Showing papers on "Absorption (logic) published in 1970"

Electroabsorption in Semiconductors: The Excitonic Absorption Edge

Abstract: Numerical calculations of the optical-absorption coefficient for direct, excitonic transitions in a uniform applied electric field are presented. The electron-hole scattering is treated within the effective-mass approximation and leads to an absorption coefficient which differs markedly in size and shape from the Franz-Keldysh absorption spectrum. A detailed numerical study of the shape of the absorption-edge spectrum at photon energies somewhat below the zero-field absorption threshold suggests that for small field strengths the dominant asymptotic form of the absorption coefficient is $\mathrm{exp}(\ensuremath{-}\frac{{C}_{0}|E\ensuremath{-}{{E}_{0}}^{\ensuremath{'}}|}{f})$, where $f=\frac{|e|\mathrm{Fa}}{R}$ is the electric field strength in units of exciton Rydbergs per electron-exciton Bohr radius. This result contradicts the existing belief that the electron-hole interaction does not alter the asymptotic form of the Franz-Keldysh shape: $\mathrm{exp}(\ensuremath{-}\frac{{{C}_{0}}^{\ensuremath{'}}{|E\ensuremath{-}{{E}_{0}}^{\ensuremath{'}}|}^{\frac{3}{2}}}{f})$. Physical arguments are presented to show why the exciton effects should be important. A discussion is presented of the interrelationships among the present treatment of electro-absorption and various one-electron, exciton, and many-body formalisms.

Optical and Magnetic Investigations of the Localized States in Semiconducting Glasses

Abstract: We measured optical absorption and magnetic susceptibility of amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ as a function of temperature. An exponential variation of absorption constant with photon energy was found in the range $0.09l\ensuremath{\alpha}l0.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$. A Curie term in the susceptibility was shown to be characteristic of disorder in the vitreous material. A model relating the weak absorption tail to the susceptibility requires highly localized states having an exponential energy distribution in the gap.

Charge-Transfer Spectra of Transition-Metal Ions in Corundum

Abstract: The ultraviolet absorption spectrum of corundum (${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$) containing dilute concentrations of the $3d$-series transition-metal ions ${\mathrm{Ti}}^{3+}$, ${\mathrm{V}}^{3+}$, ${\mathrm{Cr}}^{3+}$, ${\mathrm{Mn}}^{4+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Ni}}^{3+}$ was measured at room temperature and at liquid-nitrogen temperature for photon energies in the range 3-9 eV. The most important features of the observed spectra are (a) the peak positions are characteristic of the particular impurity ion; (b) the peak positions and widths (\ensuremath{\sim}0.7 eV) are independent of temperature over the range studied; and (c) the integrated intensities are strong. The spectra are identified with the allowed transition whereby an electron is transferred from a nonbonding orbital, localized predominantly on the ${\mathrm{O}}^{=}$ ligands, to either the ${t}_{2g}({\ensuremath{\pi}}^{*})$ or ${e}_{g}({\ensuremath{\sigma}}^{*})$ antibonding orbital, localized predominantly on the metal ion. The position in energy of the first absorption peak for the various ions of the series relative to one another and to the position of the intrinsic absorption edge of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ is in good agreement with the charge-transfer model.

Wannier Exciton in an Electric Field. I. Optical Absorption by Bound and Continuum States

Abstract: A comprehensive study of electric field effects on optical absorption by Wannier excitons is presented, showing field effects on both bound and continuum states. The calculations and results have been simplified by defining appropriate dimensionless parameters such that the eigenvalues are independent of field when expressed in terms of these parameters. A general normalization procedure for wave functions with continuous eigenvalues is outlined. The effect of the electron-hole interaction on the electric-field-induced oscillations is demonstrated, with the result that the electron-hole interaction enhances these oscillations near an ${M}_{0}$-type edge (positive effective masses) and quenches these oscillations near an ${M}_{3}$-type edge (negative effective masses). This effect would inhibit the observation of ${M}_{3}$-type edges in electroreflectance.

Jahn-Teller Effect on the Structure of the Emission Produced by Excitation in the A Band of KI: Tl-Type Phosphors. Two Kinds of Minima on the Γ 4 − ( T 1 u 3 ) Adiabatic Potential-Energy Surface

