systems to conduction electrons, and the parameters of the theory are obtained from experiment. Relaxational absorption and dispersion in this amorphous metal are interpreted in terms of a tunneling model with a broad spectrum of decay rates.

Theory of nonlinear oscillating dipolar excitations in one-dimensional condensates

Abstract: We find that for a one-dimensional condensate in the presence of an applied ac field, a significant absorption peak should occur for values of the external frequency just below the pinning frequency ${\ensuremath{\omega}}_{F}$. This power absorption is due to those nonlinear normal modes of the condensate known as "breathers," which are shown to phase lock with the applied field.

Pinning and conductivity of two-dimensional charge-density waves in magnetic fields

Abstract: The dynamical properties of the impurity pinning of phasons of the charge-density wave are investigated for two-dimensional electron systems in the presence of magnetic fields applied perpendicular to the system. The effect of magnetic fields is treated classically. It is found that there exists a finite size of domains if the impurity potential overwhelms the long-range part of mutual Coulomb interactions, and the magnetoconductivity tensor is evaluated in such cases. The cyclotron resonance is shown to be shifted and broadened. It is predicted that absorption of electromagnetic waves exists at the pinned-mode frequency though the oscillator strength will be smaller by a factor of ${({\ensuremath{\omega}}_{c}\ensuremath{\tau})}^{\ensuremath{-}2}$ than the cyclotron resonance if ${\ensuremath{\omega}}_{c}\ensuremath{\tau}\ensuremath{\gg}1$, where ${\ensuremath{\omega}}_{c}$ and $\ensuremath{\tau}$ are the cyclotron frequency and the damping of the resonance, respectively.

Photon statistics of a bistable absorber

Abstract: We discuss the quantum-statistical properties of a system of absorbing atoms in a ring cavity under the action of a resonant classical field. At a semiclassical level, this system exhibits a bistable behavior in the sense that the transmitted light ${E}_{T}$ is a discontinuous function of the incident light ${E}_{I}$ with a hysteresis cycle, which resembles a first-order phase transition. Here we specify this analogy stressing the nonthermodynamic properties of this coherently driven system. The quantized field ${E}_{T}$ is described by a Glauber $P$ function, which obeys to a Fokker-Planck equation. This equation is derived from a very general master equation in the case of a good quality cavity by neglecting derivatives of order higher than second. A remarkable feature of this equation is that the diffusion coefficient is a nonlinear function of the field intensity. The steady-state solution of the Fokker-Planck equation has the following features. The $P$ function can be single or double peaked according to the value of ${E}_{I}$. In fact, our Fokker-Planck equation is like the one of a Brownian particle moving in a potential which presents one minimum or minima separated by a barrier, depending on the value of ${E}_{I}$. The peaks of the $P$ function correspond to the minima of the potential. In the double-peaked case, the peaks have different widths. The maxima of the $p$ function occur at values of ${E}_{T}$, which coincide with the classical stationary solutions that are stable according to the linear stability analysis. These semiclassical solutions give a discontinuous two-valued function of ${E}_{T}$ vs ${E}_{I}$ (hysteresis cycle). On the contrary, the mean value $〈{E}_{T}〉$ is a continuous single-valued function of ${E}_{I}$, which behaves similarly to the Maxwell construction of a first-order phase transition: $〈{E}_{T}〉$ jumps from low to high values in a very sharp transition region of ${E}_{I}$, which lies between the discontinuity points of the hysteresis cycle. However, the Maxwell rule that we obtain is quite different from that of equilibrium thermodynamics. The mean-square fluctuations of ${E}_{T}$ are always very small except in the transition region where they are quite remarkable. We also present an exact expression of the stationary $P$ function, which takes into account all higher-order derivatives, and we show that the Fokker-Planck approximation is very good for calculating mean values and mean-square fluctuations. Finally, we discuss the connections which link bistable absorption to the laser with injected signal and to the laser with a saturable absorber.

