Showing papers on "Absorption (logic) published in 1973"

Complete Hydrogen and Helium Particle Spectra from 30- to 60-MeV Proton Bombardment of Nuclei with A = 12 to 209 and Comparison with the Intranuclear Cascade Model

Abstract: Differential cross sections for the production of proton, deuteron, triton, helium-3, and $\ensuremath{\alpha}$ particles from as many as 10 targets ($A=12\ensuremath{-}209$) were measured using 29-, 39-, and 62-MeV incident protons. The particles were detected, with $\ensuremath{\approx}0.2$-MeV [full width at half maximum (FWHM)] energy resolution for protons, over a secondary energy range of $\ensuremath{\approx}2\ensuremath{-}6 \mathrm{to} 62$ MeV in a total absorption telescope composed of three solid-state detectors. Representative results are shown for cross sections differential in energy and angle, as well as for angle-and energy-integrated cross sections. For incident 60-MeV protons the integral magnitude of the nonevaporation charged-particle production is found to be $\ensuremath{\sim}{10}^{2}{A}^{\frac{1}{3}}$ mb. Fewer protons but more complex particles were measured for carbon and oxygen targets than expected from an ${A}^{\frac{1}{3}}$ dependence for either component alone. The continuum cross sections for $z=1$ particles at a given angle (mb ${\mathrm{sr}}^{\ensuremath{-}1}$ Me${\mathrm{V}}^{\ensuremath{-}1}$) are nearly independent of incident energy when measured with incident protons in the 30- to 60- MeV energy range. Nonevaporation production of complex particles ($A\ensuremath{\ge}2$) is 25-40% of that for protons. The proton spectra have been compared with predictions from the intranuclear cascade model. Differential spectral predictions compare well with the measured spectra for angles in the range \ensuremath{\sim}25-60\ifmmode^\circ\else\textdegree\fi{}, and relatively poor predictions for small and large angles are more favorable when reflection and refraction by the potential well are included. Evidence is given that predictions for backward angles are greatly improved by allowing proton scattering from nucleon pairs within the model nucleus, but the $A$-dependent underprediction at extreme forward angles is not understood at all. The calculated angle-integrated spectra reproduce the measured spectral shape but consistently predict $\ensuremath{\approx}30%$ too few nonevaporation protons for targets with $A\ensuremath{\ge}27$.[NUCLEAR REACTIONS $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{27}\mathrm{Al}$, $^{54}\mathrm{Fe}$, $^{56}\mathrm{Fe}$, $^{60}\mathrm{Ni}$, $^{89}\mathrm{Y}$, $^{120}\mathrm{Sn}$, $^{197}\mathrm{Au}$, $^{209}\mathrm{Bi}$, ($p,{p}^{\ensuremath{'}}X$), ($p,dX$), ($p,tX$), ($p,^{3}\mathrm{He}X$), ($p,\ensuremath{\alpha}X$), $E=62,39,29$ MeV; semi; measured $\ensuremath{\sigma}(E;{E}_{{p}^{\ensuremath{'}}},{E}_{d},{E}_{{3}_{\mathrm{He}}},{E}_{\ensuremath{\alpha}},\ensuremath{\theta})$; deduced $\ensuremath{\sigma}(E)$. $2\ensuremath{\lesssim}{E}_{{p}^{\ensuremath{'}}},{E}_{d},{E}_{t},{E}_{{3}_{\mathrm{He}}},{E}_{\ensuremath{\alpha}}\ensuremath{\lesssim}70$ MeV. Comparisons with intranuclear cascade model.]

Optical Spectra of Core Electrons in Metals with an Incomplete Shell. I. Analytic Features

Abstract: Analytic features of the spectra due to excitation of a core electron in metal with an incomplete shell are studied with a simple model. The incomplete shell (denoted by d state, representatively) is treated as a nondegenerate localized state interacting with conduction band through the s – d mixing. It is assumed that in the initial state of the absorption the unoccupied d level is well above the Fermi energy e F while in the final states the d level of the excited atom is lowered down to e d due to the core hole left behind. When e d > e F , the singular absorption edge appears at \(\tilde{\varepsilon}_{\text{F}}\) as expected, with an antiresonance around \(\tilde{\varepsilon}_{d}\). When e d < e F , the singular absorption edge appears at \(\tilde{\varepsilon}_{d}\), the Fermi level threshold being blurred out by the life time effect of the d hole. Emission spectra are also calculated. The results are compared qualitatively with experimental data.

