Showing papers on "Absorption (logic) published in 1992"

Infrared absorption strength and hydrogen content of hydrogenated amorphous silicon.

Abstract: We have used infrared transmission and nuclear-reaction analysis to determine the ir absorption strength of the Si-H wagging and stretching modes in hydrogenated amorphous silicon (a-Si:H). The films were deposited by plasma-assisted chemical vapor deposition and reactive magnetron sputtering. We show that the widely used ir-data-analysis method of Brodsky, Cardona, and Cuomo can lead to significant errors in determining the absorption coefficients, particularly for films less than \ensuremath{\sim}1 \ensuremath{\mu}m thick. To eliminate these errors we explicitly take into account the effects of optical interference to analyze our data. We show that the hydrogen content can be determined from the stretching modes at \ensuremath{\omega}=2000 and 2100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ as well as the wagging mode at \ensuremath{\omega}=640 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. By assigning different oscillator strengths to the 2000- and 2100-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ modes, we show that the absorption strength of the stretching modes does not depend on the details of sample preparation, contrary to hypotheses previously invoked to explain experimental data. We obtain ${\mathit{A}}_{640}$=(2.1\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, ${\mathit{A}}_{2000}$=(9.0\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, and ${\mathit{A}}_{2100}$=(2.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ for the proportionality constants between the hydrogen concentration and the integrated absorbance of the wagging and stretching modes. The value of ${\mathit{A}}_{640}$ is \ensuremath{\sim}30% larger than the generally used value. We show that previously published data for both the wagging and stretching modes are consistent with the proportionality factors determined in the present study.

Nature and origin of the 5-eV band in SiO2:GeO2 glasses

Abstract: The sources of an absorption band at \ensuremath{\sim}5 eV observed in ${\mathrm{SiO}}_{2}$:${\mathrm{GeO}}_{2}$ and ${\mathrm{GeO}}_{2}$ glasses have not been unambiguously identified. Results reported here are consistent with the source of two types of neutral oxygen vacancies. Samples of ${95\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$:${5\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ and ${90\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$:${10\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ were prepared by a chemical vapor deposition soot-remelting method. Optical-absorption and electron paramagnetic resonance spectra were measured. An absorption band centered at 5 eV in as-prepared ${\mathrm{SiO}}_{2}$:${\mathrm{GeO}}_{2}$ glasses is composed of two components. One has a peak at 5.06 eV and a FWHM (full width at half maximum) of 0.38 eV. Illumination with uv light bleached this band, and generated Ge E' centers. A linear relation was found between the decrement in the intensity of the 5.06-eV component and the concentrations of uv-induced Ge E' centers. This relation is a basis for attributing the defect responsible for this component to the precursors of uv-induced Ge E' centers. We propose that the 5.06-eV band is due to neutral oxygen monovacancies (NOV's) coordinated by two Ge ions. The oscillator strength of this band was evaluated to be approximately 0.4\ifmmode\pm\else\textpm\fi{}0.1 assuming that the NOV's are converted into Ge E' centers by absorption of uv quanta. The activation energy for this conversion process was of the order of ${10}^{\mathrm{\ensuremath{-}}2}$ eV.The second component of the absorption spectra has a peak at 5.16 eV and a FWHM of 0.48 eV. This band is not bleached but emits luminescence bands at 3.2 eV (intense) and 4.3 eV (weak) when irradiated with 5-eV light. Based on other research, we assign this band to ${\mathrm{Ge}}^{2+}$ ions coordinated by two oxygens and having two lone pair electrons (neutral oxygen divacancies). The concentrations of ${\mathrm{Ge}}^{2+}$ ions were much larger than those of the NOV's and the ratio of the NOV's to ${\mathrm{Ge}}^{2+}$ ions increases with increasing ${\mathrm{GeO}}_{2}$ content. A similarity was found in the characteristics of these two types of oxygen-deficient defects to those in ${\mathrm{SiO}}_{2}$ glasses.

Paramagnetic Meissner effect in Bi high-temperature superconductors.

Abstract: For certain ceramic samples of the Bi-based high-temperture superconductors, the dc-field-cooling signal becomes paramagnetic in fields below a few 100 mOe This effect correlates with an anomaly in the low-field microwave absorption The data are consistent with orbital paramagnetic moments due to the appearance of spontaneous supercurrents in fields smaller than the lower critical field ${\mathit{H}}_{\mathit{c}1\mathit{a}}$ parallel to the CuO planes

Point-defect energies in the nitrides of aluminum, gallium, and indium.