Charge transfer in collisions of atomic hydrogen with O/sup 8 +/, He/sup + +/, and H/sup +/

Abstract: The charge-transfer process ${\mathrm{O}}^{8+}+\mathrm{H}(1s)\ensuremath{\rightarrow}{\mathrm{O}}^{7+}+{\mathrm{H}}^{+}$ is considered for ${\mathrm{O}}^{8+}$ impact energies from 0.025 to 200 keV/amu, using the atomic base $S$ matrix formulation represented by time-dependent bases which denote moving atomic orbitals. In the evaluation of the $S$ matrix, models of two extreme types, the unitarized model and the absorption model, are introduced. The numerical results show that there is only a small difference between the cross sections obtained using the respective models for impact energies above 0.5 keV/amu, while the cross section due to the unitarized model becomes smaller than one-half of that obtained with the absorption model at an impact energy of 0.05 keV/amu. The data are also compared with other calculations of the same process. For the investigation of the validity of our formula, the unitarized formula is also applied to the processes ${\mathrm{H}}^{+}+\mathrm{H}(1s)\ensuremath{\rightarrow}\mathrm{H}+{\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}+\mathrm{H}(1s)\ensuremath{\rightarrow}{\mathrm{He}}^{+}+{\mathrm{H}}^{+}$. The results are in good agreement with the experimental data.

Rigorous theoretical investigation of the ground state of H 2

Abstract: A variety of results are reported for the ground state ($X^{1}\ensuremath{\Sigma}_{g}^{+}$) of the hydrogen molecule. They include improved Born-Oppenheimer calculations, a study of the convergence of energies with basis-set size, adiabatic calculations, and the first successful ab initio nonadiabatic calculation. The latter, which treats ${\mathrm{H}}_{2}$ as a four-body problem, leads us to predict that the dissociation energy of ${\mathrm{H}}_{2}$ is 36117.92 ${\mathrm{cm}}^{\ensuremath{-}1}$. This value may be compared with 36 117.8\ifmmode\pm\else\textpm\fi{}0.4 ${\mathrm{cm}}^{\ensuremath{-}1}$ (the value derived from the experimental ionization potential), with 36118.6\ifmmode\pm\else\textpm\fi{}0.5 ${\mathrm{cm}}^{\ensuremath{-}1}$ (the value derived from the experimental vibrational levels of the $B^{1}\ensuremath{\Sigma}_{u}^{+}$ state), or with 36118.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ (the upper-bound value derived from the experimental absorption limit).

Unconfined quarks and gluons

Abstract: We explore the possibility that fractionally charged colored quarks and electrically neutral colored gluons may exist as real particles. Our framework is a renormalizable, spontaneously broken version of color dynamics [quantum chromodynamics (QCD)], in which the gluons are given a common mass $\ensuremath{\mu}$ in the Lagrangian. Color SU(3) remains an exact global symmetry, and for small $\ensuremath{\mu}$ our understanding of color-singlet hadrons in QCD is unaltered. For small $\ensuremath{\mu}$ the masses of physical quarks and gluons are large: $O({(2\ensuremath{\pi}{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\mu})}^{\ensuremath{-}1})$ with ${\ensuremath{\alpha}}^{\ensuremath{'}}$ the slope of Regge trajectories. There is at least one stable hadron for every representation of color SU(3). In the unbroken QCD limit ($\ensuremath{\mu}\ensuremath{\rightarrow}0$) the conventional picture of color confinement is recovered. To implement quasiconfinement in detail we use the MIT bag model. For $\ensuremath{\mu}=0$ this model describes confinement. For $\ensuremath{\mu}\ensuremath{ e}0$ it describes quasiconfinement with no further modification. Production cross sections for quarks and gluons turn out to be very small. Quark-nucleon and gluon-nucleon cross sections, on the other hand, are very large. Quarks and gluons have a large nuclear appetite: Sequential absorption of nucleons by a free quark is exothermic up to large values of total baryon number. If quarks are very heavy, primordial quarks left over from the big bang will be found in superheavy quark-nucleon complexes with large nonintegral charge and baryon number. Primordial gluons will have the appearance of integrally charged superheavy nucleon complexes.

