Additional Sharp Peaks Research Articles

Strong fluorescence emissions by H-aggregates of the dye thiacyanine in the presence of the surfactant aerosol-OT

The cationic dye 3,3 ′-diethylthiacyanine iodide (THIA) in dilute aqueous solution shows weak fluorescence with a broad peak around 470 nm, and the anionic surfactant bis(2-ethylhexyl) sodium sulphosuccinate, called aerosol-OT (AOT) enhances the fluorescence of the dye. When excited at 422 nm ( λ max of THIA), the fluorescence peak of THIA, in the presence of increasing AOT concentrations up to its critical micellar concentration (CMC), is gradually reduced to a shoulder around 475 nm, and two additional sharp peaks at 494 and 601 nm are progressively developed. Above the CMC of AOT, the original 470 nm peak starts reappearing with simultaneous disappearance of the 494 and 601 nm peaks. At appropriate AOT concentrations (below CMC), THIA forms two metachromatic species (H-aggregates) showing strong absorption peaks at 377 and 366 nm. When excited at these metachromatic peaks, the fluorescence of the THIA/AOT system is, instead of being quenched, largely enhanced about 8–10 times compared to when excited at 422 nm, and above the CMC, fluorescence diminishes. Application of the principal component analysis method to the observed fluorescence spectra results in three pure component spectra with peaks at 470, 494 and 601 nm, which have been assigned to monomer, normal H-aggregates and twisted H-aggregates emission peaks, respectively. The twisted H-aggregates originate from the excited preexisting hexamer as well as from the excimer formed between excited and ground state trimers.

Ligand K-Edge and Metal L-Edge X-ray Absorption Spectroscopy and Density Functional Calculations of Oxomolybdenum Complexes with Thiolate and Related Ligands: Implications for Sulfite Oxidase

X-ray absorption spectra have been measured at the S K-, Cl K-, and Mo L3- and L2-edges for the d 0 dioxomolybdenum(VI) complexes LMoO2(SCH2Ph), LMoO2Cl, and LMoO2(OPh) (L ) hydrotris(3,5- dimethyl-1-pyrazolyl)borate) to investigate ligand -metal covalency and its effects on oxo transfer reactivity. Two dominant peaks are observed at the S K-edge (2470.5 and 2472.5 eV) for LMoO2(SCH2Ph) and at the Cl K-edge (2821.9 and 2824.2 eV) for LMoO2Cl, demonstrating two major covalent contributions from S and Cl to the Mo d orbitals. Density functional calculations were performed on models of the three Mo complexes, and the energies and characters of the Mo 4d orbitals were interpreted in terms of the effects of two strong cis-oxo bonds and additional perturbations due to the thiolate, chloride, or alkoxide ligand. The major perturbation effects are for thiolate and Cl - ﷿ mixed with the dxz orbital and U mixed with the dz 2 orbital. The calculated 4d orbital energy splittings for models of these two major contributions to the bonding of thiolate and Cl ligands (2.47 and 2.71 eV, respectively) correspond to the splittings observed experimentally for the two dominant ligand K-edge peaks for LMoO2(SCH2Ph) and LMoO2Cl (2.0 and 2.3 eV, respectively) after consideration of final state electronic relaxation. Quantification of the S and Cl covalencies in the d orbital manifold from the pre-edge intensity yields, 42% and 17% for LMoO2(SCH2Ph) and LMoO2Cl, respectively. The Mo L2-edge spectra provide a direct probe of metal 4d character for the three Mo complexes. The spectra contain a strong, broad peak and two additional sharp peaks at higher energy, which are assigned to 2p transitions to the overlapping t2g set and to the dz 2 and dxy levels, respectively. The total peak intensities of the Mo L 2- edges for LMoO2(OPh) and LMoO2Cl are similar to and larger than those for LMoO2(SCH2Ph), which agrees with the calculated trend in ligand-metal covalency. The theoretical and experimental description of bonding developed from these studies provides insight into the relationship of electronic structure to the oxo transfer chemistry observed for the LMoO2X complexes. These results imply that anisotropic covalency of the Mo- Scys bond in sulfite oxidase may promote preferential transfer of one of the oxo groups during catalysis.