Experimental Measurements of the C 12 + C 12 Nuclear Reactions at Low Energies

Abstract: The nuclear reactions $^{12}\mathrm{C}(^{12}\mathrm{C}, \ensuremath{\alpha})^{20}\mathrm{Ne}$ and $^{12}\mathrm{C}(^{12}\mathrm{C}, p)^{23}\mathrm{Na}$ have been observed from 5-MeV down to 2.45-MeV center-of-mass energy. Angular distributions and energy distributions of the protons and $\ensuremath{\alpha}$ particles were analyzed to obtain the total cross sections and other nuclear information. The Coulomb and angular momentum barrier penetrability was factored out to elucidate the intermediate resonance structure in the nuclear factor $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{S}$. The observed rise in the nuclear factor at the lowest energies may be interpreted as "absorption under the barrier" as proposed by Michaud and Vogt. The importance of these reactions for the carbon burning era of nucleosynthesis and energy generation in the later evolution of stars is mentioned and reaction rates are estimated for various burning temperatures.

Absorption and Magnetic-Circular-Dichroism Spectra of Octahedral Ce 3 + in Cs 2 NaY Cl 6

Abstract: The $4{f}^{1}\ensuremath{\rightarrow}5{d}^{1}$ transitions of ${\mathrm{Ce}}^{3+}$, ${E}_{u}^{\ensuremath{'}\ensuremath{'}}(^{2}F_{\frac{5}{2}})\ensuremath{\rightarrow}{U}_{g}^{\ensuremath{'}}(^{2}T_{2g})$ and ${E}_{u}^{\ensuremath{'}\ensuremath{'}}(^{2}F_{\frac{5}{2}})\ensuremath{\rightarrow}{E}_{g}^{\ensuremath{'}\ensuremath{'}}(^{2}T_{2g})$ have been studied at precisely octahedral sites using absorption and magnetic-circular-dichroism (MCD) techniques at liquid-helium temperature. The no-phonon lines of the two transitions are located at 28 196 and 29435 ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively. Considerable vibronic structure is resolved in both bands characterized by long progressions in ${\ensuremath{ u}}_{1}({a}_{1g})$ of the Ce${\mathrm{Cl}}_{6}^{3\ensuremath{-}}$ moiety with low-energy lattice progressions superimposed. The ${U}_{g}^{\ensuremath{'}}$ and ${E}_{g}^{\ensuremath{'}\ensuremath{'}}$ excited states show a spin-orbit splitting of \ensuremath{\sim} 1240 ${\mathrm{cm}}^{\ensuremath{-}1}$ versus a predicted free-ion value of $\frac{3{\ensuremath{\zeta}}_{d}}{2}\ensuremath{\approx}1500$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The MCD spectrum strongly supports the proposed assignments. There are no indications of a significant Jahn-Teller effect in the ${U}_{g}^{\ensuremath{'}}(^{2}T_{2g})$ excited state. Since no further ${\mathrm{Ce}}^{3+}$ absorption is detected out to 50 000 ${\mathrm{cm}}^{\ensuremath{-}1}$, $\ensuremath{\Delta}(\ensuremath{\equiv}10Dq)\ensuremath{\ge}20000$ ${\mathrm{cm}}^{\ensuremath{-}1}$ in the excited state.

Absorption Coefficients for the Wings of the First Two Resonance Doublets of Cesium Broadened by Argon

Abstract: Measurements are reported of the absolute absorption coefficients in the wings of the 8944-, 8521-, 4593-, and 4555-\AA{} resonance lines of cesium in the presence of argon at densities between about 6 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ and 2 \ifmmode\times\else\texttimes\fi{} ${10}^{19}$ atom ${\mathrm{cm}}^{\ensuremath{-}3}$. The data extend from about 0.7 \AA{} to as much as 600 \AA{} from the line center and were obtained using cesium densities between 7 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ and 2 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ atom ${\mathrm{cm}}^{\ensuremath{-}3}$ and temperatures between 390 and 470\ifmmode^\circ\else\textdegree\fi{}K. The measured absorption coefficients are proportional to the product of the cesium and argon densities as expected from line broadening produced by binary collisions between cesium and argon atoms. Using the quasistatic theory as a guide for interpretation, the line profiles for wings of the first resonance doublet show a transition from a wavelength dependence determined by long-range van der Waals forces to a dependence determined by the short-range internuclear potentials discussed by Hedges, Drummond, and Gallagher (HDG). The far-wing absorption profiles obtained in the present experiments are in good agreement with those calculated from the fluorescence data of HDG.