Abstract: Experimental data on the nature and energetic location of levels associated with native point defects in the group-III metal nitrides are critically reviewed and compared with theoretical estimates. All three show strong evidence of the existence of a triplet of donorlike states associated with the nitrogen vacancy. Ground states are at about 150, 400, and 900 meV from the conduction-band edge in InN, GaN, and AlN, respectively, with their charged derivatives lying closer to the band edge. These values agree with both modified-hydrogenic and deep-level calculations, surprisingly well in view of the inherent approximations in each in this depth range. The InN donor ground state is both optically active and usually occupied, showing a distinctive absorption band which is very well described by quantum-defect analysis. Variation of threshold with electron concentration shows a Moss-Burstein shift commensurate with that observed in band-to-band absorption. In both GaN and AlN, levels have been identified at about 1/4${\mathit{E}}_{\mathit{G}}$ and about 3/4${\mathit{E}}_{\mathit{G}}$, which correlate well with predictions for the antisite defects ${\mathrm{N}}_{\mathit{M}}$ and ${\mathit{M}}_{\mathrm{N}}$, respectively, while similar behavior in InN is at odds with theory. The metal-vacancy defect appears to generate a level somewhat below midgap in AlN and close to the valence-band edge in GaN, but has not been located experimentally in InN, where it is predicted to lie very close to the valence-band edge. A tentative scheme for the participation of two of the native defects in GaN, namely ${\mathit{V}}_{\mathrm{N}}$ and ${\mathrm{N}}_{\mathrm{Ga}}$, in the four broad emission bands found in Zn-compensated and undoped GaN is offered.

Total (elastic plus inelastic) cross sections for electron scattering from diatomic and polyatomic molecules at 10-5000 eV: H2, Li2, HF, CH4, N2, CO, C2H2, HCN, O2, HCl, H2S, PH3, SiH4, and CO2.

Abstract: By employing a previously proposed model [A. Jain, Phys. Rev. A 34, 3703 (1986); J. Phys. B 22, 905 (1988)], we report calculations on the total (elastic plus inelastic) electron-scattering cross sections in a wide energy range (10--5000 eV) from several diatomic and polyatomic molecular targets (${\mathrm{H}}_{2}$ ${\mathrm{Li}}_{2}$, HF, ${\mathrm{CH}}_{4}$, ${\mathrm{N}}_{2}$, CO, ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$, HCN, ${\mathrm{O}}_{2}$, HCl, ${\mathrm{H}}_{2}$S, ${\mathrm{PH}}_{3}$, ${\mathrm{SiH}}_{4}$, and ${\mathrm{CO}}_{2}$). A model complex optical potential (composed of static, exchange, polarization, and absorption terms) is calculated for each collision system from the corresponding molecular wave function at the Hartree-Fock level. The resulting complex optical potential, free from any adjustable parameter, is treated exactly in a variable-phase approach to yield scattering complex phase shifts and the total cross sections. In the intermediate- and high-energy region, the small contribution due to the nonspherical nature of the target is treated perturbatively in the first-order Born approximation. The present method is quite simple in nature and is able to reproduce fairly well the experimental total cross sections in the present energy region. Results are also given for individual elastic and absorption (accounting for all energetically possible inelastic processes in an approximate way) cross sections. In addition, we provide Born-Bethe parameters for all the above molecules including those of ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$. We have also examined the correlation between molecular properties and the total-cross-section parameter. For molecules possessing a permanent dipole or quadrupole moment, the present results are only roughly reliable above 100 eV.

Stark effect of one-dimensional Wannier excitons in polydiacetylene single crystals.

Abstract: Electric-field-modulated reflectivity of three different polydiacetylenes---PTS, poly[2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate)]; PFBS, poly[2,4-hexadiyne-1,6-diol-bis(p-fluorobenzene sulfonate)]; and DCHD, poly[1,6-di(n-carbazolyl)-2,4-hexadiyne]---has been measured and is analyzed with respect to the underlying mechanism for the observed sensitivity of \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ transitions to electric fields. Excitons of high oscillator strength and their vibronic satellites respond to fields along the polymer backbone by a large quadratic Stark shift, revealing a large polarizability for this direction. About 0.5 eV above the excitonic absorption edge, in a region of relatively low absorption, electroreflectance signals of different origin are observed, which, contrary to the excitonic signals, vary strongly in size among different specimens of the same composition. Line-shape analysis and its dependence on the field strength identify this signal as the Franz-Keldysh effect of free-electron states, the continuum of the excitons, and exclude an assignment to forbidden exciton transitions. The large polarizability of the excitonic states, which results from unusually strong coupling to their continuum, is consistent with a Wannier exciton extending over about ten conjugated bonds and with a small reduced mass of the order 0.1${\mathit{m}}_{0}$. The large binding energy (0.5 eV) and oscillator strength (f\ensuremath{\approxeq}0.6) of the excitons and their extremely strong coupling to continuum states are attributed to the one-dimensional character of the electron states.