Abstract: Emission spectra, excitation spectra, and decay times of ${\mathrm{Ga}}^{+}$, ${\mathrm{In}}^{+}$, and ${\mathrm{Ti}}^{+}$ in KI, KBr, KCl, and NaCl have been investigated at 1.8-300\ifmmode^\circ\else\textdegree\fi{}K under excitation in the $A$-absorption band. There are three types of temperature variation of the emission spectra; these types are represented by KI:Tl, KBr:Tl, and KCl:Tl, respectively. In case of types (1) and (2), two emission bands, ${A}_{T}$ (high energy) and ${A}_{X}$ (low energy), are observed. The excitation spectrum for the ${A}_{T}$ emission is slightly different from that for the ${A}_{X}$ emission. The decay times of the ${A}_{X}$ emission are complex and temperature dependent, as exemplified in KI:Ga and KI:In. The ${A}_{T}$ emission is surely due to the inverse process of the $A$ absorption, ${{\ensuremath{\Gamma}}_{4}}^{\ensuremath{-}}(^{3}T_{1u})\ensuremath{\rightarrow}{{\ensuremath{\Gamma}}_{1}}^{+}(^{1}A_{1g})$. Four reasons are given why the ${A}_{X}$ emission is not due to the inverse process of the $D$ absorption as proposed by Illingworth and supported by Donahue and Teegarden. To explain the ${A}_{X}$ emission, the Jahn-Teller effect on the $^{3}T_{1u}$ state has been considered by assuming Russell-Saunders coupling. Since the correlation of polarization indicates the existence of tetragonal minima, adiabatic potential-energy surfaces (APES's) in the ${E}_{g}({Q}_{2}, {Q}_{3})$ configuration coordinate space have been obtained for the ${{\ensuremath{\Gamma}}_{4}}^{\ensuremath{-}}(^{3}T_{1u})$ and ${{\ensuremath{\Gamma}}_{1}}^{\ensuremath{-}}(^{3}T_{1u})$ states. When the spin-orbit interaction is of suitable value as compared with the Jahn-Teller (electron-lattice) interaction, the obtained APES's have two kinds of minima. Emission is considered to occur from these minima to the ground state, at least at low temperatures. The APES's can explain why there exist the three types of the emission spectra as well as the characteristic features of the emission spectra, excitation spectra, decay times, and polarization. The importance of these APES's for analyzing the stress, Stark, and Zeeman effects on the ${A}_{T}$ and ${A}_{X}$ emission is pointed out.

Lifetimes of Bound Excitons in CdS

Abstract: Weakly bound excitons in CdS have giant oscillator strengths which lead to exceedingly fast radiative lifetimes. We have been able to measure the lifetime of the ${I}_{2}$ line, an exciton bound to a neutral donor, and the lifetime of the ${I}_{1}$ line, an exciton bound to a neutral acceptor. We find ${\ensuremath{\tau}}_{{I}_{2}}=0.5\ifmmode\pm\else\textpm\fi{}0.1$ nsec and ${\ensuremath{\tau}}_{{I}_{1}}=1.03\ifmmode\pm\else\textpm\fi{}0.1$ nsec. The measurements were made at 1.6\ifmmode^\circ\else\textdegree\fi{}K. The lifetimes are measured by exciting the luminescence with an argon laser modulated at 100 Mc/sec, and measuring the time delay of the luminescence with a 100-Mc/sec phase-sensitive detector. Previous calculations by Rashba and Guygenishvili predicted radiative lifetimes which were an order of magnitude shorter than our measured values. They used an incorrect value for the exciton mass. When corrected, their theory gives ${\ensuremath{\tau}}_{{I}_{2}}=0.56$ nsec and ${\ensuremath{\tau}}_{{I}_{1}}=1.86$ nsec, in reasonably good agreement with our measurements. Thomas and Hopfield measured the absorption oscillator strength of the ${I}_{2}$ line. Their measurements predict a radiative lifetime for the ${I}_{2}$ line of 0.4\ifmmode\pm\else\textpm\fi{}0.1 nsec. This is very close to our measured value and shows that the ${I}_{2}$ line decays radiatively. We conclude that the nonradiative Auger effect is negligible for the ${I}_{2}$ line and either negligible or small for the ${I}_{1}$ line. We also calculate the lifetime for donor-acceptor pair recombination to be 2.2 nsec as the pair separation goes to zero. This agrees with Colbow's experimental value of 2.5\ifmmode\pm\else\textpm\fi{}1 nsec. Using the same method, we calculate the lifetime of the Te isoelectronic trap to be 27 nsec. This agrees poorly with Cuthbert and Thomas's measured value of 300 nsec.

Optical and Ferroelectric Properties of Barium Sodium Niobate

Abstract: Optical absorption, refractive index, dielectric constant, nonlinear optical coefficients, and linear electro-optic coefficients are reported for single-domain crystals of ${\mathrm{Ba}}_{2}$Na${\mathrm{Nb}}_{5}$${\mathrm{O}}_{15}$ between room temperature and the Curie temperature. Orthorhombic ($\mathrm{mm}2$) barium sodium niobate, a filled tungsten-bronze structure, is stable to intense laser radiation, and its phasematchable nonlinear coefficients are 3 times those of LiNb${\mathrm{O}}_{3}$ and LiI${\mathrm{O}}_{3}$. For the 1.064-\ensuremath{\mu}m laser fundamental, the observed phase-match temperatures and the angular half-widths of the phase-matched second-harmonic intensity due to the coefficients ${d}_{31}$ and ${d}_{32}$ are found to be in good agreement with the values calculated from refractive-index data. The dielectric constant and electro-optic half-wave voltage data indicate that ${\mathrm{Ba}}_{2}$Na${\mathrm{Nb}}_{5}$${\mathrm{O}}_{15}$ is a useful electro-optic modulator material. The temperature variation of the spontaneous polarization ${P}_{S}$ is deduced from the birefringence, electro-optic, nonlinear optical, and pyroelectric data. It is concluded that the ferroelectric transition in ${\mathrm{Ba}}_{2}$Na${\mathrm{Nb}}_{5}$${\mathrm{O}}_{15}$ is of the first order.

