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Showing papers on "Absorption (logic) published in 1969"


Journal ArticleDOI
TL;DR: In this article, the singularities of x-ray absorption or emission in metals are studied by a new "one-body" method, which describes the scattering of conduction electrons by the transient potential due to the deep hole.
Abstract: The singularities of x-ray absorption or emission in metals are studied by a new "one-body" method, which describes the scattering of conduction electrons by the transient potential due to the deep hole. Using the linked-cluster theorem, the net transition rate in the time representation is expressed as the product of two factors: a one-electron transient Green's function $L$, and the deep-level Green's function $\mathcal{G}$. These factors obey simple Dyson equations, which can be solved asymptotically by using Muskhelishvili's method. The x-ray transition rate is found to behave as $\frac{1}{{\ensuremath{\epsilon}}^{\ensuremath{\alpha}}}$, where $\ensuremath{\epsilon}$ is the frequency measured from the threshold, and $\ensuremath{\alpha}$ an exponent involving the various phase shifts ${\ensuremath{\delta}}_{l}$ which describe scattering by the deep hole. $\ensuremath{\alpha}$ may be g0 (infinite threshold) or 0 (zero threshold). The experimental implications of these results and their relation to the Friedel sum rule are briefly discussed.

903 citations


Journal ArticleDOI
TL;DR: Interstellar formaldehyde has been detected in absorption against numerous galactic and extragalactic radio sources by means of the ground-state rotational transition at 4830 MHz as mentioned in this paper.
Abstract: Interstellar formaldehyde (${\mathrm{H}}_{2}$CO) has been detected in absorption against numerous galactic and extragalactic radio sources by means of the ${1}_{11}$-${1}_{10}$ ground-state rotational transition at 4830 MHz. The absorbing regions often correspond in velocity with 18-cm OH features. ${\mathrm{H}}_{2}$CO is the first organic polyatomic molecule ever detected in the interstellar medium and its widespread distribution indicates that processes of interstellar chemical evolution may be much more complex than previously assumed.

329 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured stopping cross sections of particles from 400 keV to 2 MeV with an accuracy of 3.6 to 4.4% in 17 elements (Be, C, Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ge, Pd, Ag, In and Sn).
Abstract: Stopping cross sections of $\ensuremath{\alpha}$ particles from 400 keV to 2 MeV have been measured to an accuracy of \ifmmode\pm\else\textpm\fi{}3.6 to \ifmmode\pm\else\textpm\fi{}4.9% in 17 elements (Be, C, Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ge, Pd, Ag, In, and Sn). The experimental method consists of the elastic scattering of $\ensuremath{\alpha}$ particles from a thick Ta backing onto which a thin layer of target element has been evaporated. The energy loss of $\ensuremath{\alpha}$ particles in the target film is determined by the difference in energy between $\ensuremath{\alpha}$ particles scattered from clean Ta and $\ensuremath{\alpha}$ particles scattered from Ta after having gone through the thin layer of target element. The results are compared with measurements by Porat and Ramavataram and by Gobeli and with estimates by Whaling; the discrepancies range from 1 to 20%. Structure in and a decrease of the $\ensuremath{\alpha}$-particle stopping cross section ${\ensuremath{\epsilon}}_{\ensuremath{\alpha}}$ with stopping element atomic number ${Z}_{2}$ are noticed in the region ${Z}_{2}=22\ensuremath{-}29$. This dependence is not predicted by the Bethe-Bloch formalism valid at higher velocities, nor by the Firsov or the Lindhard formalism valid at lower velocities. The oscillatory structure of ${\ensuremath{\epsilon}}_{\ensuremath{\alpha}}$ on ${Z}_{2}$ is discussed qualitatively by comparing ${\ensuremath{\epsilon}}_{\ensuremath{\alpha}}({E}_{\ensuremath{\alpha}})$ versus ${Z}_{2}$ with a Hartree-Fock-Slater potential $\ensuremath{\varphi}(r)$ versus $Z$, with ${E}_{\ensuremath{\alpha}}$ related to the radius $r$ by a velocity comparison. An empirical formula for ${\ensuremath{\epsilon}}_{\ensuremath{\alpha}}={\ensuremath{\epsilon}}_{\ensuremath{\alpha}}({E}_{\ensuremath{\alpha}}, {Z}_{2})$ has been constructed from the present measurements.