Studies of photoexcited states in polyacetylene and poly(paraphenylenevinylene) by absorption detected magnetic resonance: The case of neutral photoexcitations.

Abstract: Photoexcited states in transpolyacetylene [t-(CH${)}_{\mathit{x}}$], and in alkoxy derivatives of poly(paraphenylenevinylene) (PPV) nondegenerate ground-state polymers, have been studied by the technique of absorption detected magnetic resonance. In addition to the stable charged photoexcitations, which are charged solitons in t-(CH${)}_{\mathit{x}}$ and bipolarons in PPV, we clearly identify neutral photoexcitations with nonzero spins. In t-(CH${)}_{\mathit{x}}$ these are spin-1/2 neutral solitons ${\mathit{S}}^{0}$(S\ifmmode\bar\else\textasciimacron\fi{} $^{0}$) with an optical transition at 1.35 eV. The neutral photoexcitations in PPV form triplet excitons which cannot dissociate into ${\mathit{S}}^{0}$S\ifmmode\bar\else\textasciimacron\fi{} $^{0}$ pairs due to the confinement caused by the nondegenerate backbone structure.

Unoccupied electronic structure and core-hole effects in the x-ray-absorption spectra of Cu2O.

Abstract: X-ray-absorption measurements at the Cu ${\mathit{L}}_{2,3}$ and O K edges of ${\mathrm{Cu}}_{2}$O reveal the presence of unoccupied states of predominantly Cu d and O p character at the bottom of the conduction band. We find that spectral features up to 25 eV from threshold can be assigned to structures in the calculated unoccupied density of states, projected on the Cu and O sites. However, for a satisfactory description of the anomalous line shape and threshold intensity of the Cu edges, the core hole created in the absorption process must be included in the calculation. We have investigated this effect by a modified Clogston-Wolff impurity model, based on the calculated band structure. We conclude that, possibly because of the peculiar Cu-O coordination, the Cu 2p core-hole potential is stronger in ${\mathrm{Cu}}_{2}$O than in other Cu(I) materials, and that the Cu ${\mathit{L}}_{2,3}$ spectra present both band features, typical of monovalent Cu materials and of copper metal, and a resonant contribution, typical of divalent compounds such as CuO.

Lithium intercalation into WO3 and the phase diagram of LixWO3.

Abstract: We report studies of the electrochemical intercalation of lithium in ${\mathrm{WO}}_{3}$ powder and in polycrystalline evaporated ${\mathrm{WO}}_{3}$ films Electrochemical cells with beryllium x-ray windows were used to study the structure changes in the ${\mathrm{WO}}_{3}$ host in situ Rietveld profile refinement was used to analyze the x-ray-diffraction patterns of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$ after correcting for the absorption of the beryllium window Upon the intercalation of Li, the monoclinic structure of ${\mathrm{WO}}_{3}$ proceeds to higher and higher symmetry First a first-order phase transition to a tetragonal phase with x\ensuremath{\sim}01 in ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$ occurs Then another first-order phase transition to a cubic phase with x\ensuremath{\sim}036 occurs The cubic phase can accommodate further Li, up to a maximum of about x=05 All the phase transitions can be reversed, although some hysteresis is observed; upon complete deintercalation, monoclinic ${\mathrm{WO}}_{3}$ is recovered We report the structural changes, prove that the transitions are first order, and give a phase diagram for ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$ as a function of x Our results suggest that the formation of the tetragonal phase upon intercalation of Li in ${\mathrm{WO}}_{3}$ is responsible for the electrochromatic properties of crystalline ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$, and that conventional interpretations that ignore the existence of this phase are most likely in error

Excited-state tautomerization of 7-azaindole in water

Abstract: The authors have studied the solvent dependence of the steady-state absorption, emission, and time-resolved emission of 7-azaindole (7AI) and two nonreactive analogues, N-methyl-7-azaindole (NMAI) and 7-methyl-7H-pyrrolo[2,3-b]pyridine (7MPP), in an effort to understand the apparently anomalous behavior of 7AI in water. The author`s find that 7AI undergoes the same tautomerization reaction via solvent-catalyzed double proton transfer in water as it does in alcohol solvents. Kinetic modeling shows that the unusual features of the 7AI emission in water arise mainly from quantitative changes in two key rate parameters. In water the rate constant for tautomerization (H{sub 2}O 1.2 x 10{sup 9} s{sup {minus}1} at 24 {degrees}C) is much slower and simultaneously the nonradiative decay rate of the product is much faster (5 x 10{sup 9} s{sup {minus}1} in both H{sub 2}O and D{sub 2}O) than in most alcohols, making observation of the reaction difficult. The reason the reaction rate in water is unusually slow appears to result from differences in the hydrogen bonding structure and dynamics of water compared to monoalcohols. 43 refs., 12 figs., 4 tabs.