F + and F ′ Centers in Magnesium Oxide

Abstract: Single crystals of MgO have been additively colored by heating them in Mg vapor at temperatures from 1600 to 1800\ifmmode^\circ\else\textdegree\fi{}C and pressures up to 4000 Torr. The ${F}^{+}$ and ${F}^{\ensuremath{'}}$ optical absorption bands (due to anion vacancies containing one and two electrons, respectively) were observed to be extremely similar but distinguishable, and were observed at liquid-nitrogen temperature to peak at 4.96 and 5.03 eV, respectively. The oscillator strength for the ${F}^{\ensuremath{'}}$ center is estimated to be 0.70\ifmmode\pm\else\textpm\fi{}0.05. A photoconversion process ${F}^{+}\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}$ was studied. The luminescence bands for these centers were observed at 3.1 and 2.4 eV, respectively. The density of ${F}^{\ensuremath{'}}$ centers ${N}_{{F}^{\ensuremath{'}}}$ produced by the additive coloring process was observed to be related to the density of atoms in the vapor ${N}_{\mathrm{Mg}}$ by the equation ${N}_{{F}^{\ensuremath{'}}}=1270{N}_{\mathrm{Mg}}{e}^{\ensuremath{-}\frac{Q}{\mathrm{kT}}}$, where $Q=1.53$ eV. The temperature dependence of these two bands was measured and is discussed.

Optical Gap of Strontium Titanate (Deviation from Urbach Tail Behavior)

Abstract: The absorption of SrTi${\mathrm{O}}_{3}$ below the fundamental edge shows, near liquid-nitrogen temperature, an oscillatory behavior, for which an explanation is proposed in terms of indirect transitions ${\ensuremath{\Gamma}}_{15}\ensuremath{\rightarrow}{X}_{3}$, centered around 3.27 eV and involving a 51-meV LO phonon. There is no evidence of excitonic effects. The first direct transition is probably ${X}_{{5}^{\ensuremath{'}}}\ensuremath{\rightarrow}{X}_{3}$ and is located at \ensuremath{\sim}3.46 eV. Broadening effects mask the indirect structure at higher temperatures where the absorption curves gradually evolve into normal Urbach tailing.

An Investigation of Trapped Holes and Trapped Excitons in Alkaline Earth Fluorides

Abstract: The e.p.r. and optical absorption spectra of the self-trapped hole (V$\_{K}$) centres in CaF$\_{2}$, SrF$\_{2}$ and BaF$\_{2}$ have been measured and it is shown that the molecular axes of these centres are alined along $\langle $100$\rangle $. Preferential alinement of the V$\_{K}$ centres along a particular cube edge is readily produced by bleaching into the optical absorption bands of the V$\_{K}$ centre at 77 K with linearly polarized light. The anisotropy created in the crystals by preferential alinement is detected in both the e.p.r. and optical spectra. The thermally induced decay of the optically alined V$\_{K}$ centres is monitored by e.p.r. methods and it is shown that, in CaF$\_{2}$, linear (0 degrees) motion of the V$\_{K}$ centres predominates over the entire decay range (85 to 138 K). In SrF$\_{2}$ linear motion predominates in the early stages of decay (ca. 110 K) but random (90 degrees) motion sets in at higher temperatures (ca. 120 K). In BaF$\_{2}$ only random motion is observed. The kinetics of reorientation and decay of the V$\_{K}$ centre are investigated. A band observed in the X-ray fluorescence of CaF$\_{2}$ (at 279 nm), SrF$\_{2}$ (at 300 nm) and BaF$\_{2}$ (at 310 nm) is assigned to emission of the self-trapped exciton by comparison with a similar band in the recombination luminescence of the V$\_{K}$ centre (due to e$^{-}$ + V$\_{K}$). The temperature dependence of the X-ray fluorescence intensity is described. The polarization properties of the recombination luminescence are investigated on crystals containing optically alined V$\_{K}$ centres. The e.p.r. and optical absorption spectra of trapped hole centres involving other lattice defects are also described. One such centre, with its molecular axis close to $\langle $111$\rangle $, involves an interstitial fluorine ion and is named the V$\_{H}$ centre. Other centres with molecular axes close to $\langle $100$\rangle $ appear to be stabilized by impurity ions and are labelled V$\_{KA}$ and V$\_{KA^{\prime}}$ centres. The V$\_{H}$ centre occurs in CaF$\_{2}$, SrF$\_{2}$ and BaF$\_{2}$, the V$\_{KA}$ centre in SrF$\_{2}$ and BaF$\_{2}$, and the V$\_{KA^{\prime}}$, centre only in BaF$\_{2}$. The kinetics of reorientation and decay of the V$_{KA}$ centre have been investigated. We give a general discussion of the properties of hole centres, including the H centre, in alkaline earth fluorides.