Question of Gauge: Nonresonant Two-Photon Absorption

Abstract: The transition probability per unit time for nonresonant two-photon absorption from the $1s$ to the $2s$ state of atomic hydrogen is obtained analytically using both the electric-dipole interaction $\ensuremath{-}e\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}$ and the Coulomb-gauge interaction $\ensuremath{-}(\frac{e}{m})\stackrel{\ensuremath{\rightarrow}}{\mathrm{A}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$. The two rates are different, except in the case of resonance. In situations like this, gauge invariance can be used to show that results calculated from the $\ensuremath{-}e\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}$ interaction are always correct.

Temperature dependence of exchange narrowing in the one-dimensional antiferromagnet N ( C H 3 ) 4 Mn Cl 3 (TMMC)

Abstract: A quantitative analysis of the temperature and angular dependence of the exchange-narrowed EPR linewidth $\ensuremath{\Delta}H(\ensuremath{\theta},T)$ at $K$ band is presented between $40lTl300$ \ifmmode^\circ\else\textdegree\fi{}K for the one-dimensional antiferromagnet ${\mathrm{N}(\mathrm{C}{\mathrm{H}}_{3})}_{4}$Mn${\mathrm{Cl}}_{3}$ (TMMC). The $s=(\frac{5}{2})$ Mn(II) moments are treated as classical spins in order to avoid decoupling four-spin correlation functions arising from the antiferromagnetic coupling at either short or long time. The proper treatment of spin correlations accounts for the $T$ dependence of $\ensuremath{\Delta}H$ and provides a 20% correction to the high-$T$ limit even at 300\ifmmode^\circ\else\textdegree\fi{}K. No adjustable parameters are needed for a quantitative fit at $\ensuremath{\theta}=54\ifmmode^\circ\else\textdegree\fi{}$ (between the applied field and the chain axis), where the absorption is Lorentzian. The dominant broadening arising from intrachain dipolar interactions ${\mathcal{H}}^{\ensuremath{'}}$ limits diffusive behavior and leads to an adjustable parameter which controls the magnitude of $\ensuremath{\Delta}H(0,T)$ and the non-Lorentzian profile at $\ensuremath{\theta}=0$. The increasing importance of short-time effects and the decreasing importance of diffusive contributions to $\ensuremath{\Delta}H$ are quantitatively illustrated at $T=77$ \ifmmode^\circ\else\textdegree\fi{}K. This trend continues to lower $T$, although the $T=20$ \ifmmode^\circ\else\textdegree\fi{}K and $T=4.2$ \ifmmode^\circ\else\textdegree\fi{}K data cannot be treated by the present model of only including spin correlations due to isotropic exchange. The angular anisotropy of $\ensuremath{\Delta}H(\ensuremath{\theta},300)$ and $K$ and $X$ bands shows the expected ${\ensuremath{\omega}}_{0}$ dependence on the Larmor frequency at $\ensuremath{\theta}=54\ifmmode^\circ\else\textdegree\fi{}$.

Indirect exciton fine structure in GaP and the effect of uniaxial stress

Abstract: The technique of high-resolution wavelength-modulated absorption has been used to study the indirect exciton in GaP, both with and without uniaxial stress. The existence of a camel's back in the exciton dispersion curves is confirmed, and its height at zero stress is found to be 2.4\ifmmode\pm\else\textpm\fi{}0.2 meV for the lower (${X}_{7}$) exciton ground state and slightly larger for the upper (${X}_{6}$) exciton. The valley-anisotropy splitting between these states at zero stress is found to be 1.9\ifmmode\pm\else\textpm\fi{}0.2 meV at the minima of the dispersion curves. The observed changes in exciton binding energy with stress are treated approximately using perturbation theory. The following deformation potentials are found: $b=\ensuremath{-}1.8\ifmmode\pm\else\textpm\fi{}0.2$ eV, $d=\ensuremath{-}4.5\ifmmode\pm\else\textpm\fi{}0.5$ eV, ${\mathcal{E}}_{2}=6.5\ifmmode\pm\else\textpm\fi{}0.5$ eV, ${\mathcal{E}}_{1}\ensuremath{-}a=1.6\ifmmode\pm\else\textpm\fi{}0.2$ eV. The exciton parameters obtained rest mainly on the results for [001] stress, as the other stress directions (particularly [111]) give complex line shapes which cannot be understood with a simple model. The observation of an excited state in pure material is found to imply a mean group-state binding energy of 20.5 meV, in good agreement with recent theoretical calculations, but higher than many experimental estimates. The binding energy is confirmed to be of this magnitude from level-crossing effects in the stress measurements.