Influence of H-D isotopic substitution on the protonic conductivity of LiNbO3.

Abstract: The electric dc dark conductivity in congruent ${\mathrm{LiNbO}}_{3}$ has been studied in the temperature range 80--600 \ifmmode^\circ\else\textdegree\fi{}C as a function of the proton and deuteron concentration and the driving electric field. The conductivity was found to follow an Arrhenius-like behavior with an activation energy equal for protons and deuterons, ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{act}}$=1.23 eV. The pre-exponential factor ${\mathrm{\ensuremath{\sigma}}}_{0}$ shows a linear concentration dependence for both H and D, but with different slopes. The ratio of the slopes yields (\ensuremath{\partial}${\mathrm{\ensuremath{\sigma}}}_{0}$/\ensuremath{\partial}c${)}_{\mathrm{H}}$/(\ensuremath{\partial}${\mathrm{\ensuremath{\sigma}}}_{0}$/\ensuremath{\partial}c${)}_{\mathrm{D}}$=1.36. This agrees with the mass dependence of the attempt frequencies in a hopping model of protons and deuterons and rules out models of OH-OD migration. The conductivity was found to be independent of the electric field up to values of ${10}^{5}$ V/cm, which is comparable to inner electric fields in volume holograms in ${\mathrm{LiNbO}}_{3}$. In the temperature range accessible to both methods, the dc and the holographically measured values of the conductivity agree very well, displaying the same temperature and concentration dependence. The charge-carrier concentration was determined from the integrated absorption strength of the ir OH-OD stretch-mode absorption of proton- and deuteron-exchanged waveguides with a known rate of exchange of Li by H-D, yielding an oscillator strength ${\mathit{f}}_{\mathrm{H}}$=1.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$.

Critical investigation of the infrared-transmission-data analysis of hydrogenated amorphous silicon alloys.

Abstract: Infrared-transmission spectroscopy is widely used to obtain quantitative information about hydrogen bonding in hydrogenated amorphous silicon (a-Si:H), silicon-germanium (a-SiGe:H), and silicon-carbon (a-SiC:H) alloys. To simplify the conversion of transmission spectra to absorption spectra the most commonly used methods, suggested by Brodsky, Cardona, and Cuomo (BCC) [Phys. Rev. B 16, 3556 (1977)] and Connell and Lewis (CL) [Phys. Status Solidi B 60, 291 (1973)], assume incoherent multiple reflections in the film as well as the substrate. We show that the absorption in a-Si:H alloy films on c-Si substrates is different for coherent and incoherent reflections in the film. This difference causes the BCC and CL methods to overestimate or underestimate the absorption coefficient (\ensuremath{\alpha}) in many experimental situations. The most notable feature is an overestimate of \ensuremath{\alpha} if the film thickness d is below a critical value (${\mathit{d}}_{\mathrm{min}}$). For dg${\mathit{d}}_{\mathrm{min}}$, the error in absorption coefficient is usually less than 10%. Below ${\mathit{d}}_{\mathrm{min}}$, the error in \ensuremath{\alpha} increases as d decreases. The maximum error, which occurs in the limit d\ensuremath{\rightarrow}0, increases with the refractive index of the film and is \ensuremath{\ge}30% for a-SiC:H alloys, \ensuremath{\sim}70% for a-Si:H, and \ensuremath{\le}90% for a-SiGe:H alloys. The value of ${\mathit{d}}_{\mathrm{min}}$ decreases as the refractive index of the film and the frequency of the vibrational mode increase. For a-Si:H, for example, the hydrogen content determined from the 640-${\mathrm{cm}}^{1}$ Si-H wagging-mode absorption is overestimated if d is less than \ensuremath{\sim}1 \ensuremath{\mu}m. We show that experimental data are consistent with the predictions of this analysis. In most cases it is possible to correct the results from the BCC and CL methods so that they are accurate to within 10%. For greater accuracy, infrared-transmission data should be analyzed by taking the effects of optical interference into account.

Clock synchronization and isotropy of the one-way speed of light.