Copper Diffusion into Lithium Niobate

Holographically recorded refractive index gratings in lithium niobate crystals are of potential use for applications in integrated optics. In this field commercially available wafers serve as the substrate material for waveguide fabrication. To increase the number of photorefractive centers [1] the wafers can be surface-doped by additionally indiffusing [2] a thin layer of a transition metal, e.g, iron. Iron-doped substrates have been used to fabricate reflection filters for infrared light in LiNbO3:Ti channel waveguides [3] for dense WDM applications or laser diode wavelength stabilisation. Also DBR waveguide lasers [4] have been realized in LiNbO3:Fe utilizing holographically recorded laser mirrors. A main disadvantage of the indiffusion of iron is its small diffusion constant even at high temperatures. As an alternative, in this contribution we investigate copper diffusion at T ˆ 1000 C into lithium niobate. The sample was prepared in the following way: An undoped polished lithium niobate wafer of congruently melting composition was cut into pieces of x0 z0 ˆ 8 10 mm2, the c-axis of the crystal pointing along the larger side. The direction of the y-axis was parallel to the top face normal. The thickness y0 of the sample was 1 mm. A 500 nm-thin layer of copper was deposited by thermal evaporation on the top face of the crystal. We successively indiffused the layer into the substrate at T ˆ 1000 C in air. The annealing steps were Dt ˆ 1 h, 1 h, 1 h, 2 h and 2 h, resulting in a total diffusion time of t ˆ 1 h, 2 h, 3 h, 5 h and 7 h. After each annealing step the c-face of the sample (orientated rectangular to the top face) was precisely polished. Then it was scanned in the y-direction by the focused electron beam of a microprobe (focus diameter < 1 mm, acceleration voltage 25 keV) to determine the distribution of copper atoms in the sample. As a reference the signals of oxygen and niobium atoms were also checked. One scan step of the beam was dy ˆ 1 mm. After 1000 steps the whole sample was scanned and the copper indiffusion was carried on with the next annealing step Dt. For a total annealing time below t ˆ 7 h the evaporated copper layer had not yet indiffused completely into the sample. An additional sharp peak on the top appeared in the copper signal during the microprobe scan. This indicates that up to this point indiffusion from an infinite source had taken place. The solution of Fick's laws results in

Esr Study of C60N- Anion Radicals in DMSO Solution

Abstract With the aim to stabilize reduced C60 molecules of solid fulleride as free anion radicals in solution, NaxC60 (x=l, 10) was dissolved in DMSO. The NaxC60 was chosen among the other alkali-metal fullerides due to the possibility to increase a number of intercalated Na atoms in a fee. lattice of C60 A sufficiently high concentration of C60 anion radicals of an order of magnitude larger than that usually achieved in solution by an electrochemical reduction was obtained. The sharp resonances with the linewidths of order of 0.1 G and with g=2.0004(6) −2.0012(8) of the room temperature ESR spectra of fluid solution were assigned from mono- to pentaanions of C60. An appearance of additional sharp peaks at g=2.0016–2.0019 in the spectra of solution of nominal Na10C60, but Na1C60, tentatively related to C60 n- with n>6. The results of this study showed a simple approach in an accomplishment of highly concentrated solutions of free anion radicals C60 n-, as well as in stabilization of the anion radicals with increased n. In this way a homogeneity of alkali metal intercalation and characteristics of the solvent will be important.

Specific features of hypersonic damping in relaxor ferroelectrics

Abstract A comparative analysis of the hypersonic damping behavior in a wide temperature range has been carried out in a relaxor ferroelectric PbMg1/3Nb2/3O3-(PMN) and in a related crystal Na1/2Bi1/2TiO3- (NBT). The damping of longitudinal acoustic phonons was obtained from Brillouin scattering data. The main very broad maxima in hypersonic damping are found to be shifted to higher temperatures with respect to the corresponding main dielectric anomalies. Additional sharp peaks in the hypersonic damping are connected with an inner structural rearrangement in both crystals. This inner evolutionary process contributes to the hypersonic damping regardless of whether there is a real phase transition like in NBT or such a phase transition is frustrated like in PMN. Comparing NBT with PMN we suggest that the dynamics of these materials is determined by two coupled order parameters.