Absorption of Oxygen in Silicon in the Near and the Far Infrared

Abstract: Previous work on oxygen in silicon has shown that oxygen dissolves interstitially in silicon forming a complex which may be approximately described as Si$\_{2}$O. Absorption bands of Si: O occur at 517, 1136 and 1203 cm$^{-1}$ and these have been assigned by earlier authors to the $\upsilon \_{2}$ (symmetric bending), $\upsilon \_{3}$ (antisymmetric stretch) and $\upsilon \_{1}$ (symmetric stretch) normal modes of vibration of Si$\_{2}$O. The present investigation confirms the $\upsilon \_{3}$ origin of the 1136 cm$^{-1}$ band (the well known 9$\mu $m band) but we disagree with the earlier assignments of the 517 and 1136 cm$^{-1}$ bands. The results reported here are relevant to organic siloxanes. We have extended the investigation of Si: O into the far infrared and we find sharp absorption lines at 29.3, 37.8, 43.3 and 49.0 cm$^{-1}$ which we have assigned to the $\upsilon \_{2}$ mode of Si$\_{2}$O. The isotope shift due to $^{18}$O has been observed in the far infrared spectrum. Effects of uniaxial stress on the 29.3 cm$^{-1}$ line have been investigated and are found to be consistent with the assignment to the $\upsilon \_{2}$ mode. The main features of the far infrared spectrum are accounted for with a simple anharmonic potential which ignores coupling of the Si$\_{2}$O to the crystal lattice. We have investigated effects of uniaxial stress on the 517, 1136 and 1203 cm$^{-1}$ bands of Si$\_{2}$O. Our stress results for the 1136 cm$^{-1}$ band are consistent with the earlier $\upsilon \_{3}$ assignment. Using our normal mode description, we conclude that the 1203 cm$^{-1}$ band is a combination band involving $\upsilon \_{3}$ and $\upsilon \_{2}$ excitations. We have not been able to give a clear cut assignment to the 517 cm$^{-1}$ band, but we suggest that $\upsilon _{1}$ type excitation may be involved. The appendix describes the stress splitting of the 836 cm$^{-1}$ band of the silicon A centre in electron irradiated Si: O and our results confirm an earlier model for this centre. In all cases investigated here, the stress splittings arise from raising the orientational degeneracy of the oxygen complex.

Optical Properties of Amorphous Germanium Films

Abstract: The optical constants of amorphous Ge films formed under well-defined conditions have been determined in the 0.1-25.0-eV spectral range. In the 0.1-1.8-eV range they were determined by analysis of precise reflectance and transmittance data ($\mathrm{RT}$), and in the 0.1-25.0-eV range by a Kramers-Kronig analysis of normal-incidence reflectance data. Both analyses gave the same results in the region of overlap. The absorption edge was found to be quite sharp (0.06 eV) and to occur usually at a photon energy of about 0.6 eV. The position of the edge was normally about 0.2 eV lower than the direct edge in crystalline Ge. No evidence was found for either the spin-orbit split valence band associated with crystalline Ge or a tailing and/or large number of states in the forbidden region. The smallest nonzero value of the absorption measured on the low-energy side of the absorption edge was about 10 ${\mathrm{cm}}^{\ensuremath{-}1}$. There was no evidence for free-carrier absorption further in the infrared. At 0.1 eV, the index of refraction was 3.99 \ifmmode\pm\else\textpm\fi{} 0.04 as determined by the $\mathrm{RT}$ analysis. This value was in excellent agreement with the value of 4.00 derived from the zero-frequency dielectric constant, which has been calculated using the sum rule. This is also the value determined for crystalline Ge in the infrared. Reflectance data for amorphous Ge films deposited and measured in ultra-high vacuum (in situ) are reported for the region 2.0-11.8 eV. The density, determined by weighing films of known thickness, was 12-15% less than the density of crystalline Ge.