Interpretation of the High-Energy Processes O 16 ( γ , p 0 ) N 15 and O 16 ( γ , n 0 ) O 15

Abstract: Calculations based on a two-nucleon ($n\ensuremath{-}p$) absorption mechanism show that the form factor of the residual nucleus plays an important role in determining the shape of the photoproduction cross section in a wide energy range $60 \mathrm{MeV}l~{E}_{\ensuremath{\gamma}}l~300 \mathrm{MeV}$

Far-infrared absorption and ionic conductivity of Na, Ag, Rb, and K β-alumina

Abstract: The far-infrared conductivity of Na, Ag, Rb, and K $\ensuremath{\beta}$-alumina is measured from 10 to 200 ${\mathrm{cm}}^{\ensuremath{-}1}$. A broad band of absorption is observed whose detailed structure depends on growth conditions. The frequencies observed agree quantitatively with model calculations by Wang, Gaffari, and Choi (WGC) for the vibrations of the mobile ions. The ionic conductivity of Na, Ag, Rb, and K $\ensuremath{\beta}$-alumina obtained from a melt, a ${\mathrm{Bi}}_{2}$${\mathrm{O}}_{3}$ flux, and a Co-doped ${\mathrm{Bi}}_{2}$${\mathrm{O}}_{3}$ flux is also measured. Invariably the melt-grown material which has the lowest mobile-ion content has the higher conductivity, the pure flux-grown material the lowest, while the Co-doped material lies between the two. This result points to the crucial but subtle role played by the concentration of mobile ions, compensating defect, and the interaction between the two. Despite the variation of the observed conductivities with growth conditions, convincing correlations can be made between the vibrational properties, the model calculation of WGC, and the observed conductivities, that suggest that the basic jump mechanism is the interstitialcy and it is quantitatively described by the WGC model. It is suggested that the observed variation in conductivity are due to the inhibition of this mechanism by short-range order among the mobile ions and compensating defects.

Ne i-like resonance lines and Na i-like satellites in argon and chlorine

Abstract: Using the beam-foil technique, we have measured the decay times of the resonance lines $2{p}^{6}^{1}S\ensuremath{-}2{p}^{5}3s^{1,3}P$ of Ne i-like Cl viii and Arix. We compare our results with theoretical values of the absorption oscillator strengths of these transitions and those of lighter ions of the isoelectronic sequence. We have partially resolved the Na i-like satellite spectra and suggest some identifications based on relativistic Hartree-Fock calculations. Some new observations of transitions between $n=3$ and $n=4$ levels of the Ne i-like ions are reported.

Optical transitions via the deep O donor in GaP. II. Temperature dependence of cross sections

Abstract: A detailed experimental study of temperature dependence of optical cross sections for the GaP: O one-electron state for $Tl300$ K is presented. Broadening effects on the edge as well as temperature shift of the deep-level binding energy and spectral variations of the measured cross section with temperature are treated. In our purely optical data from bulk material the overall broadening of the edge for ${\ensuremath{\sigma}}_{p1}^{0}(h\ensuremath{ u})$ and ${\ensuremath{\sigma}}_{n1}^{0}(h\ensuremath{ u})$ is well explained (\ensuremath{\sim}15% accuracy) by absorption of configuration-coordinate phonon quanta in the optical transitions, apart from an anomalous behavior below 30 K. The 0.9-eV O level is nearly pinned to the valence band below 175 K ($\frac{\ensuremath{\Delta}{E}_{i}}{\ensuremath{\Delta}{E}_{g}}=0.90\ifmmode\pm\else\textpm\fi{}0.10$) while above this temperature $\frac{\ensuremath{\Delta}{E}_{i}}{\ensuremath{\Delta}{E}_{g}}$ seems to decrease gradually. The temperature-dependent spectral behavior of the cross sections seems to be due to the presence of two different levels related to the one-electron O state where the new level is situated 80\ifmmode\pm\else\textpm\fi{}20 meV above the previously established 0.9-eV level. The apparent temperature dependence of the part of the cross sections associated with the 0.9-eV level is described in terms of a thermal activation energy of 0.4 meV.