Optical Spectra of Cr 3 + Pairs in the Spinel Zn Ga 2 O 4

Abstract: Exchange interactions of nearest-neighbor ${\mathrm{Cr}}^{3+}$ pairs in Zn${\mathrm{Ga}}_{2}$${\mathrm{O}}_{4}$ have been studied in detail. An analysis of the emission, absorption, and luminescence excitation spectra yield the ($^{4}A_{2}$, $^{4}A_{2}$) ground-state level splittings and the ($^{4}A_{2}$, $^{2}E$) excited-state splittings. The first are described by $\ensuremath{-}J{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}_{2}+j{({\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}_{2})}^{2}$, with ${S}_{1}={S}_{2}=\frac{3}{2}$, $J=\ensuremath{-}22.2\ifmmode\pm\else\textpm\fi{}0.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$, and $j=\ensuremath{-}1.7\ifmmode\pm\else\textpm\fi{}0.3$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The ($^{4}A_{2}$, $^{2}E$) level splittings are compared with the values calculated from the orbital-dependent exchange Hamiltonian ${\mathcal{H}}_{\mathrm{ex}}=\ensuremath{-}{\ensuremath{\Sigma}}_{j}{J}_{\mathrm{ij}}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{s}}}_{i}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{s}}}_{j}$, where ${s}_{i}={s}_{j}=\frac{1}{2}$ and the sum is over the ${t}_{2g}$ orbitals of the ${\mathrm{Cr}}^{3+}$ ion. In the local ${C}_{2\ensuremath{ u}}$ symmetry of the pair, there are four independent ${J}_{\mathrm{ij}}$ parameters which have been determined as ${J}_{c}=+39$ ${\mathrm{cm}}^{\ensuremath{-}1}$, $J_{c}^{}{}_{}{}^{\ensuremath{'}}=+117$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${J}_{d}=\ensuremath{-}561$ ${\mathrm{cm}}^{\ensuremath{-}1}$, and ${J}_{\ensuremath{\pi}}=\ensuremath{-}105$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The large value of ${J}_{d}$ is a consequence of the direct overlap between ${t}_{2}\ensuremath{\zeta}$ functions, which is possible in the 90\ifmmode^\circ\else\textdegree\fi{} exchange configuration of nearest-neighbor Cr pairs in the spinel structure. The relative intensities of the different pair lines are compared with values calculated from the mechanism of the exchange-induced dipole moment and reasonable agreement is found.

Total Hadronic Photoabsorption Cross Sections on Hydrogen and Complex Nuclei from 4 to 18 GeV

Abstract: Final total cross sections are given for a counter experiment at SLAC on hadronic photon absorption in hydrogen, deuterium, carbon, copper, and lead at incident energies from 3.7 to 18.3 GeV. Some of the nucleon cross sections have been revised and the C, Cu, and Pb data from 3.7 to 7.4 GeV have not been reported previously. The cross sections for complex nuclei vary approximately as ${A}^{0.9}$ in our energy range, indicating that the photon interacts, at least partially, as a strongly interacting particle. The energy dependences of the proton and neutron cross sections are also similar to those of hadron-nucleon cross sections and hence may be fitted by a typical Regge parametrization, yielding ${\ensuremath{\sigma}}_{T}(\ensuremath{\gamma}p)=(98.7\ifmmode\pm\else\textpm\fi{}3.6)+(65.0\ifmmode\pm\else\textpm\fi{}10.1){\ensuremath{ u}}^{\ensuremath{-}\frac{1}{2}}$ \ensuremath{\mu}b and ${\ensuremath{\sigma}}_{T}(\ensuremath{\gamma}n)=(103.4\ifmmode\pm\else\textpm\fi{}6.7)+(33.1\ifmmode\pm\else\textpm\fi{}19.4){\ensuremath{ u}}^{\ensuremath{-}\frac{1}{2}}$ \ensuremath{\mu}b, where $\ensuremath{ u}$ is the photon energy in GeV. These extrapolate to the same value at infinite energy, consistent with Pomeranchukon exchange, and the energy-dependent part yields an isovector-to-isoscalar-exchange ratio of 0.18 \ifmmode\pm\else\textpm\fi{} 0.06. While these observations are qualitatively consistent with vector meson dominance, quantitatively vector dominance fails in relating our results to $\ensuremath{\rho}$ photo-production on hydrogen or to experiments determining the $\ensuremath{\rho}$-nucleon cross section. Vector dominance cannot be rescued by assuming that the $\ensuremath{\rho}$-photon coupling constant depends on the photon mass. Instead, an additional short-range interaction is apparently required, possibly due to a heavy (\ensuremath{\gtrsim} 2 GeV / ${\mathit{c}}^{2}$) vector meson or to a bare-photon interaction. The additional interaction accounts for approximately 20% of the total photoabsorption cross section.

Optical Measurements of Surface Plasmons in Gold

Abstract: The absorption associated with surface plasmons at a gold-air interface is measured using an evanescent wave coupler. The measurements cover the energy range 1.8-3 eV. Scans made for different values of surface wave vector in the range 1.2 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$-2.7 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ give the dispersion of the surface plasmon both for the gold-air interface and for a gold-silica interface, and also yield the optical constants of gold. Good agreement is obtained with lower-energy measurements of the dispersion by Ritchie et al. An analysis of the range of surface plasmons is presented using the present data and the Drude approximation for extrapolation into the infrared where the range increases to several centimeters.