Exciton-Optical Properties of TlBr and TlCl

Abstract: The effect of strain on thallous-halide films has been investigated, and a technique developed for producing essentially strain-free films. The optical absorption of strain-reduced TlBr and TlCl films has been measured down to 4.2\ifmmode^\circ\else\textdegree\fi{}K, and the results analyzed for the optical constants. Relatively narrow (2-meV) firstexciton lines were found in the near ultraviolet accompanied by longitudinal-optical-phonon sidebands. The Faraday rotation pattern was measured through the first peak in TlBr and found to be linear in $H$. An analysis of the rotation pattern together with the known $n$ and $\ensuremath{\kappa}$ yields an exciton effective $g$ value of 0.7 for TlBr. Oscillatory magnetoabsorption has been observed in both TlBr and TlCl originating from the edge of the sideband. The exciton binding energies are 6.5\ifmmode\pm\else\textpm\fi{}1 meV in TlBr and 11\ifmmode\pm\else\textpm\fi{}2 meV in TlCl. The exciton-optical transition is discussed and the importance of lattice and polaron effects emphasized.

Cooperative and Stepwise Excitation of Luminescence: Trivalent Rare-Earth Ions in Yb 3 + -Sensitized Crystals

Abstract: The probability of cooperative energy transfer from two excited ions to a nearby ion is computed and compared with the probability of excitation by stepwise energy transfer. For ${\mathrm{Yb}}^{3+}$ sensitization of ${\mathrm{Er}}^{3+}$, ${\mathrm{Ho}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ activated hosts, the cooperative transition rate from two excited ${\mathrm{Yb}}^{3+}$ ions is estimated to be in the range ${10}^{7}$-${10}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, depending on the degree of overlaps of absorption and emission bands. In the case of the ${\mathrm{Er}}^{3+}$+${\mathrm{Yb}}^{3+}$ and ${\mathrm{Ho}}^{3+}$+${\mathrm{Yb}}^{3+}$ systems, the dependence of output intensity on the exciting light intensity ${I}_{E}$ and the concentration of Yb cannot discriminate between cooperative and serial transfer, but estimates of the transfer probabilities show that the stepwise process dominates. It is suggested that the ${\mathrm{Tb}}^{3+}$+${\mathrm{Yb}}^{3+}$ system would be an excellent system in which to observe the cooperative effect unambiguously. In the ${\mathrm{Tm}}^{3+}$+${\mathrm{Yb}}^{3+}$ system also, we expect stepwise transfer to dominate except for unfeasibly low ${I}_{E}$.

Absolute Signs of Second-Harmonic Generation Coefficients of Piezoelectric Crystals

Abstract: This paper describes the measurement of the absolute signs of the nonlinear optical coefficients ${d}_{\mathrm{ij}}$ that give rise to second-harmonic generation in piezoelectric crystals. The measurements were made with a repetitively $Q$-switched Nd-yttrium aluminum garnet (Nd-YAG) laser (1.064 \ensuremath{\mu}) and involve relative sign determinations between two crystals followed by a comparison of at least one of them with ${d}_{14}$ of GaP, which has been shown previously to be positive with respect to the +[111]. To determine the unique directions in the crystals studied, several different methods were used, including: the piezoelectric effect, the pyroelectric effect, the etching behavior, and the electric field aligned spontaneous polarization. The crystals investigated included the following: zinc-blende crystals (ZnSe, ZnTe, and CdTe), wurtzite crystals (ZnO, BeO, CdS, and CdSe), ferroelectric crystals [LiTa${\mathrm{O}}_{3}$, LiNb${\mathrm{O}}_{3}$, BaTi${\mathrm{O}}_{3}$, ${\mathrm{Ba}}_{2}$Na${\mathrm{Nb}}_{5}$${\mathrm{O}}_{15}$, ${\mathrm{Gd}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$, and ${\mathrm{Tb}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$], and several other crystals (LiI${\mathrm{O}}_{3}$, LiGa${\mathrm{O}}_{2}$, LiCH${\mathrm{O}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{H}}_{2}$O, and quartz). Strong absorption of the second harmonic complicates the interpretation of the experimental results for ZnTe, CdTe, and CdSe. The present results and those of several recent theoretical predictions are in good agreement.

Pressure-Induced Metal-Semiconductor Transition and 4 f Electron Delocalization in Sm Te

Abstract: The pressure variation of resistivity and optical absorption in SmTe has been studied. A continuous pressure-induced semiconductor-to-metal transition is observed, which we ascribe to the promotion of electrons from the $4f$ level into the conduction band as the gap between them shrinks with pressure and finally vanishes. The gap deduced from the saturation resistivity ratio $\frac{\ensuremath{\rho}{(P)}_{\mathrm{sat}}}{\ensuremath{\rho}(0)}$ is in good agreement with the gap of 0.62 \ifmmode\pm\else\textpm\fi{} 0.02 eV obtained from infrared absorption data.