Photoelectron spectroscopic study of SrxNbO3.

X-ray and ultraviolet photoelectron spectroscopic (XPS and UPS) data were obtained for ${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NbO}}_{3}$ (x=0.8,0.85,0.9). The valency of the Nb ion was found to be between pentavalent and tetravalent. No shifts of the XPS core-level spectra, such as O 1s, Sr 3p, and Sr 3d, were observed in this system even when x was varied. For all samples, sharp Fermi edges were observed with He I (21.2 eV) photon excitation. The results indicate that this valence band was mainly caused by the hybridization between the O 2p and Nb 4d orbitals, as had been proposed for the tungsten bronze by Goodenough. Eu substituted samples, (Sr,Eu${)}_{0.9}$${\mathrm{NbO}}_{3}$, were also studied. An additional sharp peak, due to Eu 4f state, was clearly observed in the valence-band region close to the Fermi level.

Optical spectra of ultrathin II–VI quantum wells

Ultrathin wide band gap II–VI quantum wells (QW's) feature a large inhomogeneous broadening of exciton transitions in the optical absorption and photoluminescence (PL) spectra. When the photon energy of the excitation laser approaches, or lies within the broadened PL band, a series of additional sharp peaks appear at low energy. The laser induced linear polarisation of the sharp peaks enables the clear demonstration of evenly spaced phonon-assisted transitions. The relative intensity of the phonon-assisted transitions and the mean phonon energy depend strongly on the photon energy of the exciting light. A new model of phonon-assisted relaxation between classically localized states explains these results well.

Cryopreservation of Desiccation-Sensitive Axes of Camellia sinensis in Relation to Dehydration, Freezing Rate and the Thermal Properties of Tissue Water

The inter-relationships between water content, rate of freezing to −196 °C, thermal properties of water, and survival were studied in excised embryonic axes of tea seeds. Three freezing rates were used: 10°C min −1 , about 200°C min −1 and rapid freezing (attained by plunging samples into nitrogen slush). Differential scanning calorimetry yielded three categories of melting endotherms: those without endothermic peaks (no freezable water), those with a broad endothermic peak with onset temperature varying with water content, and those with an additional sharp peak at about −2°C. When axes were cooled slowly, the sharp peak was present at high water contents (> 1.8 g H 2 O g −1 dw) and was diminished as axes were dried. The sharp peak was not apparent when axes were subjected to rapid freezing. Axes at moisture contents and subjected to freezing rates such that the sharp peak was present in the melting endotherms did not survive in tissue culture. Axes rapidly dried to water contents between 1.1 and 1.6 g H 2 O g −1 dry mass prior to rapid freezing showed 100 % survival in tissue culture. Ultrastructural studies of freeze fractured replicas showed considerable freezing damage at intermediate freezing rates, but good preservation of subcellular detail under rapid freezing conditions. It is suggested that the sharp peak observed on melting endotherms represents the melting of pure water arising from ice crystals formed during freezing. If axes with a sufficiently high water content are cooled at a freezing rate that prevents the formation of large ice crystals, damage is minimized and survival enhanced. If axes are dried to water content close to the level of non-freezable water, the additional stress of freezing is deleterious.