Deep inelastic pion-induced nuclear reactions in the isobar model

Abstract: Deep inelastic pion-induced nuclear reactions are calculated in the isobar model, which assumes that an interacting resonant pion and nucleon form an unstable particle, the $\ensuremath{\Delta}$, which can propagate through the nucleus and either decay or be absorbed by interacting with other nucleons in the nucleus. The propagation of the particles is treated classically and the interactions are assumed to be incoherent. The lifetime of the $\ensuremath{\Delta}$ is taken to be energy dependent as prescribed by measured pion-nucleon scattering and the cross section of the $\ensuremath{\Delta}$ absorption is determined by measured pion production in two nucleon collisions. The formation of the $\ensuremath{\Delta}$ by a pion and nucleon and the subsequent absorption of the $\ensuremath{\Delta}$ provide a two-step mechanism for pion absorption. These calculations indicate that the pion is absorbed mostly on the inside forward edge of the nuclear surface where the nuclear density has almost reached central density. The calculations are compared with measured pion absorption cross sections, proton spectra, and spallation products in pion-induced reactions. All of these data indicate that pion absorption is underestimated in this model by perhaps as much as 35%, particularly for low energy (\ensuremath{\sim} 100 MeV) pions. Possible explanations for these discrepancies are discussed.[NUCLEAR REACTIONS Calculated absorption cross section, proton spectra, and spallation products for pion-induced reactions on various nuclei.]

Optical absorption and mössbauer spectra of purple and green yoderite, a kyanite-related mineral

Abstract: Mossbauer and polarized optical absorption spectra of the kyanite-related mineral yoderite were recorded. Mossbauer spectra of the purple (PY) and green yoderite (GY) from Mautia Hill, Tanzania, show that the bulk of the iron is Fe3+ in both varieties, with Fe2+/(Fe2++Fe3+) ratios near 0.05. Combining this result with new microprobe data for PY and with literature data for GY gives the crystallochemical formulae: $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.95}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{0}}{\text{.01}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.34}}}^{{\text{3 + }}} {\text{Mn}}_{{\text{0}}{\text{.07}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.57}}} )_{5.97}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.98}}} {\text{P}}_{{\text{0}}{\text{.03}}} ){\text{O}}_{{\text{18}}{\text{.02}}} ({\text{OH)}}_{{\text{1}}{\text{.98}}} ] \hfill \\ \end{gathered}$$ and PY and $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.98}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{< 0}}{\text{.001}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.45}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.56}}} )_{6.02}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.91}}} {\text{O}}_{{\text{17}}{\text{.73}}} {\text{(OH)}}_{{\text{2}}{\text{.27}}} ] \hfill \\ \end{gathered}$$ for GY. The Mossbauer spectra at room temperature contain one main doublet with isomer shifts and quadrupole splittings of 0.36 (PY), 0.38 (GY) and 1.00 (PY), 0.92 (GY) mm s−1, respectively. These values correspond to Fe3+ in six or five-fold coordination. The doublet components have anomalously large half widths indicating either accomodation of Fe3+ in more than one position (e.g., octahedraA1 and five coordinatedA2) or the yet unresolved superstructure.

Systematics of fusion barriers obtained with a modified proximity potential

Abstract: A systematic semiempirical analysis is presented for excitation functions for complete fusion of complex nuclei. Barrier penetration is included as is the $l$ dependence of the fusion radius. The general form of the proximity potential is assumed and small but important parameter adjustments are made to achieve a fit to experimental data. A correction is presented of $s$-wave barrier heights and radii for complete fusion. For ${Z}_{t}{Z}_{p}l500$ these fusion radii are greater than the empirical reaction radii obtained at energies well above the barrier. This result implies weak surface absorption for ${Z}_{t}{Z}_{p}l500$ and the contrary for higher ${Z}_{t}{Z}_{p}$.NUCLEAR REACTIONS Semiempirical analysis of complete fusion excitation functions with a modified proximity potential; correlation between $s$-wave barrier heights and fusion radii; fusion radii exceed empirical interaction radii for ${Z}_{t}{Z}_{p}l500$ and the contrary.