Abstract: Experimental tests of the isotropy of the speed of light using one-way propagation are analyzed using a test theory of special relativity. It is shown that, when properly expressed in terms of measurable quantities, the results of such experiments are independent of the method of global synchronization of clocks. Experiments analyzed include a Jet Propulsion Laboratory time-of-flight measurement, a resonant two-photon absorption experiment, the Smithsonian Astrophysical Observatory-NASA 1976 rocket gravitational redshift experiment, and M\"ossbauer rotor experiments. If the characteristic anisotropy is proportional to $\ensuremath{\alpha}w$, where $w$ is the velocity of the Earth relative to the cosmic background radiation, the best bound on $\ensuremath{\alpha}$ from these experiments is $|\ensuremath{\alpha}|l9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$.

Optical transitions of C 60 films in the visible and ultraviolet from spectroscopic ellipsometry

Abstract: Optical spectra for ${\mathrm{C}}_{60}$ films, obtained by elliposmetry, are reported for the energy range from 1.9 to 9.5 eV. Our results are compared with related results from other optical techniques as well as photoemission results. Strong similarity to absorption measurements of dissolved ${\mathrm{C}}_{60}$ results from the strong molecular character of the solid films. We fit our spectra with Lorentzian line shapes to determine energy positions and shapes of the transitions. We observe the onset of optical absorption near 1.9 eV. We also report initial results from alkali-metal doping and a useful method for incorporating K into the bulk of film samples.

Absolute frequency measurement of the hydrogen 1S-2S transition and a new value of the Rydberg constant.

Abstract: We have compared the frequency of the hydrogen 1S-2S transition (resonance width \ensuremath{\Delta}\ensuremath{ u}/\ensuremath{ u}=2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$) with a reference frequency synthesized from an 88-THz infrared ${\mathrm{CH}}_{4}$-stabilized HeNe laser. In this first measurement of an optical frequency that overcomes the uncertainty limits of the visible ${\mathrm{I}}_{2}$-stabilized He-Ne laser standard, we find a 1S-2S frequency of 2 466 061 413.182(45) MHz with an 18-fold improved precision of 1.8 parts in ${10}^{11}$. A new value of the Rydberg constant, ${\mathit{R}}_{\mathrm{\ensuremath{\infty}}}$=109 737.315 684 1(42) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, is deduced.

Observation of the (2 p , nd ) 1 P o double-excitation Rydberg series of helium

Abstract: Double-excitation Rydberg resonances of He below the {ital N}=2 threshold of He{sup +} were studied with an energy resolution of {congruent}4 meV by photoionization using synchrotron radiation. All three Rydberg series of {sup 1}{ital P{ital o}} resonances accessible by photoabsorption, including the hitherto missing'' (2{ital p},{ital nd}){sup 1}{ital P{ital o}} series, were resolved. This settles a long-standing discrepancy between experiment and theoretical predictions and establishes a firm basis for testing the accuracy of atomic multiconfiguration calculations.

Electronic states and optical properties of porphyrins in van der Waals contact: thorium(IV) sandwich complexes

Abstract: Ground-state and time-resolved excited-state absorption spectra and fluorescence and phosphorescence spectra of three Th{sup IV} sandwich complexes, Th{sup IV}(TPP){sub 2}, Th{sup IV}(OEP){sub 2}, and Th{sup IV}(OEP)(TPP), are reported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinate, TPP = 5,10,15,20-tetraphenylporphyrinate). These complexes, in which the nitrogen planes of the two porphyrin macrocycles are {approximately}2.9 {Angstrom} apart, exhibit a number of prominent optical characteristics: (1) monoporphyrin-like Q and B absorption bands, (2) a new absorption between the Q and B bands, (3) a weak, low-energy absorption that is substantially red-shifted relative to the Q bands of analogous monoporphyrin complexes, (4) fluorescence and phosphorescence emission bands that are even further red-shifted relative to typical emission bands from porphyrin monomers, and (5) a moderately intense near-infrared {sup 3}({pi},{pi}*) excited-state absorption not observed in monomeric porphyrins. These characteristic optical properties of the sandwich complexes are all accounted for by a relatively simple molecular orbital configuration-interaction model. Additionally, the spectral data and molecular orbital model identify the energies of charge-transfer configurations and delineate their contribution to the electronic states of these strongly-coupled {pi} systems. These results provide insights into the interactions that can take place between other pairs of chromophores brought within van der Waals contact, such as the bacteriochlorophyll dimermore » of the photosynthetic reaction center. 49 refs., 9 figs., 1 tab.« less

Charged-phonon absorption in doped C60.