Crystallographic and electronic structure of ultra thin Ag/Cr/Ag(100) sandwiches

We have investigated the electronic and crystallographic structure of thin Ag/Cr/Ag(100) sandwiches using angle-resolved ultraviolet (ARUPS) and X-ray (ARXPS) photoemission. First an ordered two-dimensional (2D) Cr monolayer is formed by deposition of one layer equivalent (LE) Cr onto a clean Ag(100) single crystal kept at 440 K under UHV conditions. The relevant ARXPS curves for Cr2p12 are structureless as expected for a single monolayer. Then Ag films (from the submonolayer range to 5 LE) are deposited at RT onto this Cr monolayer. Even after deposition of 3 LE Ag, the ARUPS spectra show the persistence of Cr3d induced features characteristic of the 2D Cr monolayer. However after Ag deposition the valence band spectra also reveal an additional sharp Cr3d peak at 1.2 eV binding energy, and the Cr2p12 ARXPS curves are structured and exhibit strong forward scattering features characteristic of a face-centered cubic environment. It is concluded that only a part of the Cr monolayer is destroyed and dissolves in the deposited Ag which grows in the form of 3D islands.

Spectrum of Scattered Light by an Ionized Two-level Atom

Abstract The spectrum of scattered light by an ionized two-level atom is investigated. The ionization is considered as a damping out of the ordinary two-level system and its effect on the spectrum is extracted. Because of the ionization damping, the central peak in the spectrum changes significantly according to the parameters involved. For a large ionization damping, a central dip is observed; for a small ionization damping, an additional sharp peak appears on top of the central peak.

Low temperature specific heat of R 2Cu 2O 5 (R = Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Specific heat of rare earth cuprates R 2 Cu 2 O 5 (R = Y, Tb-Lu) was measured in the temperature range 2.0 to 30 K. Specific heat anomalies corresponding to antiferromagnetic ordering of Cu 2+ ions have been found. The Neel temperatures vary from T N =10 K (R = Dy) to T N = 25 K (R = Er). For Yb 2 Cu 2 O 5 and Dy 2 Cu 2 O 5 compounds additional sharp peaks are observed at temperatures T = 9.5 K (R = Dy) and T = 6.8 K (R = Yb). These peaks may be associated with a spin-reorientation transition.

Low temperature heat capacity of R 2Cu 2O 5 (R = Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Heat capacity of rare earth cuprates R 2Cu 2O 5 (R = Y, Tb-Lu) has been measured in the temperature range from 2.0 to 30 K. Specific heat anomalies corresponding to antiferromagnetic ordering of Cu 2+ ions are found. The Néel temperatures vary from T N = 10 K ( R = Dy) to T N = 25 K ( R = Er). For Yb 2Cu 2O 5 and Dy 2Cu 2O 5 compounds, additional sharp peaks are observed at temperatures T = 9.5 K ( R = Dy) and T = 6.8 K ( R = Yb). These peaks may be associated with a spin-reorientation transition. Samples with Ho, Tm, Tb and Yb reveal Schottky anomalies due to crystal field splitting of rare earth ions' ground multiplet.

High quality zinc sulfide epitaxial layers grown on (100) silicon by molecular beam epitaxy

ZnS films were successfully grown by molecular beam epitaxy on (100) Si substrates. These high quality films, grown at a substrate temperature of 340 °C with a molecular beam flux ratio of Zn to S of unity, exhibited high crystallographic quality and revealed a flat surface. Secondary ion mass spectroscopic analysis showed that the film contained little impurity. In photoluminescence, a broad peak was observed at 2.6 eV at room temperature while an additional sharp peak at 3.6 eV appeared at 77 K. The latter is considered to be band-to-band emission and the former may be due to the Cu-blue emission.

Novel spin-flop effects in FexMn1−xF2

High-pulsed-field magnetization measurements in the randomly mixed antiferromagnet FexMn1−xF2, with 0.15≤x≤0.40, have revealed an extremely broad spin-flop transition, with a width of ΔH≂60 kOe. An unusual square shape, with rounded edges, is found in dM/dH, with an asymmetry which depends on x. For the largest values of x, an additional sharp peak is also seen. A four-sublattice molecular-field model has been used to interpret the results, and predicts a number of interesting effects. For 0.01≲x≲0.10, the usual first-order spin-flop is suppressed, and a new canted-intermedi ate (CI) phase appears, in which the spins rotate continuously from the antiferromagnetic (AF) to spin-flop (SF) phase. For 0.10≲x≲0.6, the CI–SF transition becomes first order, and for x≳0.6 the CI phase disappears, leaving a relatively normal first-order spin-flop. The model shows that small misalignments cause considerable broadening and rounding of the transition, and convincingly reproduces the experimental shapes and widths.