122 citations


Journal ArticleDOI
TL;DR: The results of a series of reflectivity and absorption measurements performed to clarify respectively the excitonic transitions in the intrinsic region to 13.5 eV and the thermal and spectral dependences of absorptions in the tail region of LiF, NaF, KF, CaF 2 SrF 2 and BaF 2, each being cleaved from Harshaw blocks of single crystal, are presented in this article.
Abstract: The results of a series of reflectivity and absorption measurements performed to clarify respectively the excitonic transitions in the intrinsic region to 13.5 eV and the thermal and spectral dependences of absorptions in the tail region of LiF, NaF, KF, CaF 2 SrF 2 and BaF 2 , each being cleaved from Harshaw blocks of single crystal, are presented. The tails of Harshaw LiF, being expressed by exponential functions of energy in the range of 10 0 cm -1 \({\lesssim}A{\lesssim}5{\cdot}10^{2}\) cm -1 and 300°K≤ T ≤573°K, have turned out to be extrinsic to LiF. The present experiments could not provide sufficiently conclusive evidences in clarifying the question whether the tails of Harshaw alkaline earth fluorides, to Urbach rule is applicable for the range of 10 0 cm -1 \({\lesssim}A{\lesssim}10^{2}\) cm -1 and 78°K≤ T ≤573°K, are intrinsic. In the first peak of reflectivity spectra, the fluorine doublet with ∼0.05 eV separation is detected for KF and quintet structure for SrF 2 and BaF 2 . In the latter, th...

105 citations


Journal ArticleDOI
TL;DR: The theory of atomic structure developed in the two preceding papers which treats electron correlation accurately in excited as well as ground states is applied to the evaluation of multiplet absorption oscillator strengths for a number of transitions of the type of $1{ s}^{2}2{s}^{^{ 2}2s 2{p}^{n}2
Abstract: The theory of atomic structure developed in the two preceding papers which treats electron correlation accurately in excited as well as ground states is applied to the evaluation of multiplet absorption oscillator strengths for a number of transitions of the type of $1{s}^{2}2{s}^{2}2{p}^{n}\ensuremath{\rightarrow}1{s}^{2}2s2{p}^{n+1}$ in CII, NI, NII, NIII, OII, OIII, OIV, FII, NeII, and NaIII. Those types of correlation effects necessary to obtain accurate oscillator strengths are clearly indicated by the theory. The usual improvement on the Restricted Hartree-Fock (RHF) calculation, the mixing of those few configurations nearly degenerate with the RHF configuration, is by itself incapable of bringing the oscillator strengths into agreement with experimental values. All the nondynamical correlation effects given in the first paper of this series must be considered. Very detailed wave functions which contain those important nondynamical correlations were obtained in that paper and here are used to compute oscillator strengths. The results are compared extensively with recent experiments. The calculated values are usually in very good agreement with experimental data. Many more transitions for which no experimental results are yet available are also tabulated here.

85 citations


Journal ArticleDOI
TL;DR: The O\char22{}H stretching band and its immediate sidebands have been studied for O${\mathrm{H}}^{\ensuremath{-}1}$ impurities in NaCl, KCl, RbCl and KBr, KI, and NaBr from 1.4 to 300\ifmmode^\circ\else\textdegree\fi{}K as discussed by the authors.
Abstract: The O\char22{}H stretching band and its immediate sidebands have been studied for O${\mathrm{H}}^{\ensuremath{-}}$ impurities in NaCl, KCl, RbCl, KBr, KI, and NaBr from 1.4 to 300\ifmmode^\circ\else\textdegree\fi{}K. Sideband splittings range from 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ in NaCl:O${\mathrm{H}}^{\ensuremath{-}}$ to 37 ${\mathrm{cm}}^{\ensuremath{-}1}$ in KBr:O${\mathrm{H}}^{\ensuremath{-}}$, and correlate with dips in the low-temperature thermal conductivity and with far-infrared absorption bands. Preliminary results on the electric-field-induced dichroism in KCl:O${\mathrm{H}}^{\ensuremath{-}}$ and RbCl:O${\mathrm{H}}^{\ensuremath{-}}$ give a dipole transition moment perpendicular to the O\char22{}H axis for the main sideband. There is evidence that several sidebands in NaCl:O${\mathrm{H}}^{\ensuremath{-}}$ are themselves combination bands with one or more 2-3-${\mathrm{cm}}^{\ensuremath{-}1}$ tunneling levels. The first overtone yields values of ${\ensuremath{\omega}}_{e}=3790$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{e}{x}_{e}=85.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for O${\mathrm{H}}^{\ensuremath{-}}$ in KBr. These values explain very well the O${\mathrm{H}}^{\ensuremath{-}}$/O${\mathrm{D}}^{\ensuremath{-}}$ isotope shift of the main stretching band in KBr and KCl. The main-band oscillator strengths are small, on the order of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ for most crystals, but this value is consistent with a theoretical calculation of the dipole moment by Cade. The main bands broaden rapidly with increasing temperature, possibly indicating a prominent lifetime-limiting role played by the sideband levels. The sideband levels themselves are tentatively suggested to be due to a center-of-mass resonance in KBr:O${\mathrm{H}}^{\ensuremath{-}}$ and to a librational resonance or fast tunneling level in KCl:O${\mathrm{H}}^{\ensuremath{-}}$.