Study of the Interaction of Light with Rough Metal Surfaces. I. Experiment

Abstract: This paper describes measurements of the reflectivity, scattering, and transmission of light by metals with rough surfaces. For surfaces whose roughness is very short ranged, the ratio of rough-surface reflectivity to smooth-surface reflectivity varies exponentially as ${\ensuremath{\lambda}}^{\ensuremath{-}2}$ both above the plasmon frequency and in regions where ${\ensuremath{\epsilon}}_{2}\ensuremath{\gg}|{\ensuremath{\epsilon}}_{1}|$. In the latter regions for these rough surfaces, the scattered intensity follows a ${\ensuremath{\lambda}}^{\ensuremath{-}4}$ wavelength variation. For surfaces which are more wavy, the reflectivity and scattered light vary less rapidly with wavelength. Well below the plasmon frequency, additional fields not present on smooth surfaces, but coming from induced extra currents and dipoles on rough surfaces, add coherently to the specular beam, with a resonant wavelength variation. Near the plasmon frequency there is extra absorption. We have studied the angular variation of the scattered light, and have observed additional incoherent light associated with these extra dipoles and currents. We compare the experimental results with the scalar scattering theory and make some general comments about the properties of surface dipoles and currents.

Crystal Size Effects on the Exciton Absorption Spectrum of W Se 2

Abstract: The thickness dependence of the optical absorption spectrum of thin single crystals of W${\mathrm{Se}}_{2}$ (500 \AA{} to a unit-cell layer) was studied in the wavelength region 8000-5000 \AA{} at 77\ifmmode^\circ\else\textdegree\fi{}K A significant shift (015 eV) to higher energies in the position of the exciton absorption peaks was observed with decreasing thickness Such a shift is shown to follow a $\frac{1}{{t}^{2}}$ dependence on the thickness ($t$) down to 40 \AA{} This fact suggests the presence of a quantum size effect A deviation from the $\frac{1}{{t}^{2}}$ law has been observed for crystals less than 40 \AA{} thick

Interstellar absorption of cosmic x rays.

Abstract: Improved results on the total photo-ionization cross section of atomic helium are given. The total cross section includes contributions from simultaneous ionization and excitation of ${\mathrm{He}}^{+}$ and double ionization; the combined effect of these two processes adds about 10% to the normal photo-ionization process where ${\mathrm{He}}^{+}$ is left in the ground state. A lower abundance (10.92 based on 12.00 for hydrogen) is adopted for helium, based on recent radio determinations. In calculating the opacity due to $K$-shell photo-ionization of heavy elements, a lower abundance (8.00) is also adopted for neon. Brief mention is made of the effects of irregularities in the density distribution of the interstellar gas on the problems of both x-ray and radio-wave absorption.

Spontaneous-Raman-Scattering Efficiency and Stimulated Scattering in Silicon

Abstract: The absolute spontaneous-Raman-scattering efficiency and linewidth of the 521-${\mathrm{cm}}^{\ensuremath{-}1}$ optical mode of silicon have been measured at 77 K using a continuous laser (Nd in yttrium aluminum garnet) operating at 1.064 \ensuremath{\mu}m. The measured scattering efficiency (5.1 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$/cm sr for unpolarized forward scattering along the crystal [111] direction) and narrow linewidth yield a calculated value of the stimulated Raman gain coefficient which is considerably larger than those reported for other media, both solid and liquid. Stimulated Raman scattering in Si at 77 K has also been observed using a focused multimode $Q$-switched YAG: Nd laser. Inaccuracy in the measured stimulated gain resulted mainly from the uncertainty in the effective focal volume inside the silicon. Multiphoton absorption at the incident laser frequency has been considered and found to modify the measured stimulated gain by a significant amount. The estimated gain from the stimulated Raman effect was found to be in satisfactory agreement with that calculated from the absolute spontaneous-Raman-scattering efficiency.

Optical Constants of Rubidium and Cesium from 0.5 to 4.0 eV

Abstract: Measurements are reported for ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$, the real and imaginary parts of the dielectric constant, of Rb and Cs at room temperature in the frequency range 0.5-4.0 eV. The experiments were performed by multiple reflection at metal-silica interfaces employing the technique used earlier on Na and K. The optical absorption of Rb shows structure which can be attributed to conventional interband transitions within the nearly free-electron model. The optical absorption of Cs also shows structure, but the threshold occurs at a lower frequency than that predicted by the nearly free-electron model. Both metals show a leveling off and a perceptible rise in the absorption at the high-frequency end of the range. The possibility of absorption due to many-body effects or transitions to $d$ bands above the Fermi level is discussed. The results for ${\ensuremath{\epsilon}}_{1}$ are used to deduce values for the infrared optical masses which are found to be 1.16 m for Rb and 1.19 m for Cs.