Experimental evidence for impulsive energy transfer in electronically adiabatic low-energy collison of Na + with CF 4

Time-of-flight measurement of the inelastic scattering of Na + ions from CF 4 reveal two energy-loss mechanisms which have been observed at center-of-mass collison energies between 4 and 24 eV. At small angles θ 1ab≲20° a single energy transfer peak is observed and attributed to normal mode excitation. At larger angles an additional sharp peak is observed out to θ lab = 70° with nearly double the energy transfer. the final velocities and angular dependence agree with an impulsive interaction involving only one of the F atoms with an effective mass which approaches that of the F atoms with increasing collison energy. This is the first evidence for an impulsive mechanism in inelastic collisons at energies below the threshold for electonic excitation.

Simultaneous excitation of rotations and surface phonons in the scattering of D 2 from a NaF crystal

Abstract Angular distributions and time-of-flight spectra have been measured for the scattering of n-D 2 from NaF(001) along the 〈100〉 direction at energies between 30 and 90 meV. The angular distributions reveal, in addition to the elastic diffraction peaks, additional sharp peaks due to the rotational transitions 0→2,1→3, and 0→4. The relative rotational transition probabilities show an anomalous behavior which can in part be explained by a kinematic rainbow. An enhancement of rotational transitions in grazing collisions (large θ i ) is also observed. The time of flight spectra confirm the assignments and reveal that phonon processes also are occurring simultaneously with the rotational transitions.

Simultaneous excitation of rotations and surface phonons in the scattering of D 2 from a NaF crystal

Angular distributions and time-of-flight spectra have been measured for the scattering of n-D 2 from NaF(001) along the 〈100〉 direction at energies between 30 and 90 meV. The angular distributions reveal, in addition to the elastic diffraction peaks, additional sharp peaks due to the rotational transitions 0→2,1→3, and 0→4. The relative rotational transition probabilities show an anomalous behavior which can in part be explained by a kinematic rainbow. An enhancement of rotational transitions in grazing collisions (large θ i) is also observed. The time of flight spectra confirm the assignments and reveal that phonon processes also are occurring simultaneously with the rotational transitions.

Anisotropic exciton polaritons

Some aspects of exciton polariton behaviour in anisotropic crystals are investigated for a specific model of the response function of an exciton split by a uniaxial field. The extraordinary-polariton dispersion scheme shows an additional branch for propagation in a nonprincipal direction appearing in the anomalous dispersion gap of the cubic case. The reflectivity spectrum obtained for this case is in good agreement with measurements performed on the basal plane of the uniaxial crystal PbI2 under oblique incidence. For a consideration of spatial-dispersion effects (finite exciton mass) the theory of Hopfield and Thomas is extended to uniaxial crystals. It is found that three simultaneous waves exist for every frequency and that a sharp extra reflection peak appears for every long-wavelength polariton mode of nonzero frequency. Reflection curves are calculated for different values of the exciton massm and the thicknesst of the Hopfield layer. The experimental normal-incidence «extraordinary” spectrum obtained by Biellmannet al. on the $$(10\bar 11)$$ of PbI2 which shows two additional sharp peaks can be reproduced by appropriate choice of the parametersm andt.