Incoherent Resonance Fluorescence from a Rb Atomic Beam Excited by a Short Coherent Optical Pulse

Abstract: Fluorescence has been detected from Rb atoms following narrow-line excitation by a short coherent optical pulse The fluorescence oscillates as a function of the pulse area $A$ with maxima for $A=\ensuremath{\pi},3\ensuremath{\pi},5\ensuremath{\pi},\dots{}$ and minima for $A=0,2\ensuremath{\pi},4\ensuremath{\pi},\dots{}$ In addition to demonstrating one of the simplest coherent optical effects, this experiment dramatizes the potential of the technique for determining dipole moments, pulse areas, relaxation times, and lifetimes The details of the experimental apparatus are described Results of a computer simulation of the experiment including the actuai excitation pulse shape, absorption frequency width, finite absorption (5-10% of pulse), and loss to a third level are given

Photoionization from the Selectively Excited 3 s 3 p P 1 1 ∘ State to the 3 p 2 S 0 1 Autoionization Level of Mg i

Abstract: The $3s3p{^{1}P_{1}}^{\ensuremath{\circ}}$ level of Mg i has been selectively excited by the second harmonic of the output of a high-power, narrow-band, frequency tunable dye laser. The excited-state absorption series $3s3p{^{1}P_{1}}^{\ensuremath{\circ}}\ensuremath{-}3\mathrm{snd}^{1}D_{2}$ has been recorded to $n=24$. A strong autoionization resonance has been obtained at $\ensuremath{\lambda}=300.9$ nm, with a photoionization cross section of \ensuremath{\sim}5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$, in very good agreement with the values predicted by calculation.

Generation of Vacuum-Ultraviolet and Soft—X-Ray Radiation Using High-Order Nonlinear Optical Polarizabilities

Abstract: The harmonic or sum-frequency power generated in the last coherence length of a low-density atomic species is calculated subject to the condition that the applied electric field be bounded by the multiphoton absorption or ionization limit. It is shown that higher-order polarizations may equal or exceed lower-order polarizations. Calculations are given for generation at 1773 and 1064 \AA{} in Xe, and at 236, 169, and 177 \AA{} in ${\mathrm{Li}}^{+}$.

Properties of Crystalline and Amorphous Silicon Telluride

Abstract: Single crystals of silicon telluride, which has an unusual 2:3 stoichiometry, have been grown by a chemical vapor deposition technique. A photoluminescence level at 1.3 eV has been found as well as strong EPR signals, suggesting defects in the crystal which also could explain other unusual features. Thin films and bulk samples of silicon telluride glasses, ${\mathrm{Si}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$, $0.02\ensuremath{\le}x\ensuremath{\le}0.25$, were prepared and examined by optical absorption, photoconductivity, infrared transmission, electrical conductivity, Hall effect, differential thermal analysis, EPR, and x-ray experiments. The values of the energy gap (0.7 to 1.0 eV), resistivity ($\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ \ensuremath{\Omega} cm), and glass transition temperature (120 to 175 \ifmmode^\circ\else\textdegree\fi{}C) were found to increase throughout the glass-forming region, with increasing silicon. Some microscopic structural features are shared by crystal and glass, including the presence of Si${\mathrm{Te}}_{4}$ tetrahedra, interatomic distances and reststrahlen absorption bands, even though the bonding seems to change from largely ionic in the crystal to largely covalent in the glasses. No EPR signal could be detected in the bulk glasses down to 77\ifmmode^\circ\else\textdegree\fi{}K. The Hall mobility of the glasses is approximately 1.0 ${\mathrm{cm}}^{2}$/V sec, independent of room temperature from 200 to 300 \ifmmode^\circ\else\textdegree\fi{}K, and indicates the predominance of hole conduction. No polaron effects are evident. Structurally, a covalent model consisting of tetravalent silicon and divalent tellurium is consistent with all the data for the amorphous system.

Optical Absorption in Vapor-Quenched Aluminum

Abstract: An aluminum film formed by evaporation at 25 K onto a similar film previously deposited at room temperature displayed a featureless optical spectrum in the range 0.6-3.5 eV, the characteristic (200) peak being completely absent. A closer analysis showed the absorption to consist of a Drude term ($\ensuremath{\hbar}{\ensuremath{\omega}}_{p}=7.45$ eV; $\ensuremath{\tau}=3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ sec) and what is believed to be interband absorption, possibly associated with a residual (111) absorption band.

