Change in the states of optically excited Rb atoms near sapphire surface

The changes in the states of excited Rb atoms approaching a single-crystal sapphire surface have been investigated by methods of laser-excitation spectroscopy and luminescence of Rb vapor in a cell with sapphire windows, the gap between which was varied from 250 to 500 nm. Upon resonant excitation of Rb atoms by two semiconductor lasers with powers of 20 and 40 mW, luminescence from optically excited 5D _3/2 and 5D _5/2 states and optically unexcited 6P _1/2 and 6P _3/2 states is observed. It is established that the luminescence intensity from unexcited states is only a few times lower than that from excited states, with allowance for the fact that excited atoms are rapidly and almost completely quenched on the sapphire surface. The found anomalously strong luminescence from optically unexcited 6P _J states is explained by their nonradiative occupation near the sapphire surface from optically excited 5P _J states, in which atoms fail to reach the sapphire surface because of the repulsion from it. This repulsion is due to the polarization interaction between sapphire and the atoms in the 5P _J states near the surface. Nonradiative transition from the 5P _J state to the 6P _J ^?1 state is accompanied by excitation of two optical phonons in sapphire.     

Laser probing of OH production in catalytic oxidation of NH3byNO2 and no over a polycrystalline Pt surface at high temperatures

Catalytic ammonia denox reactions over a polycrystalline Pt surface in the temperature range 800–1330 K have been investigated by laser-induced-flourescence detection of the OH radical produced on and subsequently desorbed from the surface. For the NO₂-NH₃ system, strong curvature is observed in the low temperature region of the Arrhenius plots, which could be attributed to the presence of different desorption processes. The two extracted apparent OH desorption energies increase from 30 to 49 kcal/mol and from a surprisingly low value of 2 to 27 kcal/mol as the NO₂NH₃ reactant mixture ratio is decreased from 70 to 0.037. On the other hand, for the NO-NH₃ system, no such curvature in the Arrhenius plots is observed and the OH desorption energy decreases from 38 to 26 kcal/mol as the NO/NH₃ ratio is decreased from 70 to 0.4. The different trends in the apparent OH desorption energies as a function of the NO_x/NH₃ ratio in the present two systems can be rationalized by the degree of coverage of chemisorbed O atoms relative to that in the earlier studied O₂-NH₃ system.     

Wavefunction symmetries and binding energies in GaSxSe1−x compounds

We report on a synchrotron radiation photoemission spectroscopy study of mixed GaS_xSe_1?x crystals. The most prominent composition effect in the valence band is an increase by ～ 0.4 eV of the energy separation between the p_z states and the p_x, p_y states as x increases from 0 to 1. The repulsion in energy is accompanied by a progressive mixing of these states in k-space regions far from the zone boundaries.     

Exciton spectra, valence band splitting, and energy band structure of CuGaXIn1−XS2 and CuGaXIn1−XSe2 crystals

Exciton spectra are studied in CuGa_XIn_1−XS₂ solid solutions by means of photoreflectivity and wavelength modulation spectroscopy at liquid nitrogen temperature. The exciton parameters, dielectric constants, and free carrier effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The crystal field and spin orbit valence band splitting is calculated as a function of X taking into account the energy position of excitonic lines. The energy band structure of CuGa_XIn_1−XS₂ and CuGa_XIn_1−XSe₂ compounds is derived from optical spectra at photon energies higher than the fundamental band gap. The energies of optical transitions are tabulated for X values from 0 to 1.     

Enhancement of Jc by Hf-doping in the superconductor MgB2: A hyperfine interaction study

Measurements of the critical current density (J_c) by magnetisation and the upper critical field (H_c2) by magnetoresistance have been performed for hafnium-doped MgB₂. There has been a remarkable enhancement of J_c as compared to that by ion irradiation without any appreciable decrease in T_c, which is beneficial from the point of view of applications. The irreversibility line extracted from J_c shows an upward shift. In addition, there has been an increase in the upper critical field which indicates that Hf partially substitutes for Mg. Hyperfine interaction parameters obtained from time differential perturbed angular correlation (TDPAC) measurements revealed the formation of HfB and HfB₂ phases along with the substitution of Hf. A possible explanation is given for the role of these species in the enhancement of J_c in MgB₂ superconductor.     

Elastic photoelectron-scattering effects in quantitative X-ray photoelectron spectroscopy

We present improved formulae for the correction parameters Q_x and β_eff that are used to account for elastic scattering of photoelectrons in quantitative X-ray photoelectron spectroscopy (XPS). The new formulae are based on new Monte Carlo simulations for 584 photoelectrons in 39 elemental solids that could be excited by Mg Kα and Al Kα X-rays in 315 different XPS configurations. The new simulations differed from similar earlier calculations in that differential elastic-scattering cross sections calculated from the Dirac–Hartree–Fock potential were utilized rather than those from the Thomas–Fermi–Dirac potential, a smaller analyzer acceptance angle was chosen, and the number of trajectories in each simulation was an order of magnitude larger. New values of Q_x and β_eff were obtained for each photoelectron line, each X-ray source, and each XPS configuration. These Q_x and β_eff values could be fitted to simple two-parameter expressions, each a function of the single-scattering albedo and the photoelectron emission angle. Values of Q_x from the new predictive formula differed from the previous expression by less than 1%. Larger deviations in the values of β_eff, up to 2.5%, were found from the new fit to the β_eff parameter. The new expressions for Q_x and β_eff provide a convenient means for correction of elastic-scattering effects in XPS.     