Magnetic Structure of Manganese Chromite

An investigation of the magnetic structure and properties of Mn${\mathrm{Cr}}_{2}$${\mathrm{O}}_{4}$ has been carried out by means of neutron diffraction and an evaluation of the suitability of various proposed theoretical models has been made. From room-temperature diffraction patterns it is established that Mn${\mathrm{Cr}}_{2}$${\mathrm{O}}_{4}$ is a normal spinel with a $u$ parameter of 0.3892\ifmmode\pm\else\textpm\fi{}0.0005 and with less than one percent of the ${\mathrm{Mn}}^{++}$ ions present on $B$ sites. The Curie temperature, as determined from diffraction data, is \ensuremath{\sim}43\ifmmode^\circ\else\textdegree\fi{}K. Below this temperature the magnetic contribution to the fundamental spinel peaks arising from aligned spins increases as the temperature is lowered and is effectively saturated at about 20\ifmmode^\circ\else\textdegree\fi{}K. At 18\ifmmode^\circ\else\textdegree\fi{}K, additional sharp peaks appear at positions which cannot be indexed either on the original unit cell or on any reasonably enlarged cell. These extra reflections persist with unchanged intensities down to 4.2\ifmmode^\circ\else\textdegree\fi{}K. No change in either the positions or intensities of the fundamental lines is observed in going through the transition. Above 18\ifmmode^\circ\else\textdegree\fi{}K, a diffuse peak is present in the region where the principal extra lines develop. This diffuse peak decreases with increasing temperature, but is still observable above the Curie point. Application of a magnetic field along the neutron scattering vector decreases the magnetic contributions to the fundamentals, but increases the intensities of the extra reflections.The N\'eel model and the Yafet-Kittel model fail to account for major qualitative features of the diffraction results. On the other hand, the ferrimagnetic spiral model of Lyons, Kaplan, Dwight, and Menyuk is in good qualitative accord with all the observations. A detailed comparison with the theoretical predictions is presented and attention is called to broad areas of agreement as well as to certain discrepancies. The general conclusion reached is that the spiral model is a good first approximation to the ground-state spin configuration.

Magnetic Structure of Manganese Chromite

Numerous attempts have been made to explain the anomalous magnetic properties of chromites and of mixed spinels containing chromium on the basis of the theoretical models of Néel and of Yafet and Kittel. Recent calculations by T. A. Kaplan and co-workers, using a classical Heisenberg model, have shown that over an appreciable range of the ratio of the BB to the AB interaction, the ground state is a ferrimagnetic spiral, i.e., a conical spin configuration in which the transverse components progress in spiral fashion along a fixed direction in the crystal. The object of the present study is to determine the suitability of the various proposed theoretical models in the case of manganese chromite, for which rather complex magnetic behavior has been observed by means of neutron diffraction. From diffraction patterns taken at room temperature, it is readily established that MnCr2O4 is a normal spinel with less than 1% of the Mn2+ ions present on B sites. The Curie temperature, as determined from diffraction data is ∼43°K. Below this temperature the magnetic contributions to the fundamental spinel peaks arising from aligned spins increases as the temperature is lowered and is effectively saturated at about 20°K. At 18°K, additional sharp peaks appear at positions which cannot be indexed either on the original unit cell or on any reasonably enlarged cell. These extra reflections persist with unchanged intensities down to 4.2°K. No change in either the positions or intensities of the fundamental lines is observed in going through the transition. Above 18°K a broad, diffuse peak is present in the region where the principal extra lines develop. This diffuse peak decreases with increasing temperature, but is still observable above the Curie point. Application of a magnetic field along the neutron scattering vector decreases the magnetic contributions to the fundamentals, but increases the intensities of the extra reflections. Analysis of the data shows that while little difficulty is encountered in explaining the observed saturation moments, the Néel model and the Yafet—Kittel model fail to account for major qualitative features of the diffraction results. Agreement is not significantly improved by modifications of the Néel model in which reversed spins (either random or ordered) are introduced, or in which some of the spins remain paramagnetic even at low temperatures. On the other hand, all the qualitative aspects of the diffraction results can be explained in terms of the ferrimagnetic spiral model of Kaplan and co-workers. The theoretical calculations give reasonably good quantitative agreement with the positions and intensities of the extra lines as well as the intensities of the fundamentals, while retaining the expected values for the individual ionic moments and the macroscopic saturation moment. Although some discrepancies still remain, there is every reason to believe that these can be removed by further refinement of the theory and by the extension of experimental measurements to single crystals.