85 citations


Journal ArticleDOI
TL;DR: In this article, the reflectance and electroreflectance spectra of single crystals of lead iodide (Pb${\mathrm{I}}_{2}$) were measured between 2.2 and 5.0 eV at 4.5 and 77
Abstract: The reflectance and electroreflectance spectra of single crystals of lead iodide (Pb${\mathrm{I}}_{2}$) were measured between 2.2 and 5.0 eV at 4.5 and 77\ifmmode^\circ\else\textdegree\fi{}K, respectively. A Kramers-Kronig dispersion analysis of the data was made, and the real and imaginary parts of the dielectric constant were calculated together with their field-induced modulation. The first three lines of the hydrogenic exciton series associated with the absorption edge are well resolved in reflectivity. In the same energy range, the electoreflectance spectrum is interpreted as the Stark shift of the first exciton line from which the binding energy of the exciton is deduced. In the region of fundamental absorption, the existence of several metastable exciton states gives rise to a new and large contribution to the electroreflectance spectra.

81 citations


Journal ArticleDOI
TL;DR: In this paper, the Ising-basis-function (IBF) method was used to obtain the energy of the two-magnon bound state originally found by Orbach for general longitudinal exchange anisotropy.
Abstract: The recent observation of two-, three-, four-, and five-magnon bound states in the linear chains of Co${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O has prompted a theoretical examination of such states in an anisotropic linear ferromagnetic chain with $S=\frac{1}{2}$. A new method called the Ising-basis-function (IBF) method is developed. This method treats the conventional, localized Ising wave functions as Wannier functions, from which a complete, orthonormal set of Bloch functions (IBF's) is formed. Using these IBF's as basis functions, we obtain the expression for the energy of the two-magnon bound state originally found by Orbach for general longitudinal exchange anisotropy. Furthermore, we can calculate the energy of the ($ng2$)-magnon bound states for the case of strong longitudinal anisotropy. The method is also applied to describe the effect of transverse exchange anisotropy. It is shown that this anisotropy causes an interaction between bound states, particularly important near zero field, and gives rise to a finite probability of exciting the bound states by photon absorption. The generalization of this method to treat bound states in two and three dimensions and for $Sg\frac{1}{2}$ is also discussed. The method is simple and has a direct physical interpretation. As an example, a physical description of the two-magnon bound state in a general system is given. Since the IBF method automatically contains some of the magnon-magnon interactions in zero order, it should be useful in other problems where these interactions are important.

79 citations


Journal ArticleDOI
TL;DR: In this paper, the energy and $A$ dependence of the cross sections for nuclear photoabsorption and inelastic electron-nucleus scattering are given, assuming that the photon interactions are mediated by hadrons of sufficiently low mass and that the momentum transfers in forward photoproduced hadrons are small compared to the reciprocals of their mean free paths in nuclear matter.
Abstract: Quantitative predictions for the energy and $A$ dependence of the cross sections for nuclear photoabsorption and inelastic electron-nucleus scattering are given. In general, the nucleons do not contribute equally to the total photon-nucleus cross section when coherent contributions of photoproduced hadrons are taken into account. At low energies (${E}_{\ensuremath{\gamma}}\ensuremath{\sim}1$ BeV), the cross sections are proportional to nuclear number $A$, but at high energies, they become proportional to the number of surface nucleons---provided that the photon interactions are mediated by hadrons of sufficiently low mass. The condition on the masses is that the momentum transfers in forward photoproduction of these states should be small compared to the reciprocals of their mean free paths in nuclear matter. In the case of $\ensuremath{\rho}$ dominance, the real-photon photoabsorption cross section has the same $A$ dependence as hadron-nucleus total cross sections for photon energies above $\ensuremath{\approx}10$ BeV, whereas the cross section for virtual photon absorption at that energy, obtained from inelastic electron scattering is nearly proportional to $A$ for spacelike momentum transfer $|{Q}^{2}|\ensuremath{\gtrsim}5$ Be${\mathrm{V}}^{2}$. We then generalize to an arbitrary spectrum of intermediate particles, and discuss the sensitivity of feasible experiments to various models in which the spectrum contains important structure beyond the $\ensuremath{\rho}$. Measurements of the photon-nucleus cross sections will provide a fundamental test of "hadron dominance" in general, and of $\ensuremath{\rho}\ensuremath{-}\ensuremath{\omega}\ensuremath{-}\ensuremath{\varphi}$ dominance in particular, as well as help to determine the basic parameters of photon-nucleon and $\ensuremath{\rho}$-nucleon interactions. We also calculate the photon-deuteron cross section and discuss the multiple scattering approach to photon-nucleus interactions. This discussion provides insight into the many-body processes which underlie the eikonal, optical-model calculations; it is also relevant to the determination of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}n}$ at high energies.