Role of γ+γ→e++e-+e++e- in photoproduction, colliding beams, and cosmic photon absorption

Abstract: We consider the photon-photon channel, $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{e}^{+}+{e}^{\ensuremath{-}}+{e}^{+}+{e}^{\ensuremath{-}}$, with respect to the possible relevant experimental roles it plays in (i) the photoproduction process $\ensuremath{\gamma}+Z\ensuremath{\rightarrow}Z+{e}^{+}+{e}^{\ensuremath{-}}+{e}^{+}+{e}^{\ensuremath{-}}$, (ii) the colliding beam process ${e}^{\ifmmode\pm\else\textpm\fi{}}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ifmmode\pm\else\textpm\fi{}}+{e}^{\ensuremath{-}}+{e}^{+}+{e}^{\ensuremath{-}}+{e}^{+}+{e}^{\ensuremath{-}}$, and (iii) the absorption of high-energy cosmic photons. Discussion of the first role includes calculation of rates at different energies, error estimates, and effects of screening and threshold cuts. In the second role, emphasis is placed on scale changes due to energy and angle kinematic cuts. Competition with the single-pair absorption process (i.e., $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{e}^{+}+{e}^{\ensuremath{-}}$) at the higher cosmic $\ensuremath{\gamma}$-ray energies \ensuremath{\approx} ${10}^{20}$ eV is discussed in the consideration of the third role.

Parametric Decay of Obliquely Incident Radiation

Abstract: An electromagnetic wave obliquely incident on an inhomogeneous plasma has a resonance near ${\ensuremath{\omega}}_{0}={\ensuremath{\omega}}_{\mathrm{pe}}(x)$. It is shown that temporally growing modes exist for parametric decay into ion-acoustic and electron-plasma waves in the vicinity of this resonance in spite of the density gradient. For typical laser fusion parameters the maximum growth rate is comparable to the ion-acoustic frequency. The mechanism can contribute to significant power absorption.

Acceptor-to-Band Transitions in Semiconductors: Photoluminescence, Exponential Absorption Edges, and Final-State Interactions

Abstract: The effects of disorder and of final-state electron-hole interactions on acceptor-to-band absorption and photoluminescence spectra of semiconductors are treated theoretically within the effective-mass approximation The results of the calculations are used to analyze photoluminescence spectra from lightly doped GaAs: Cd measured by Williams and Bebb It is shown that although a proper one-electron calculation of optical absorption ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ should include the effects of electron-hole spatial correlations, the excitonic Coulomb final-state interaction exactly cancels the spatial correlations in the case of infinite hole mass Therefore the usual one-electron approximation, which neglects both spatial and Coulombic correlations, is valid for small electron-hole mass ratios (${m}_{e}\ensuremath{\ll}{m}_{h}$); for GaAs: Cd, the finite-mass-ratio corrections are nearly independent of energy and can be neglected The major portion of the discrepancies between the Williams-Bebb data and Eagles's one-electron theory cannot be attributed to final-state interactions, but can be attributed to small internal microfields of strength ${10}^{3}$ to ${10}^{4}$ V/cm generated by disorder The sources of these microfields are most likely to be piezoelectric phonons, but ionized impurities or excitons could possibly generate such fields It is shown that photoluminescence lines are particularly sensitive to disorder and that the band-to-acceptor luminescence edge $L(\ensuremath{\omega})$ should not be a precisely exponential function of photon energy $\ensuremath{\hbar}\ensuremath{\omega}$ Instead the microfield model predicts that the edge shape should be approximately given by ln $L(\ensuremath{\omega})\ensuremath{\propto}{(\ensuremath{\hbar}{\ensuremath{\omega}}_{0}\ensuremath{-}\ensuremath{\hbar}\ensuremath{\omega})}^{\ensuremath{\alpha}}$, with $1l\ensuremath{\alpha}l\frac{3}{2}$ The existing data are compatible with $\ensuremath{\alpha}=\frac{3}{2}$, but the possibility of $\ensuremath{\alpha}=1$ cannot be ruled out without additional photoluminescence measurements further down the edge If such measurements indicate that $\ensuremath{\alpha}$ is precisely equal to unity, the electric-microfield theory of exponential absorption edges may require revision