Optical Properties of Antiferromagnetic Chromium and Dilute Cr-Mn and Cr-Re Alloys

Abstract: The antiferromagnetic energy gap and optical properties of single crystals of Cr and of Cr alloyed with 0.45, 0.94, 2.5, and 4.5% Mn and with 1.05 and 2.0% Re have been investigated by measuring the optical absorption over a photon energy range of 0.08-5 eV. The data were taken at 4.2 K by a calorimetric technique. It is found that at 4.2 K, approximately 1% Mn or Re will cause commensurate antiferromagnetic ordering, and the energy gap shifts abruptly from 0.13 to 0.36 eV near this concentration of diluent. Evidence is given for indirect transitions occurring across a double energy gap in the noncommensurate samples, and the results are explained in terms of a band model. The widths of the absorption peak and the peak in ${\ensuremath{\epsilon}}_{2}$ in the commensurate samples are discussed in terms of a gap anisotropy and impurity broadening. Optical absorption peaks near 1, 2, and 3.4 eV are associated with direct interband transitions originating from the $N_{1}^{\ensuremath{'}}$, $\ensuremath{\Gamma}_{25}^{\ensuremath{'}}$, and ${P}_{4}$ symmetry points, with a contribution to the 1-eV absorption from transitions from the Fermi surface to the flat unoccupied band extending through ${N}_{4}$, ${\ensuremath{\Gamma}}_{12}$, ${P}_{3}$, and $H_{25}^{\ensuremath{'}}$.

Intra- and Interband Free-Carrier Absorption and the Fundamental Absorption Edge in n-Type InP

Abstract: The infrared absorption in $n$-type InP was studied as a function of free-carrier concentration from 3.5\ifmmode\times\else\texttimes\fi{}${10}^{17}$ to \ensuremath{\sim} ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in the spectral range of 10 \ensuremath{\mu} to the fundamental absorption edge. The Burstein shifted edge, the normal intraband free-carrier absorption, and the interband free-carrier absorption were analyzed. The fundamental edge shifts to higher energy with increasing free-carrier concentration but only by 47% of the expected Burstein shift. The energy separation between the central-conduction-band minimum and the next higher conduction valleys in InP was found to be 0.90 \ifmmode\pm\else\textpm\fi{} 0.02 eV.

Photoabsorption near the L I I , I I I Edge of Silicon and Aluminum

Abstract: The absorption coefficients of Al, Si, and SiO were determined in the extreme uv with high spectral resolution, using synchrotron radiation. A detailed comparison of the ${L}_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edge in the metal and in the semiconductor shows important differences. The spikes observed at the edge in Al are at least consistent with the predictions of recent theory. The silicon $L$ threshold shows the expected spin-orbit splitting of the initial states, but the shape just beyond the edge is not in good agreement with the results of one-electron band theory. The ${L}_{I}$ edges are located at 117.4 eV in Al and at 151 eV in Si. Great similarities are noted in the broad structure, 15 or more eV beyond the ${L}_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edges in the three materials studied and also in Na and Mg films. It is suggested that electron-hole transitions accompanied by collective excitations are involved.

Optical Properties of the Rubidium and Cesium Halides in the Extreme Ultraviolet

Abstract: The absorption spectra of evaporated thin films of all rubidium and cesium halides in the 50- to 250-eV region are reported. In this range, transitions from the $3d$ shell of ${\mathrm{Rb}}^{+}$ and from the $4d$ and $4p$ shells of ${\mathrm{Cs}}^{+}$ can be seen, as well as some transitions from inner shells of the halogen ions. Besides the absorption fine structure near the threshold for inner-shell transitions, broad absorption structure is observed and explained as due to $d\ensuremath{\rightarrow}f$ continuum transitions. The number of effective electrons whose oscillator strength has been exhausted in our spectral region has been computed from the absorption data; it is particularly strong for materials containing either Cs or I. Differences in the spectra of materials with NaCl and with CsCl structure are discussed. The measurements were performed using the DESY electron synchrotron as a light source.

Pulse Radiolysis of Pyrrolidine

Abstract: The formation, decay, and absorption spectra of transients formed from pyrrolidine in aqueous solutions were studied. The solvated electron ( ${\rm e}_{{\rm aq}}^{-}$) reacts with the protonated and the nonprotonated pyrrolidine forming the ${\rm C}_{4}{\rm H}_{8}{\rm N}$· radical. The rate constants are: $k({\rm e}_{{\rm aq}}^{-}+{\rm C}_{4}{\rm H}_{8}{\rm NH}_{2}{}^{+})=(7.5\pm 1.5)\times 10^{6}\ M^{-1}\ \text{second}^{-1}$ and $k({\rm e}_{{\rm aq}}^{-}+{\rm C}_{4}{\rm H}_{8}{\rm NH})=(1.1\pm 0.5)\times 10^{6}\ M^{-1}\ \text{second}^{-1}$. The ${\rm C}_{4}{\rm H}_{8}{\rm N}$· radical has an absorption maximum at 3200 A, $\epsilon =510\ M^{-1}\ {\rm cm}^{-1}$, and disappears in a second-order reaction, $k=3.1\times 10^{9}\ M^{-1}\ \text{second}^{-1}$. The rate constants for the reaction of aqueous pyrrolidine with OH radicals, $k=9.6\times 10^{9}\ M^{-1}\ \text{second}^{-1}$ for pH 2 and $k=1.45\times 10^{10}\ M^{-1}\ \text{second}^{-1}$ for pH 8, were also determined. The pyrrolidine transients formed b...