64 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the absorption spectra of MgO powders obtained from three sources and interpreted in terms of both Fr\"ohlich-type (surface) modes and classical electromagnetic scattering theory.
Abstract: Magnesium oxide powders obtained from three sources exhibit strikingly different infrared absorption spectra. These differences are correlated herein with variations in the size and shape of the particles in the powders, and are interpreted in terms of both Fr\"ohlich-type (surface) modes and classical electromagnetic scattering theory. (1) Reagent-grade MgO powder exhibits a strong and broad absorption centered at 550 ${\mathrm{cm}}^{\ensuremath{-}1}$, the position of which is determined by the Fr\"ohlich-type fundamental frequency for spherical particles that are small compared with the radiation wavelength. The absorption broadening results from variations in particle size and shape. The presence of double particles gives rise to a secondary absorption near 460 ${\mathrm{cm}}^{\ensuremath{-}1}$. (2) MgO smoke in various matrices absorbs strongly at the bulk-mode frequency (401 ${\mathrm{cm}}^{\ensuremath{-}1}$) owing to the presence of both individual large cubical particles and long chains of small cubical particles. A second strong absorption occurs at 490 ${\mathrm{cm}}^{\ensuremath{-}1}$ in Nujol, or at 546 ${\mathrm{cm}}^{\ensuremath{-}1}$ on polyethylene plates; these frequencies represent a surface mode of small MgO cubes. A shoulder at still higher frequencies is ascribed to surface modes of rod-shaped particles. (3) MgO powder obtained from thermally decomposed MgC${\mathrm{O}}_{3}$ absorbs strongly at the bulk-mode frequency (401 ${\mathrm{cm}}^{\ensuremath{-}1}$), because of the presence of large flakelike particles having a very fine grain structure. With increasing grain size, the principal absorption shifts to lower frequencies, and a secondary absorption, near 490 ${\mathrm{cm}}^{\ensuremath{-}1}$ in Nujol, appears as a result of absorption by small MgO particles.

61 citations


Journal ArticleDOI
TL;DR: In this article, the resonant scattering of monochromatic light in gases is analyzed and explicit expressions for the differential cross section are obtained in the limits (Doppler width) \ensuremath{\gg} (natural width), (collision width), and (Collision width)
Abstract: The resonant scattering of monochromatic light in gases is analyzed. Explicit expressions for the differential cross section are obtained in the limits (Doppler width) \ensuremath{\gg} (natural width) \ensuremath{\gg} (collision width) and (collision width) \ensuremath{\gg} (natural width) \ensuremath{\gg} (Doppler width). In the former limit, the frequency distribution of scattered light is narrowed in the forward and backward directions, and has the full absorption width for right-angle scattering. In the collision-dominated limit, the cross section separates into a coherent and a fluorescent component. The motion of the atoms broadens the $\ensuremath{\delta}$ function distribution characterizing coherent scattering from a stationary target. The interference between light scattered by different atoms is assessed.

Journal ArticleDOI
TL;DR: The ultraviolet-absorption spectra of europium and ytterbium ions in Ca${\mathrm{F}}_{2}$ crystals have been measured at room and liquid-nitrogen temperatures.
Abstract: The ultraviolet-absorption spectra of europium and ytterbium ions in Ca${\mathrm{F}}_{2}$ crystals have been measured at room and liquid-nitrogen temperatures. Their spectra are similar to each other in the general location of their absorption bands. Both spectra consist of: (i) $4f\ensuremath{\rightarrow}5d$ bands, between about 24 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ and about 50 000 ${\mathrm{cm}}^{\ensuremath{-}1}$, due to isolated R${\mathrm{E}}^{2+}$ with crystal field strength \ensuremath{\sim}17 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ between ${e}_{g}$ and ${t}_{2g}$ bands; (ii) $4f\ensuremath{\rightarrow}5d$ bands, shifted \ensuremath{\sim}9 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ to higher energy, due to R${\mathrm{E}}^{2+}$ ions surrounded by R${\mathrm{E}}^{3+}$ neighbors; (iii) a $4f\ensuremath{\rightarrow}6p$ broad and weak band of ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_{2}$ with maximum at \ensuremath{\sim}71 000 ${\mathrm{cm}}^{\ensuremath{-}1}$; and (iv) $4f\ensuremath{\rightarrow}5d$ bands above \ensuremath{\sim}64 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ due to isolated R${\mathrm{E}}^{3+}$ and cluster-ion R${\mathrm{E}}^{3+}$. The structure in the absorption spectra of both isolated ${\mathrm{RE}}^{2+}(4{f}^{n})$ and ${\mathrm{RE}}^{3+}(4{f}^{n})$ ions can be interpreted as formed through interaction between a $5d$ electron having ${e}_{g}$ or ${t}_{2g}$ crystal field symmetry and electrons in the ground multiplet of the $4{f}^{n\ensuremath{-}1}$ core.

Journal ArticleDOI
TL;DR: In this paper, the combined resonance absorption line involving a change in both Landau and spin quantum numbers was studied in several samples of $n$-type InSb having carrier concentrations between 10 and 100 kG.
Abstract: The combined resonance absorption line involving a change in both Landau ($L$) and spin ($S$) quantum numbers ($\ensuremath{\Delta}L=1$; $\ensuremath{\Delta}S=\ensuremath{-}1$) has been studied in several samples of $n$-type InSb having carrier concentrations between ${10}^{14}$ and ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Experiments were performed at liquid-nitrogen and liquid-helium temperatures in magnetic fields between 10 and 100 kG. Measurements of the integrated absorption of this line as a function of magnetic field and carrier concentration are compared with calculations based on the non-parabolicity and inversion-asymmetry mechanisms. Results are in good agreement with the predictions of the nonparabolicity mechanism, indicating that this is the primary mechanism allowing such transitions in InSb. The observed combined resonance transition in conjunction with spin-up and spin-down cyclotron resonance measurements have yielded the $g$ values of the $L=0$ and $L=1$ Landau levels over a wide range of magnetic fields. None of the inversion-asymmetry-allowed transitions were observed. From this negative result, an upper bound is placed on the inversion-asymmetry parameter ${\ensuremath{\delta}}_{0}$.