Theory of Absolute Intensities for Vibration-Rotation Transitions in Methane

Abstract: Transition dipole moments have been calculated for lines in the ${\ensuremath{ u}}_{3}$, ${\ensuremath{ u}}_{4}$, $2{\ensuremath{ u}}_{3}$, and $3{\ensuremath{ u}}_{3}$ vibration-rotation bands of $^{12}\mathrm{C}$${\mathrm{H}}_{4}$. The values of ${\ensuremath{\mu}}^{2}$ are 0.76 \ifmmode\pm\else\textpm\fi{} 0.10, 0.69 \ifmmode\pm\else\textpm\fi{} 0.04, (1.86 \ifmmode\pm\else\textpm\fi{} 0.07) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$, and (2.96 \ifmmode\pm\else\textpm\fi{} 0.16) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ ${\mathrm{D}}^{2}$, respectively. These results have been applied to predict absolute line intensities. In particular, for the $P7F$ line at 2947.912 ${\mathrm{cm}}^{\ensuremath{-}1}$, the calculated intensity is (1.34 \ifmmode\pm\else\textpm\fi{} 0.17) ${\mathrm{cm}}^{\ensuremath{-}2}$ ${\mathrm{atm}}^{\ensuremath{-}1}$ at 300\ifmmode^\circ\else\textdegree\fi{} K. Furthermore, ultraweak absorption features observed in the $3{\ensuremath{ u}}_{3}$ region may be accounted for on the basis of transitions with $J\ensuremath{\approx}30$.

NMR Studies of Dilute Alloys of Ni in Cu

Abstract: We have studied the nuclear magnetic resonance of Cu atoms which are near neighbors to Ni atoms in dilute $\mathrm{Cu}$ Ni alloys. The experiments were performed at liquid-helium temperatures and at magnetic fields from 6 to 60 kG. The resonances show up as weak satellites to the main absorption of more distant Cu. By varying the magnetic field, we can show that several lines arise from a single shell of neighbors having a quadrupole coupling ${\ensuremath{ u}}_{1}=1.1\ifmmode\pm\else\textpm\fi{}0.1$ MHz, asymmetry parameter $\ensuremath{\eta}=0.20\ifmmode\pm\else\textpm\fi{}0.05$, and a magnetic shift $\frac{\ensuremath{\Delta}H}{H}=\ensuremath{-}0.27\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Comparing the quadrupole coupling with estimates made by B\'eal-Monod and Tompa for CuNi by measuring wipe-out number, we conclude the lines arise from first neighbors. The magnetic shift is smaller than we had previously observed for CuCo in almost exactly the ratio of the $\ensuremath{\chi}J$'s of the two impurities, where $\ensuremath{\chi}$ is the impurity susceptibility and $J$ is the $s\ensuremath{-}d$ exchange coupling.

Experimental Evidence for Repulsive Cores in Heavy-Ion Reactions

Abstract: The simultaneous study of the $^{12}\mathrm{C}$+$^{12}\mathrm{C}$, $^{12}\mathrm{C}$+$^{16}\mathrm{O}$, and $^{16}\mathrm{O}$+$^{16}\mathrm{O}$ total reaction cross sections at low energy strongly suggests that the optical potential of the heavy ions has a central soft repulsive core. Conventional Woods-Saxon potentials can fit the $^{12}\mathrm{C}$+$^{12}\mathrm{C}$ and $^{16}\mathrm{O}$+$^{16}\mathrm{O}$ reactions on the one hand or the $^{12}\mathrm{C}$+$^{16}\mathrm{O}$ reaction on the other but only the potentials with the repulsive core can fit all three reactions with the parameters varying smoothly from reaction to reaction. These reactions also determine accurately the surface thickness of the imaginary part of the potential; it varies from 0.35 fm for the $^{16}\mathrm{O}$+$^{16}\mathrm{O}$ reaction to 0.55 fm for the $^{12}\mathrm{C}$+$^{12}\mathrm{C}$ reaction. At very low energies most of the absorption takes place while the two nuclei are far from each other.

Temperature Dependence of Multiphonon Infrared Absorption

Abstract: Measurements of Harrington and Hass and of Barker indicate that the temperature dependence of the infrared absorption coefficient $\ensuremath{\beta}$ in the $n$-phonon region is considerably weaker than $\ensuremath{\beta}\ensuremath{\sim}{T}^{n\ensuremath{-}1}$, which had been predicted for the high-temperature limit of multi-phonon absorption. This discrepancy is resolved by taking into account the temperature dependence of the phonon frequencies and the lattice constant. The agreement between the experimental and theoretical results with no adjustable parameters is good. A new evaluation of the multiphonon sums yields $\ensuremath{\beta}\ensuremath{\sim}\mathrm{exp}(\ensuremath{-}\ensuremath{\omega}\ensuremath{\tau})$ directly, rather than as a sum on $n$.

Screening-Enhanced Optical Absorption and the Search for M 3 Critical Points

Abstract: The optical absorption spectrum near an ${M}_{3}$ critical point of a model doped semiconductor or metal is calculated as a function of screening wave vector, using a realistic effective-mass model and a statically screened final-state interaction, but neglecting exchange. Wavelength-modulation line shapes are derived by direct (analytic) differentiation, and it is shown that screening enhances the absorption spectrum and its derivatives near an ${M}_{3}$ threshold. Experiments to detect ${M}_{3}$ critical points should concentrate on materials with heavy band masses near the conduction-band minimum and light (negative) masses at the band maximum.