Abstract: We interpret the giant infrared resonances observed in ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ (A=K, Rb) by Fu et al. as being due to a classical charged-phonon effect in which the ${\mathit{T}}_{1\mathit{u}}$ molecular vibration modes of ${\mathrm{C}}_{60}$ acquire electronic oscilator strength via couplng to virtual ${\mathit{t}}_{1\mathit{u}}$\ensuremath{\rightarrow}${\mathit{t}}_{1\mathit{g}}$ electronic transitions. For the compounds ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$ (x=3, 4, and 6), we predict, for weak electron--molecular-vibration coupling, that the softenings of the ${\mathit{T}}_{1\mathit{u}}$ modes are proportional to x, but that the integrated oscillator strengths of their absorption bands are proportional to the square of x.

Excited states of the two-dimensional D- center in magnetic fields.

Abstract: Excited states of the two-dimensional ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ center (or ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion) are considered in the low-field and high-field limits. It is shown that in two dimensions there are no bound analogs of the large-radius bound excited states that spring into existence when an arbitrarily weak magnetic field is applied to a three-dimensional ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion. Asymptotically exact wave functions and energies are obtained in the limit of infinite magnetic field. In that limit, only four bound states are found in two dimensions: one spin singlet (symmetric space wave function), which is the ground state, and three spin triplets (antisymmetric space wave functions). It is pointed out that even those states that are unbound have, nevertheless, discrete energy levels in two dimensions (due to the Landau quantization of the planar motion) and play an essential role in the absorption of radiation by the ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ center. Connections between the two-dimensional ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ center and ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ centers in quantum wells are discussed.

Relativistic tight-binding calculations of x-ray absorption and magnetic circular dichroism at the L2 and L3 edges of nickel and iron.

Abstract: The soft-x-ray absorption and magnetic circular dichroism (MCD) cross sections at the {ital L}{sub 2} and {ital L}{sub 3} core-level edges of Ni and Fe have been calculated using a one-electron tight-binding band-structure approach. The tight-binding scheme is taken from the work of Papaconstantopoulos, supplemented with the inclusion of spin-orbit coupling in the {ital d} bands and fully relativistic dipole selection rules. In Ni, a fit to the various {ital L}{sub 2} and {ital L}{sub 3} intensity ratios can be achieved, but only with values of the {ital d}-band spin-orbit parameter {xi} and exchange parameter {Delta}{sub ex} at variance with the ground-state band-structure values. For Fe, there is no plausible value of {xi} capable of explaining the intensity ratios; also, the predicted substructure within the {ital L}{sub 2} and {ital L}{sub 3} white lines is not seen in experiment. These failures of the one-electron approach are qualitatively consistent with expected many-body electronic rearrangements associated with core-hole creation. Some discussion is offered on sum rules and on orbital versus spin magnetic moments.

Liquid-vapor density profile of helium: An x-ray study.

Abstract: The average liquid-vapor density profiles 〈\ensuremath{\rho}(z)〉 of thick $^{4}\mathrm{He}$ films adsorbed onto a silicon substrate were measured using x-ray reflectivity The results are well represented by a 90%-10% interfacial width of 92\ifmmode\pm\else\textpm\fi{}1 \AA{} at 113 K which extrapolates to a T=0 K, 90%-10% interfacial width of 76+1,-2 \AA{} The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed

Interband dielectric function of C60 and M6C60 (M=K,Rb,Cs).

Abstract: We report the room-temperature optical properties of ${\mathrm{C}}_{60}$ and ${\mathit{M}}_{6}$${\mathrm{C}}_{60}$ determined from the near-normal-incidence reflection and transmission experiments. The observed peak positions in ${\mathrm{\ensuremath{\epsilon}}}_{2}$(\ensuremath{\omega}) for ${\mathrm{C}}_{60}$ and ${\mathit{M}}_{6}$${\mathrm{C}}_{60}$ compare well with recent band-structure calculations and suggest that the ${\mathit{M}}_{6}$${\mathrm{C}}_{60}$ compounds exhibit a weak hybridization between the ${\mathrm{C}}_{60}$ and M states, consistent with the Raman-scattering studies of ${\mathit{M}}_{6}$${\mathrm{C}}_{60}$. The onset of absorption across the energy gap between the highest occupied molecular orbital (MO) and the lowest unoccupied MO in ${\mathrm{C}}_{60}$ is measured to be 1.7 eV.

Interaction of excitons with an incompressible quantum liquid.