Electrical conductivity of germanium as a function of optically injected carrier density and temperature

Abstract: The effects of carrier-carrier scattering on the transport properties of high-density electron-hole plasmas in germanium were studied both experimentally and theoretically. Experimentally, the dc electrical conductivities of nearly uniform optically injected bipolar plasmas in high-purity single crystals of germanium were measured for a broad range of carrier densities (\ensuremath{\sim} ${10}^{14}$ - \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) and temperatures (21-298 K). Concurrent measurements of the absorption of 3.39-\ensuremath{\mu}m radiation due to inter-hole-band transitions aided the calibration of the carrier densities at some temperatures. The previous theory of Appel, which used Kohler's variational principle to solve the Boltzmann equation, was generalized with respect to the inclusion of interband hole scattering, electron mass anisotropy, degeneracy, and use of the RPA $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$-dependent screening approximation (although not with respect to all at once). The most crucial generalization was the accounting for the presence of nonconducting excitons in the plasma. The scattering of free carriers by excitons was also considered, using a neutral-impurity scattering theory similar to that of Erginsoy. Incorporation of all the above considerations led to excellent agreement between experiment and theory at densities below the Mott criterion for transition to the exciton-free phase. The transition itself appeared to be gradual, however, and behavior of the conductivity in the high-density region was consistent with the recent assertion by Thomas and Rice that excitons are present at densities significantly above the Mott criterion.

Phonon-induced fine structure of excitons in solid nitrogen

Abstract: Results of quantitative high-resolution absorption measurements in the range of the $w^{1}\ensuremath{\Delta}_{\mathrm{u}}\ensuremath{\leftarrow}X{^{1}\ensuremath{\Sigma}_{\mathrm{g}}}^{+}$ and $a^{1}\ensuremath{\Pi}_{\mathrm{g}}\ensuremath{\leftarrow}X{^{1}\ensuremath{\Sigma}_{\mathrm{g}}}^{+}$ derived exciton progressions in solid ${\mathrm{N}}_{2}$ are reported. The newly observed detailed fine structure of the vibrational bands is analyzed in terms of a sharp zero-phonon line and phonon-assisted exciton transitions. A theoretical analysis based on a strong-exciton-phonon-coupling model leads to good agreement with the observed experimental line shape.

Experimental evidence for spin diffusion in the quasi-two-dimensional Heisenberg paramagnet ( C 2 H 5 N H 3 ) 2 Mn Cl 4

Abstract: High-temperature spin dynamics in the quasi-two-dimensional Heisenberg system ethylammonium-manganesechloride have been investigated by EPR with very high sensitivity Two different experimental methods are presented which can be used to analyze the resonances of the respective memory spectrum occurring at the Larmor frequency ${\ensuremath{\omega}}_{0}$ and at $2{\ensuremath{\omega}}_{0}$ Around ${\ensuremath{\omega}}_{0}$ the memory spectrum is obtained from the absorption and dispersion part of the transversal susceptibility The resonance behavior at $2{\ensuremath{\omega}}_{0}$ can be obtained directly from a satellite structure at half EPR field which has been observed in the transversal and in the longitudinal susceptibility For both resonances a logarithmic shape is determined which\char22{}as theory predicts\char22{}is due to spin diffusion There is some experimental evidence for a decoupling of the 4-spin correlation function for long times The spin-diffusion constant is evaluated to be $\frac{D}{{a}^{2}}=13\ifmmode\pm\else\textpm\fi{}2$ K

Threshold pion absorption in nuclei

Abstract: We attempt to explain threshold pion absorption in nuclei using simple pion rescattering mechanicsms. While the model is successful in the case of the deuteron it leads to 10-30% too small absorption rates in other nuclei. We also study a $\ensuremath{\rho}\ensuremath{\omega}$ exchange absorption mechanism that does not act in the deuteron, but find that it can only remove a small part of the remaining discrepancy. The two-nucleon absorption mechanism accounts for 75% of the imaginary part of the $S$-wave pion nucleus optical potential.NUCLEAR REACTIONS Pion absorption calculated at threshold in light nuclei $^{2}\mathrm{H}$, $^{4}\mathrm{He}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{20}\mathrm{Ne}$, and nuclear matter.