Further Measurements and Calculations of the Far-Infrared Anharmonic Optical Properties of KI between 12 and 300 °K

Abstract: Measurements and calculations of the far-infrared optical properties of ${\mathrm{K}}^{39}$I at 300, 77, and 12 \ifmmode^\circ\else\textdegree\fi{}K are presented. The measurements are mainly those of absorption in crystals of various thicknesses. The calculation assumed cubic anharmonicity only, with nearest-neighbor coupling, and the input lattice-dynamical data were obtained from a shell-model program. These data were generated with a wave-vector density of 32000 points per zone, which was sufficient to give a 2-3-${\mathrm{cm}}^{\ensuremath{-}1}$ resolution. The over-all agreement between experiment and theory, in both the intensity and the structure of the spectra, is good. The magnitude of certain calculated features is, however, too large, indicating a need to consider next-nearest-neighbor interactions. Evidence was also found for three-phonon damping, both beyond the two-phonon limit at all temperatures and at ${\overline{\ensuremath{ u}}}_{0}$ by 300 \ifmmode^\circ\else\textdegree\fi{}K. From these measurements it was possible to calculate portions of the three-phonon damping spectra, which were found to be reasonable. The higher-phonon effects at 300 \ifmmode^\circ\else\textdegree\fi{}K did not seem noticeably more pronounced than those found in the much harder LiF, and arguments are presented to understand this. Finally, the isotope-induced one-phonon processes which occur in natural KI were calculated. These are shown to be small away from the resonance frequency ${\overline{\ensuremath{ u}}}_{0}$ and not to be the major damping mechanism at ${\overline{\ensuremath{ u}}}_{0}$ at low temperatures, in contrast to LiF.

Incoherent and coherent pulses and multiphoton absorption probability

Abstract: We deal with the models of a coherent pulse and of an incoherent one. The former pulse is described by using a polychromatic pure coherent state and the latter is obtained by an incoherent superposition of identical polychromatic elementary pulses shifted in time by a statistically distributed delay. For the incoherent pulse the ratio between its duration and its coherence time can assume any possible value. By using these pulse models the dependence ofs-photon absorption probability on the statistical properties of the incident pulsed light is investigated. It is found that, when the two pulses have the same spectral distribution and the same energy, the ratio between the probabilities fors-photon absorption induced by the incoherent pulse and that induced by the coherent pulse is given by\(s!\left[ {\frac{{I_{inc} }}{{I_{coh} }}} \right]^{s - 1} \), where Iinc and Icoh are the intensities of the incoherent pulse and of the coherent pulse, respectively, at the time in which the field interacts with the atomic system. This ratio shows that, when the two pulses have also the same duration and, therefore, the same intensity, the probability induced by the incoherent pulse is s! times higher than that induced by the coherent pulse. Explicit calculations on the multiphoton absorption probability are carried out for the pulses when their time shapes are Gaussian.

The g-factor of the 2+ state of 180 W and quadrupole moment ratios observed for 180 W and 180 Hf by the Mössbauer technique

Abstract: By Mossbauer absorption experiments the magnetic hyperfine splitting has been observed for the 2+ states of180W and182W in a tungsten iron alloy (3.6 at%W). Since theg-factor of the 2+ state of182W is known the measured splitting of the182W line could be used for the calibration of the magnetic hyperfine field and the measurement with180W gave then for the unknowng2+-factor of180W: $$g_{2 + } (^{180} W) = 0.260 \pm 0.017.$$ By use of a WO3 absorber the electric quadrupole splittings in the same states were measured. The ratio of the quadrupole moments was derived $$\frac{{Q_{2 + } (^{180} W)}}{{Q_{2 + } (^{182} W)}} = 0.983 \pm 0.022.$$ This ratio is somewhat smaller, but more accurate than the weighted means of previous results and in disagreement with the theoretical prediction.

Theory of Absorption of Electromagnetic Radiation by Molecule-Ion Traps in n-Si and n-Ge

Abstract: A new mechanism of absorption of electromagnetic radiation at low temperatures in $n$-type silicon and germanium has been investigated. The wavelength region is from 0.01 to 0.1 cm for Si and from 0.035 to 0.5 cm for Ge. Donor concentrations up to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in Si and 6 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in Ge and compensations up to 0.1 were considered. The model used was that of the photon-induced transitions of electrons between the $1s {\ensuremath{\sigma}}_{g}$- and $2p {\ensuremath{\sigma}}_{u}$-like states of ionized donor pairs. The formula for the absorption coefficient is derived and numerical results are presented. The role of the ionized-donor-pair molecule-ion traps in hopping processes is also discussed.