Journal ArticleDOI
A. Onton1
TL;DR: In this paper, infrared transmission measurements of GaP with S or Si donor impurities at liquid-helium temperature reveal absorption lines due to excitation of the donor electron, and their spectral positions are used to determine optical ionization energies of 104.2 and 82.5 meV.
Abstract: Infrared transmission measurements of GaP with S or Si donor impurities at liquid-helium temperature reveal absorption lines due to excitation of the donor electron. Strong absorption peaks corresponding to the $1s\ensuremath{\rightarrow}2{p}_{\ifmmode\pm\else\textpm\fi{}}$ transition are observed, and their spectral positions are used to determine optical ionization energies of 104.2 and 82.5 meV (\ifmmode\pm\else\textpm\fi{}0.3 meV), respectively, for S and Si donors in GaP. A fitting of Faulkner's effective-mass calculation for donor states to the observed levels yields the effective masses of the conduction band: ${m}_{\ensuremath{\perp}}=(0.191\ifmmode\pm\else\textpm\fi{}0.005){m}_{0}$ and ${m}_{\mathrm{II}}=(1.7\ifmmode\pm\else\textpm\fi{}0.2){m}_{0}$. The $p$-like final states observed in these measurements are found to differ significantly in binding energy from the excited donor-electron states observed in the "two-electron" recombination of the exciton bound to neutral sulfur in GaP.

Journal ArticleDOI
TL;DR: In this paper, the preparation and optical properties of single-crystal films of Cd${\mathrm{Cr}}_{2}$${S}}_{4} were reported.
Abstract: We report on the preparation and optical properties of single-crystal films of Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$. The present data show no large blue shift associated with magnetic ordering near the Curie temperature contrary to that previously observed in bulk Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$ data. A large red shift correlated to the magnetic ordering is observed for a peak in the highly absorbing region. The weakness and narrowness of this absorption is inconsistent with that expected for band-to-band transitions resulting in unbound electrons and holes. We suggest an exciton model to explain this absorption.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the total absorption of radiation in the 2000-2800 \AA{} region and derived the absorption cross sections from total absorption measurements, and the linear increase in the measured effective absorption cross section with rise in gas pressure was attributed to the absorption caused by the increase in pressure.
Abstract: The total absorption of radiation in the 2000-2800 \AA{} region has been measured for ${\mathrm{O}}_{2}$. The absorption does not follow Beer's law. The linear increase in the measured effective absorption cross section with rise in gas pressure is ascribed to the absorption caused by ${\mathrm{O}}_{4}$. With methods developed and presented recently by the writer, the absorption cross sections of ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{4}$ are derived from total absorption measurements.

Journal ArticleDOI
TL;DR: In this paper, the energy and relative intensities of the allowed transitions between this ground state and the levels of the $4{f}^{2}$ configuration which were calculated using these wave functions were found to be good agreement with our optical and near-infrared absorption data.
Abstract: The absorption spectrum of cerium-doped Ca${\mathrm{F}}_{2}$ has been observed at several temperatures. When the cerium is reduced to the divalent state at room temperature, the absorption spectrum of ${\mathrm{Ce}}^{2+}$ is found to be unstable against thermal decay or bleaching by visible light; furthermore, this spectrum is found to comprise one state of a photochromic system. The absorption spectrum of Ca${\mathrm{F}}_{2}$: ${\mathrm{Ce}}^{2+}$ at low temperature is seen to consist of transitions to levels of the $4{f}^{2}$ configuration. This identification is confirmed by a conventional crystal-field calculation of the energy levels and wave functions of the $4f5d$ and $4{f}^{2}$ electronic configurations of ${\mathrm{Ce}}^{2+}$ occupying a cubic site in Ca${\mathrm{F}}_{2}$, which shows that while the ground state of the free divalent cerium ion is a level of the $4{f}^{2}$ configuration, the ground state of this ion in a sufficiently strong crystal field (${D}_{q}g1000$ ${\mathrm{cm}}^{\ensuremath{-}1}$) is a ${T}_{2}$ level of the $4f5d$ configuration. Observations of the Zeeman effect have been made which confirm the ${T}_{2}$ character of the ground state. The energies and relative intensities of the allowed transitions between this ground state and the levels of the $4{f}^{2}$ configuration which were calculated using these wave functions were found to be good agreement with our optical and near-infrared absorption data, the agreement being especially good for the near-infrared portion of the spectrum.