Abstract: Some general properties of exciton spectra of an interacting two-dimensional electron-hole system have been established. In the quantum limit both the frequencies and the matrix elements of exciton transitions are independent of environment, i.e., of filling factors of electrons and holes, if all the electron-hole interactions are equal in magnitude, ${\mathit{V}}_{\mathit{e}\mathit{e}}$=${\mathit{V}}_{\mathit{h}\mathit{h}}$=\ensuremath{\Vert}${\mathit{V}}_{\mathrm{eh}}$\ensuremath{\Vert}. Only when these symmetry restrictions are lifted, do the absorption and emission spectra acquire nontrivial properties reflecting electronic correlations in a system. We consider an exciton against the background of an incompressible quantum liquid for both symmetric and nonsymmetric systems. It is always strongly coupled to the environment. This interaction may be considered as a polaron effect that is due to the dressing of an exciton by magnetorotons, but it shows very specific features because of restrictions imposed by the Pauli exclusion principle. The energy spectrum of a dressed exciton, the electric charge distribution, and some other properties of it are investigated. When the anisotropy parameter reaches its critical value the absolute minimum of the energy spectrum shifts from the point k=0 to a circle, as a result of which abrupt changes in the emission spectrum are expected. A model of the ground state is proposed.

Sodium (Na43+) clusters in sodium sodalite

Abstract: We have measured and calculated the absorption spectrum of the Na{sub 4}{sup 3+} clusters in Na{sub 3}[AlSiO{sub 4}]{sub 3} sodalite prepared by high-vacuum deposition of sodium atoms. The samples with a Na{sub 4}{sup 3+}:Na{sub 3}{sup 3+} cluster ratio up to 1:10 show a single-absorption feature with {lambda}{sub max} = 628 nm (1.99 eV). The absorption originates from the individual sodalite cages containing the Na{sub 4}{sup 3+} cluster. For the Na{sub 4}{sup 3+}:Na{sub 3}{sup 3+} cluster ratio larger than 1:10, when some of the Na{sub 4}{sup 3+} clusters are likely to interact, the changes in the absorption spectra indicate the onset of insulator-metal transition. Time-dependent quantum mechanical calculations of the photon absorption cross section of Na{sub 4}{sup 3+} clusters in sodalite at infinite dilution were carried out. The dependence of the calculated spectra on sodalite framework charges and cluster geometry was used to determine which of the proposed charge distribution models are consistent the the absorption spectra. The best agreement between measurements and calculations is obtained for Na = +1, Si = +1.9, Al =+0.9, and O = {minus}0.95. 24 refs., 7 figs., 1 tab.

Optical response of small niobium clusters.

Abstract: The optical absorption spectra of small, isolated niobium clusters from seven to twenty atoms have been measured from 334 to 614 nm via photodepletion of niobium cluster-argon van der Waals complexes, ${\mathrm{Nb}}_{\mathit{n}}$${\mathrm{Ar}}_{\mathit{m}}$, in a moleculear beam. The absorption cross sections of all clusters in this size range increase with decreasing wavelength, as predicted by classical electrodynamic theory applied to small niobium spheres, however, the magnitudes of the cross sections are several times larger than predicted.

Use of excited-state and ground-state redox properties of polyoxometalates for selective transformation of unactivated carbon-hydrogen centers remote from the functional group in ketones

Abstract: Two types of processes are described which involve the selective transformation of unactivated carbon-hydrogen bonds in a ketone, cis-2-decalone, cis-1, which possesses conventionally far more reactive bonds. The first type of process involves irradiation of decatungstate, W{sub 10}O{sub 32}{sup 4{minus}} in the presence of cis-1 producing, trans-2-decalone, trans-1, the product resulting from epimerization of an unactivated tertiary C-H bond remote form the carbonyl group, in high selectivity at high conversion of substrate. The second type of reaction involves irradiation of the heteropolytungstate, {alpha}-P{sub 2}W{sub 18}O{sub 62}{sup 6{minus}} or {alpha}-PW{sub 12}O{sub 40}{sup 3{minus}}, in the presence of cis-1 producing two monounsaturated ketones (octalones) in high selectivity with the nonthermodynamic isomer, 2, in comparable or greater quantity than the conventional thermodynamic (conjugated) isomer, 3, eq 2. Both types of processes are independent of wavelength over the principal range of absorption of the complexes (:250-380 nm). The primary kinetic isotope of the corresponding decalin hydrocarbons were evaluated. The photochemical reaction of decatungstate with {alpha},{alpha},{alpha}{sup {prime}},{alpha}{sup {prime}}-D{sub 4}-cis-1 leads exclusively, even at moderate conversion of substrate (25%), to {alpha},{alpha},{alpha}{sup {prime}},{alpha}{sup {prime}}-D{sub 4}-trans-1. These data, an isotope crossover experiment in which decatungstate was irradiated in the presence of a 50/50 molar mixture of deuterated andmore » protiated cis-decalin in CD{sub 3}CN are consistent with initial H atom abstraction in all cases. The dramatically different products seen with the different polyoxometalate systems are dictated by the relative rates of epimerization, oxidation, and escape of the cisoid tertiary bridgehead radicals in the initial radical cage and, to a lesser extent, by the rates of conventional radical-radical reactions and other processes.« less