Electric Field Nuclear Double Resonance in Ag + -Doped NaCl

Abstract: An electric field-nuclear-magnetic-double-resonance experiment is described which enables the resolution of certain weak resonances otherwise concealed under a nearby, much stronger resonance In ${\mathrm{Ag}}^{+}$-doped NaCl, the weak resonance of the nearest-neighbor sodium nuclei to the ${\mathrm{Ag}}^{+}$ impurity is concealed by the strong resonance of the sodium nuclei far from the impurity due to the small quadrupole splitting of the nearest neighbors The technique is based on the fact that the ${\mathrm{Ag}}^{+}$ ion breaks the inversion symmetry of the near neighbors, whereas the distant nuclei remain at sites of inversion symmetry As a result an external oscillating electric field can induce $\ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}2$ transitions in the near neighbors but not in the distant ones The absorption is detected by monitoring the saturation of the magnetic resonance of the distant nuclei as a function of electric field frequency The nearest-neighbor sodium nuclei were found to have a static quadrupole coupling constant ${\ensuremath{ u}}_{Q}=125\ifmmode\pm\else\textpm\fi{}02$ kHz with an asymmetry parameter of $\ensuremath{\eta}=028\ifmmode\pm\else\textpm\fi{}004$ A simple theory is developed which explains the intensities of the quadrupole lines as a function of the orientation of the electric and magnetic fields A dynamic quadrupole coupling constant ${\ensuremath{ u}}_{D}$ is defined For the nearest-neighbor sodium nuclei ${\ensuremath{ u}}_{D}\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ Hz/(V/cm) and for the next-nearest-neighbor sodium nuclei ${\ensuremath{ u}}_{D}\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ Hz/(V/cm)

Optical Quenching of Photoluminescence in Chalcogenide Glasses; Evidence for States in Midgap

Abstract: Optical quenching of photoluminescence by radiation with photon energy less than the band-gap energy has been observed in glassy ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ and ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{As}}_{2}$${\mathrm{Te}}_{3}$. The quenching spectra provide evidence of optical absorption by a narrow band of localized states in the gap which are involved in the radiative recombination process and which are attributable to defects or impurities rather than to tails in the density of states extending into the gap.

An extension of Cramér's estimate for the absorption probability of a random walk

Abstract: Consider a real-valued random walk which is defined on a Markov chain { X n : n ≥ 0} with countable state space I . We assume that a matrix Q (.) = ( q ij (.)) is given such that

Determination of Hot-Carrier Distribution Functions in Uniaxially Stressed p-Type Germanium

Abstract: This paper gives a description of an experimental determination of distribution functions in $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ space of hot holes in uniaxially compressed germanium. The hot-carrier studies were made at 85\ifmmode^\circ\else\textdegree\fi{}K at fields up to 1000 V/cm and uniaxial stresses up to 11 800 kg/${\mathrm{cm}}^{2}$. The field and stress were always in the $〈111〉$ direction. For the highest stresses, the maximum fields were close to the threshold for current oscillations. The distribution functions were obtained from experimental modulation of intervalence-band absorption of infrared radiation. In order to interpret the results, a parametrized distribution function has been assumed. The parameters of the distribution function are then fitted to the experimental modulation. The calculation of absorption was performed numerically, using a four-band $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ model. This model was checked for consistency by comparing with piezoabsorption measurements performed in thermal equilibrium. The average carrier energy calculated from the distribution function shows a fast increase with stress and almost saturates when the strain splitting of the two ${p}_{\frac{3}{2}}$ levels reaches the optical-phonon energy. This saturation is interpreted in terms of the change in scattering probabilities with stress. A model based on the nonparabolicity of the upper ${p}_{\frac{3}{2}}$ level is proposed for the negative differential conductivity in stressed $p$-type Ge.

Green's-Function Approach to Inverse Compton Scattering

Abstract: The emission, absorption, and transfer of radiation produced through the Compton interaction of relativistic electrons with microwave photons in a homogeneous source is considered. It is shown that the intensity of Compton-scattered radiation can be conveniently expressed in terms of a Green's function, and this enables the radiation-transfer problem for arbitrarily many scatterings to be formalized in terms of a series of appropriately defined scattering vertex functions and absorption propagators. A rigorous discussion of the cross section for inverse Compton scattering is presented which is valid both in the extreme relativistic ($\ensuremath{\gamma}\ensuremath{\gg}1$) and weakly relativistic limits ($\ensuremath{\gamma}\ensuremath{\simeq}1$). Using these results the spectral behavior of the intensity of Compton-scattered radiation is evaluated explicitly for once- and twice-scattered photons; the qualitative features of the multiple-scattering problem are described. At sufficiently high frequencies the spectral behavior of the first- and all higher-order scatterings is the same.