Journal ArticleDOI
TL;DR: In this paper, the magnon sidebands and other structure in the optical spectrum of antiferromagnetic Mn${\mathrm{F}}_{2}$ have been studied under various conditions of magnetic fields, uniaxial stress, and temperature.
Abstract: The magnon sidebands and other structure in the optical spectrum of antiferromagnetic Mn${\mathrm{F}}_{2}$ have been studied under various conditions of magnetic fields, uniaxial stress, and temperature. We present the results of such experiments for the $^{4}T_{1}(\mathrm{I})$, $^{4}A_{1}$, $^{4}E(\mathrm{I})$, $^{4}T_{2}(\mathrm{II})$, $^{4}T_{1}(\mathrm{II})$, and $^{4}E(\mathrm{II})$ states of ${\mathrm{Mn}}^{++}$ in Mn${\mathrm{F}}_{2}$. The magnon sideband shapes have been fitted as well as possible by density-of-states calculations in which the normal magnon dispersion and an adjustable exciton dispersion were used. It has been possible in this way to identify and give the magnitude of the pair moment for the principal exchange mechanism responsible for each sideband absorption, and to give the exciton parameters for the best fit. The discrepancies in this fitting process are probably ascribable to exciton-magnon binding, and indeed one line appears to represent a bound state of an exciton and a magnon.

Journal ArticleDOI
TL;DR: In this paper, the free-carrier intraband absorption has a wavelength dependence of 2.8 eV and an additional narrow band near 3.1 eV is shown to be the direct interband transition from the 1c/1c conduction band to the higher 3c/3c band, and the relationship to Biedermann's work on other SiC polytypes is indicated.
Abstract: Nitrogen doping of cubic SiC increases the optical absorption in two regions. The free-carrier intraband absorption has a wavelength dependence of ${\ensuremath{\lambda}}^{2.8}$. An additional narrow band near 3.1 eV is shown to be the direct interband transition from the ${X}_{1c}$ conduction band to the higher ${X}_{3c}$ band. The relationship to Biedermann's work on other SiC polytypes is indicated.

Journal ArticleDOI
TL;DR: In this article, the rotary power for the low-temperature Raman spectrum of the 128-${\mathrm{cm}}^{\ensuremath{-}1}$ $E$ mode in the E$ mode was investigated.
Abstract: Fine structure has been observed in the low-temperature Raman spectrum of the 128-${\mathrm{cm}}^{\ensuremath{-}1}$ $E$ mode in $\ensuremath{\alpha}$-quartz. This structure is a manifestation of an allowed linear dependence of the optical-phonon frequency on wave vector. Since Raman scattering probes a small but finite wave vector, it is possible to observe these frequency shifts using high-resolution thermal or simulated Raman spectroscopy. The linear splitting of the 128-${\mathrm{cm}}^{\ensuremath{-}1}$ $E$-mode doublet is 0.86\ifmmode\pm\else\textpm\fi{}0.05\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/sec as determined by backscattering with several laser wavelengths. Such linear wave-vector shifts lead to optical activity in the far infrared (IR). The theory of the strength and dispersion of infrared rotary power is developed in order to establish the connection between the two phenomena. The rotary power for the 128-${\mathrm{cm}}^{\ensuremath{-}1}$ resonance can be estimated from the measured linear shift, lifetime, and IR oscillator strength. However, a direct IR rotation measurement would be hindered by the associated absorption.

Journal ArticleDOI
TL;DR: In this article, the authors used optical absorption lines of CN, CH, and C${\mathrm{H}+}$ to impose upper limits on the intensity of background radiation in the interstellar medium.
Abstract: From observations of interstellar optical absorption lines of CN, CH, and C${\mathrm{H}}^{+}$ we are able to impose upper limits, at three wavelengths, on the intensity of background radiation in the interstellar medium. These limits are consistent with the existence of thermal background radiation at a temperature of \ensuremath{\sim}3\ifmmode^\circ\else\textdegree\fi{}K, but can be reconciled with the results of a recent rocket observation by Shivanandan, Houck, and Harwit only if the intense flux which they report is concentrated into a sharp line which happens to avoid the molecular resonances, or if this flux is of local origin.

Journal ArticleDOI
TL;DR: In this article, the first exciton peaks in thin evaporated films of four thallium and silver halides were determined to be, in units of ${10}^{\ensuremath{--}6}$ eV/atm, -1.5 (AgCl), - 1.9 (AgBr), -18 (TlCl), and -16 (tlBr).
Abstract: The pressure shifts of the optical absorption edges in four thallium and silver halides at 10\ifmmode^\circ\else\textdegree\fi{}K were determined to be, in units of ${10}^{\ensuremath{-}6}$ eV/atm, -1.5 (AgCl), -1.9 (AgBr), -18 (TlCl), and -16 (TlBr). The pressure shifts of the first exciton peaks in thin evaporated films of these salts were measured at 80\ifmmode^\circ\else\textdegree\fi{}K. In the same units these are 6.2 (AgCl), 6.4 and 5.6 (AgBr doublet), -13.9 (TlCl), and -13.4 (TlBr). The similarity of the pressure coefficients of the absorption edge in the two silver halides is evidence that the same indirect transition is responsible for the edge in both salts, presumably ${L}_{{3}^{\ensuremath{'}}}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}$. The deformation potentials for the first direct exciton in the silver halides are approximately the same as those for the corresponding transitions in KBr and KI. The similarity of the pressure coefficients of the absorption edge and exciton peaks in the thallium salts is evidence that the absorption edge in these salts is the tail of the first direct exciton peak. The "anomalous" red shift upon cooling of the absorption edge and exciton peak in the thallium halides is shown to be the effect of thermal contraction.