Scattering of 41 MeV. cap alpha. particles and 46 MeV /sup 3/He from /sup 27/Al, /sup 28/Si, /sup 29/Si, and /sup 30/Si

Abstract: Elastic scattering angular distributions were measured for 46-MeV $^{3}\mathrm{He}$ over the \ensuremath{\sim} 30\ifmmode^\circ\else\textdegree\fi{}-110\ifmmode^\circ\else\textdegree\fi{} range and for 41-MeV $\ensuremath{\alpha}$ particles over the \ensuremath{\sim} 25\ifmmode^\circ\else\textdegree\fi{}-150\ifmmode^\circ\else\textdegree\fi{} range for the neighboring targets $^{27}\mathrm{Al}$, $^{28,29,30}\mathrm{Si}$. For $\ensuremath{\alpha}$ particles, $l=2$ inelastic scattering data were also obtained at angles beyond \ensuremath{\sim} 50\ifmmode^\circ\else\textdegree\fi{}. For $^{3}\mathrm{He}$ the three Si targets yield similar angular distributions that are different from that of $^{27}\mathrm{Al}$ at angles larger than \ensuremath{\sim} 55\ifmmode^\circ\else\textdegree\fi{}. The elastic $\ensuremath{\alpha}$-particle data supplement earlier back angle data to yield almost complete angular distributions. The $^{28}\mathrm{Si}$ $\ensuremath{\alpha}$-particle elastic angular distribution exhibits an anamoly in the 80\ifmmode^\circ\else\textdegree\fi{}-150\ifmmode^\circ\else\textdegree\fi{} region. The $^{3}\mathrm{He}$ data were fitted by optical model potentials with either volume or surface absorption. To fit all of the $\ensuremath{\alpha}$-particle elastic scattering data, potentials with both volume and surface absorption, with separate geometry parameters, were needed. Distorted-wave Born-approximation analyses of the $\ensuremath{\alpha}$-particle inelastic scattering data yield values of ${\ensuremath{\beta}}_{2}$ that are in reasonable agreement with values obtained from analyses of scattering at other energies and incident particles.NUCLEAR REACTIONS ($^{3}\mathrm{He}$, $^{3}\mathrm{He}$) at MeV and ($\ensuremath{\alpha}, {\ensuremath{\alpha}}^{\ensuremath{'}}$) at 41 MeV from, $^{27}\mathrm{Al}$, $^{28,29,30,}\mathrm{Si}$; enriched targets. Measured $\ensuremath{\sigma}(\ensuremath{\theta})$; performed optical model and DWBA analyses; deduced scattering potentials and ${\ensuremath{\beta}}_{2}$ values.

Systematic study of the magnetic circular dichroism of n s 2 ions in KBr and KI

Abstract: We report the measurement of the magnetic circular dichroism in the $A$, $B$, and $C$ bands of ${\mathrm{Ga}}^{+}$, ${\mathrm{In}}^{+}$, ${\mathrm{Tl}}^{+}$ in KI and of ${\mathrm{Ga}}^{+}$ and ${\mathrm{In}}^{+}$ in KBr. A complete analysis of the magnetic circular dichroism and absorption data is performed using the method of moments. Expressions are obtained for the coupling to the cubic vibrational modes (${A}_{1g}$) and to the noncubic modes (${E}_{g}$ and ${T}_{2g}$) both in terms of the zeroth and first moments of the absorption band and in terms of the third moment of the magnetic circular dichroism. Both methods are found to agree for ${\mathrm{In}}^{+}$ in KI and KBr. Results of the analysis of both the orbital $g$ factor and the second moments of the noncubic modes indicate that there are significant differences between the orbital states of the singlet ($C$ band) and triplet ($A$ band). Furthermore, the coupling to the cubic modes is found to be as important as coupling to the noncubic modes in the triplet states of ${\mathrm{Ga}}^{+}$ and ${\mathrm{In}}^{+}$ in KBr and KI. Lastly, electron-lattice coupling is found to increase for these ions in going from KI to KBr as the host crystals.