Experimental study of neutrino absorption on carbon

Abstract: The process of electron emission from {similar to}30 MeV neutrino absorption on carbon, {sup 12}C({nu}{sub {ital e}},{ital e}{sup {minus}}){sup 12}N, has been observed. The flux-weighted total cross section for the exclusive neutrino-induced nuclear transition {sup 12}C({nu}{sub {ital e}},{ital e}{sup {minus}}){sup 12}N(g.s.) is (1.05{plus minus}0.10(stat){plus minus}0.10(syst)){times}10{sup {minus}41} cm{sup 2}. The measured cross section and angular distribution {ital d}{sigma}/{ital d}{Omega} are in agreement with theoretical estimates. The inclusive {nu}{sub {ital e}} {sup 12}C reaction rate, which accounted for the majority of all neutrino interactions observed in this experiment, was determined from a detailed fit of energy and angular distributions for the observed electrons. The inclusive {sup 12}C({nu}{sub {ital e}},{ital e}{sup {minus}}){ital X} cross section is measured to be (1.41{plus minus}0.23(tot)){times}10{sup {minus}41} cm{sup 2}. An upper limit for the sum of the {sup 13}C({nu}{sub {ital e}},{ital e}{sup {minus}}){ital X}+{sup 27}Al({nu}{sub {ital e}},{ital e}{sup {minus}}){ital X} inclusive absorption cross sections is presented.

Intramolecular energy transfer in ruthenium(II)-chromium(III) chromophore-luminophore complexes. Ru(bpy)2[Cr(cyclam)(CN)2]24+

Abstract: A new trinuclear Ru(II)-Cr(III) chromophore-luminophore complex, Ru(bpy){sub 2}(Cr(cyclam)(CN){sub 2}){sub 2}{sup 4+}, has been synthesized and characterized. Visible light absorption by the Ru(bpy){sub 2}{sup 2+} chromophore leads to emission from the Cr(cyclam)(CN){sub 2}{sup +} luminophore, as a consequence of very efficient ({ge} 99%) and fast (subnanosecond time scale) chromophore-luminophore exchange energy-transfer process. The emission is intense ({Phi} = 5.3 {times} 10{sup {minus}3} in H{sub 2}O) and long-lived ({tau} = 260 {mu}s in H{sub 2}O). The photophysical properties of the luminophore are slightly perturbed by interaction with the chromophore, resulting in a sharper emission band shape and shorter radiative and radiationless lifetimes. The presence of a Ru(II) {yields} Cr(III) intervalence transfer state, hardly detectable in the ground-state spectrum, is clearly revealed by the excited-state absorption spectrum of the chromophore-luminophore complex.

Infrared absorption in SiO2-Ge composite films: Influences of Ge microcrystals on the longitudinal-optical phonons in SiO2.

Abstract: Infrared absorption in composite films of ${\mathrm{SiO}}_{2}$ and microcrystalline Ge has been investigated with p- and s-polarized light under 45\ifmmode^\circ\else\textdegree\fi{} oblique incidence. In the case of p-polarized incident light, pure ${\mathrm{SiO}}_{2}$ films clearly exhibit an absorption peak of the LO-phonon mode around 1240 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in addition to that of the TO mode at 1080 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, due to the so-called Berreman effect. It was found that the intensity of the LO absorption relative to that of the TO absorption decreases as the volume fraction of Ge microcrystals in the film increases. Theoretical calculations based on the Bruggeman and Maxwell-Garnett effective-medium theories have been performed. The calculations made by the Bruggeman theory could successfully reproduce the observed spectra and well explain the decrease in the intensity of the LO absorption, while those made by the Maxwell-Garnett theory disagreed with the experiments.

Response of excitonic absorption spectra to photoexcited carriers in GaAs quantum wells.

Abstract: The changes induced in the optical-absorption spectrum of a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum well due to photoexcited carrier distributions are reexamined. We use a picosecond pump-probe technique to excite carriers in a sample having wells of thickness 210 \AA{} and barriers of thickness 100 \AA{}. We find that for densities up to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ the peak of the exciton line of the lowest-energy exciton becomes bleached; however the spectrally integrated area does not change, indicating that the carriers reduce the absorption peak by collision broadening rather than by phase-space filling or exchange. We also observe shifts of the exciton line to higher energy (``blueshift'') or to lower energy (``redshift'') depending on the energy of the pump photons. The shift can be ascribed to competing effects of band-gap renormalization and the change in the binding energy of the exciton, although a detailed theoretical description is still needed.