Journal ArticleDOI
TL;DR: In this article, the Mossbauer effect has been used to study the ferroelectric transitions in potassium ferrocyanide trihydrate (KFCT) and ferric ammonium sulfate dodecahydrate (FAS).
Abstract: The M\"ossbauer effect has been used to study the ferroelectric transitions in potassium ferrocyanide trihydrate (KFCT) and ferric ammonium sulfate dodecahydrate (FAS). The recoilless fraction and isomer shift of these materials have been observed as a function of temperature. For FAS, the absorption line-width was also observed. For FAS, the recoilless fraction $f$ and absorption linewidth $w$ were found to behave anomalously at the ferroelectric transition temperature ${T}_{c}$; $f$ exhibits a minimum at ${T}_{c}$, and $w$ has a maximum there. No change in the isomer shift was detected at the FAS transition. The behavior of $f$ is consistent with a description of the transition in FAS in terms of a "soft" transverse optical (TO) lattice mode of zero wave vector (${\ensuremath{\omega}}_{\mathrm{TO}}\ensuremath{\rightarrow}O$ at $T={T}_{c}$). For KFCT, no anomalous behavior of any M\"ossbauer-effect parameter was detected. This is contrary to a previous report by other workers of a maximum in $f$ near ${T}_{c}$ for KFCT. The lack of any anomaly in KFCT is consistent with an order-disorder model of the KFCT transition, but does not preclude one particular type of displacive transition.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the hypersonic attenuation in liquid at a frequency of about 650 MHz, including the vicinity of the $\ensuremath{\lambda}$ point.
Abstract: We have measured by a new optical method the hypersonic attenuation in liquid ${\mathrm{He}}^{4}$ at a frequency of about 650 MHz In this high-frequency range absorption data are presented between 12 and 42\ifmmode^\circ\else\textdegree\fi{}K, including the vicinity of the $\ensuremath{\lambda}$ point The results are compared with the previous low-frequency measurements

Journal ArticleDOI
TL;DR: In this article, an analysis of 1268 π−p → nπ+π− interactions obtained in the 1.5 m BNHBC and in the 2 m CERN HBC exposed at the CERN proton-synchrotron to a π − beam of 11 GeV/c momentum is presented.
Abstract: In this paper we present an analysis of 1268 π−p → nπ+π− interactions obtained in the 1.5 m BNHBC and in the 2 m CERN HBC exposed at the CERN proton-synchrotron to a π− beam of 11 GeV/c momentum. The total cross-section of this channel is evaluated to be σ=(0.7±0.1) mb, while the production cross-sections for the ρ0, f0 and g0 resonances are determined as\(\sigma _{\rho ^0 } = \left( {110 \pm 14} \right)\) μb,\(\sigma _{f^0 } = \left( {75 \pm 14} \right)\) μb,\(\sigma _{g^0 } = \left( {55 \pm 14} \right)\) μb. Some experimental behaviour of the π0 production is compared with the predictions of the OPE model with Durr-Pilkuhn form factors, the OPE absorptive model, the Regge model and the absorptive Regge model.

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P. J. Dean1, Y. Yafet1, J. R. Haynes1
TL;DR: In this paper, it was shown that the fine structure of the indirect edge absorption and luminescence spectra of hyperpure silicon is due to a splitting in the energy states of the free indirect exciton rather than to transitions involving additional phonons.
Abstract: Fine structure has been observed in the indirect edge absorption and luminescence spectra of hyperpure silicon. It is shown that this structure is due to a splitting in the energy states of the free indirect exciton rather than to transitions involving additional phonons. Two free-exciton states are observed, separated by 1.8\ifmmode\pm\else\textpm\fi{}0.2 meV. The intensity ratio of the two luminescence components associated with these exciton states indicates that thermal equilibrium is achieved between them at \ensuremath{\sim}5.5\ifmmode^\circ\else\textdegree\fi{}K but not at \ensuremath{\sim}2.5\ifmmode^\circ\else\textdegree\fi{}K. This fact, together with the magnitude of the splitting, suggests that these two states do not arise from spin-spin interaction in the free exciton or from a splitting of the degenerate hole states because of coupling to the anisotropic electrons at ${\ensuremath{\Delta}}_{1}$. Instead, these experimental results, together with the intensity ratio of the two free-exciton absorption components, indicate that the splitting occurs because the binding energy of the 1s envelope state of the free exciton is significantly larger (\ensuremath{\sim}11.5 meV) when it contains a symmetric linear combination of conduction-band states; i.e., the observed splitting is due to the valley-orbit interaction. A further weakly bound free-exciton state observed in absorption is attributed to an excited envelope state.