Infrared Resonance of OH With the H 2 O Laser: A Galactic Maser Pump?

Abstract: The 79-\ensuremath{\mu}m electric dipole spectrum of OH ($^{2}\ensuremath{\Pi}_{\frac{3}{2}},J=\frac{3}{2}\ensuremath{\leftrightarrow}^{2}\ensuremath{\Pi}_{\frac{1}{2}},J=\frac{1}{2}$) has been measured by a magnetic-resonance absorption method, with a water-vapor laser as the source oscillator. Corresponding magnetic dipole spectra are calculated from the data. A near overlap of the water line on one magnetic-dipole transition suggests a possible pumping mechanism for the 18-cm maser emission of stellar OH.

The photolysis of ozone by ultraviolet radiation. IV. Effect of photolysis wavelength on primary step

Abstract: Quantum yields for the photolysis of ozone have been measured at six wavelengths in the region 248 to 334 nm. The quantum yield for the photolysis of pure ozone increases linearly with ozone concentration over the pressure range employed for $\lambda $ $\leq $ 310 nm, but is virtually independent of [O$\_{3}$] at $\lambda $ = 334 nm. At all wavelengths the quantum yield, extrapolated to zero ozone pressure, is near 4. Addition of H$\_{2}$ to the ozone markedly increases the quantum yield at all wavelengths except $\lambda $ = 334 nm. Dilution of the ozone by O$\_{2}$ reduces the quantum yield; at high [O$\_{2}$] the quantum yield tends to zero. It is shown that the results are consistent with the production of singlet molecular oxygen (in the $^{1}$\Delta $\_{\text{g}}$ or $^{1}$\Sigma $\_{\text{g}}^{+}$ states) in the primary step at all wavelengths in the ultraviolet region. For $\lambda $ $\leq $ 302 nm the atomic product is in the excited, $^{1}$D, state, O$\_{3}$ + h$ u $$\_{(\lambda \leq 302\text{nm})}$ $\rightarrow $ O$\_{2}$($^{1}$$\Sigma \_{\text{g}}^{+}$ or $^{1}$\Delta $\_{\text{g}}$) + O($^{1}$D), (1) but with increasing wavelength the efficiency of O($^{1}$D) production drops rapidly, and at $\lambda $ = 334 nm the atomic product is almost exclusively O($^{3}$P). O$\_{3}$ + h$ u $$\_{(\lambda =334\text{nm})}$ $\rightarrow $ O$\_{2}$($^{1}$$\Sigma \_{\text{g}}^{+}$ or $^{1}$\Delta $\_{\text{g}}$) + O($^{3}$P). (6) Thus it appears that ozone photolysis in the weak absorption tail of the Huggins band does not proceed with conservation of spin. Some possible implications of the laboratory observations in atmospheric photochemistry are considered.

Absorption Spectrum of Ba I in the Region of Autoionization from 2382 to 1700 Å

Abstract: The atomic absorption cross section of barium has been measured in the autoionization region at an instrumental bandwidth of 0.075 \AA{} between 2380 and 2100 \AA{} and 0.75 \AA{} between 2100 and 1700 \AA{}. The cross sections of the absorption features whose half-widths approach the bandwidth of the monochromator have been corrected to eliminate the bandwidth dependence. The observed structure is grouped into series and presented with measured wavelengths and a parameter $F$. The latter is numerically equal to the oscillator strength of the transition, except when significant perturbations from neighboring lines or the underlying continuum occur. Evidence of the presence of elements in the series $6pns^{1}P_{1}$ and $^{3}P_{1} (ng6)$ is presented.

Optical Absorption Spectra of Chromium-Doped Zinc Sulfide Crystals

Abstract: We have measured the optical absorption spectra of twinned cubic crystals of chromiumdoped zinc sulfide between 5000 and 29 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ (2-0.34 \ensuremath{\mu}m) at 4.2-500\ifmmode^\circ\else\textdegree\fi{}K. Two broad absorption bands are specific to chromium; at 77\ifmmode^\circ\else\textdegree\fi{}K, one has a maximum at 24 400 ${\mathrm{cm}}^{\ensuremath{-}1}$, the other, at 5700 ${\mathrm{cm}}^{\ensuremath{-}1}$. Weak photoconductivity is observed in the former; but none in the latter. Twelve narrow bands between 5100 and 5250 ${\mathrm{cm}}^{\ensuremath{-}1}$ are identified as zero-phonon transitions in ${\mathrm{Cr}}^{2+}$ (3${\mathrm{d}}^{4}$) substitutional at cation sites, two for the cubic structure, ten for the twin boundaries. The cubic crystal field is determined. Thirteen phonon-assisted transitions are identified and the phonon wave numbers are evaluated. We propose that the 24 400-${\mathrm{cm}}^{\ensuremath{-}1}$ band involves transitions to the conduction band edge.