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TL;DR: In this paper, the forward-scattered intensity in the spontaneous parametric process was found to decrease significantly as the idler frequency goes sufficiently far into the 5.7- and 1.4- infrared absorption bands.
Abstract: The forward-scattered intensity in the spontaneous parametric process is found to decrease significantly as the idler frequency goes sufficiently far into the 5.7-\ensuremath{\mu} and 1.4-\ensuremath{\mu} infrared absorption bands of LiNb${\mathrm{O}}_{3}$ and N${\mathrm{H}}_{4}$${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$, respectively. These results indicate that, unlike the previously studied cases in GaP and LiNb${\mathrm{O}}_{3}$, the corresponding optical nonlinear susceptibility has essentially no resonance increase and its ionic contribution remains small near these absorption bands. A formula for the forward-scattered intensity is obtained which agrees very well with the observed effects of idler attenuation on the spontaneous parametric scattering in both LiNb${\mathrm{O}}_{3}$ and N${\mathrm{H}}_{4}$${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$.

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TL;DR: In this article, the dependence of the cross-section of the pion pole on the mass of the dipion was investigated and the authors showed that the dependence can be isolated by considering certain moments in the distribution with respect to pion angles.
Abstract: The dependence of the $\ensuremath{\pi}+N\ensuremath{\rightarrow}(2\ensuremath{\pi})+N$ cross section for low invariant $\ensuremath{\pi}\ensuremath{\pi}$ mass and small $|t|$ is discussed with a view toward extrapolation to the pion pole and extraction of $\ensuremath{\pi}\ensuremath{\pi}$ phase shifts. There is a complicated $t$-dependent term in the cross section which makes extrapolation inaccurate. This term can be isolated by consideration of certain moments (in the distribution with respect to the pion angles $\ensuremath{\theta}$, $\ensuremath{\phi}$ in the dipion rest frame), if nucleon helicity flip is assumed to dominate. Indirect empirical evidence and the absorption model with $\ensuremath{\pi}$ exchange both support this assumption in the $5\ensuremath{-}10\ensuremath{-}\frac{\mathrm{BeV}}{c}$ region. The assumption is weaker than the popular assumption that the Chew-Low pion-exchange formula holds, and can also be directly tested by examining the $t$ dependence of certain moments at 0\ifmmode^\circ\else\textdegree\fi{} (forward production of the dipion). For $s$- and $p$-wave $\ensuremath{\pi}\ensuremath{\pi}$ scattering, one considers the coefficients of 1, $cos\ensuremath{\theta}$, and ${sin}^{2}\ensuremath{\theta}$ in addition to the usual $\ensuremath{\phi}$-dependent terms. The coefficient of ${sin}^{2}\ensuremath{\theta}$ is the complicated term. It is reasonable to hope that the remaining moments are smoothly varying for $0\ensuremath{\ge}\ensuremath{-}t\ensuremath{\gtrsim}4{\ensuremath{\mu}}^{2}$, and are suitable for extrapolation. They are constrained to a definite $t$ dependence at 0\ifmmode^\circ\else\textdegree\fi{} by angular momentum conservation. We feel that reliable determination of the $\ensuremath{\pi}\ensuremath{\pi}$ cross section in the $\ensuremath{\rho}$ region and below is attainable by this method in high-statistics experiments currently under way.

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TL;DR: In this article, the effect of configuration mixing between different exciton states on the oscillator strength of double stranded homopolyn nucleotides and copoly was investigated and it was shown that the oscillators strength associated with the lowest excitons decreases with configuration mixing, and that the absorption intensity in the long wave length side of the main absorption band depends linearly on G-C content.
Abstract: Singlet exciton states double stranded homopolynucleotides and copolynucleotides are determined within the framework of Frenkel exciton model taking into account the effect of configuration mixing between different exciton states. Interbase interactions are taken into account up to the distance of the order of 15 A and they are calculated directly using the generalized charge densities in the base pairs previously obtained by the authors. The exciton states of model DNA is also discussed in which two kinds of base pairs are arranged in random order. Generally the oscillator strength associated with the lowest exciton states decreases by the effect of configuration mixing. There can be found no weak π→π * transition near 280 mµ in poly (A–U) and copoly \(\left(\begin{array}{c}\text{U--A}\\ \text{A--U}\end{array}\right)\). In a model DNA the absorption intensity in the long wave length side of the main absorption band depends linearly on G–C content.

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TL;DR: In this paper, the lifetime of the 2-π resonance was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity, which was obtained using a modified zero-field level crossing (Hanle) method.
Abstract: The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the $2^{1}P$ state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of $2^{1}S$ metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the $2^{1}S$ atoms, which are excited to the $2^{1}P$ state. This state then primarily decays to the ground $1^{1}S$ state with the emission of resonance (584 \AA{}) radiation. The lifetime of the $2^{1}P$ state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.