Intracellular potential relief and size effects in the lattice of a LaF3 superionic conductor

The potential relief in the lattice of a LaF₃ crystal is calculated by quantum-mechanical methods for clusters containing from 24 to 1200 ions. For the dielectric phase, formation energy E _a for defects of the vacancy-interstitial fluorine ion type and potential barrier E _d preventing the motion of fluorine ions are found to grow from minimal values E _a = 0.12 eV and E _d = 0.22 eV for a cluster of 24 ions to maximal values E _a = 0.16 eV and E _d = 0.26 eV for clusters of 576 and 1200 ions. The values of E _a and E _d obtained in quasi-mechanical calculations are in good agreement with those obtained from Raman and quasi-elastic light scattering data.     

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Measurements of Raman scattering were performed on GaAs-In_xGa_1?xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ～ 250 Å and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-Al_xGa_1?xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk In_xGa_1?xAs alloys as the ratio of the one-layer thickness of In_xGa_1?xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.     

The residual interaction of bound nucleons-two-nucleon matrix elements deduced from transfer experiments

Matrix elements for the effective two-nucleon interaction have been deduced from the population of multiplets near closed shells as observed in direct transfer reactions. In the evaluation, the limited purity of such multiples was taken into consideration, typically by weighting the observed fractions of the two-nucleon configurations by their spectroscopic strenghts and by using the resulting energy centroids. In a few cases, off-diagonal matrix elements are available from empirical wave funcitons. The systematic errors for particle-particle matrix elements extracted directly and those obtained from Pandya transformations were found to go in opposite directions. In some cases, this feautre of the empirical mehtod could be used to suggest upper and lower “bounds” for the extracted matrix elements. Diagonal matrix elements for the empirical residual interaction show a number of features suggestive of an underlying simplicity in the interaction of bound nucleons. Within experimental uncertainties (of about 10% for T=0 matrix elements) the monopole parts of the matrix elements are fit well with a simple A?0.75 dependence, and the data available to date do not reveal any significant monopole dependence on the quantum numbers of the interacting nucleons. The usefulness of scaling is suggested. Generally, diagonal matrix elements E_J(j₁, j₂) normalized by the extracted A-dependent monopole strength agree within expected experimental uncertainties whether derived from particle-particle or particle-hole multiples and whether extracted from the beginning or the end of a major shell. For values J≠0, the diagonal E_J(j²) matrix elements seem to follow two universal functions which depend on the semi-classical coupling angles θ₁₂, but are otherwise independent on j. For j₁≠j₂ several “typical” functions ?(θ₁₂) can be constructed which fit subsets of the data and differ in a predictable way. The general features of the bound-nucleon interaction appear consistent with those of theoretical matrix elements based on a number of short-range model forces or on calculations using the G matrix approach to deal with realistic free nucleon forces. For the latter, the available theoretical numbers for j₁=j₂ agree well with the T=1 set, but they differ quantitatively from the observed matrix elements for T=0, sometimes by many (experimental) standard deviations.     

Low-field irreversibility line and its anisotropy of textured (Bi,Pb)2Sr2Ca2Cu3Oy silver-clad tapes

The irreversibility line H_irr(T) of textured (Bi,Pb)₂Sr₂Ca₂Cu₃O_y silver-clad tapes was obtained by ac susceptibility (ACS) and magnetoresistivity (MR) below 1500 G. It is found that, in the framework of flux motion, the H_irr(T) is determined by the thermally activated flux flow (TAFF) rather than the so-called giant flux creep, and the experimentally obtained H_irr(T) line is related to a certain resistivity criterion in both ACS and resistivity measurements. H_irr(T) is found to be significantly depressed by the amplitude of applied driving AC field and generally lower than that measured from the resistivity. The anisotropy in H_irr(T) is found to follow a scaling behaviour as described by the anisotropic effective mass model.     

A test of the rm method of structure determination

The r_m method of determining molecular structures from isotopic zero-point rotational constants is tested by applying it to synthetic data for OCS calculated from an assumed equilibrium structure and force field. The results show that the r_m structure is significantly different from the r_e structure, unless the substitution coordinates used in the r_m calculation are corrected for the use of finite changes of mass Δm_i by extrapolation to Δm_i = 0. The latter procedure is not usually feasible experimentally. In the ordinary r_m method the errors are generally comparable in magnitude to those in Costain's r_g method. An exception arises when the number of structural parameters equals the number of independent moments of inertia of a single isotope (e.g., a diatomic molecule or a symmetrical XY₂ molecule), in which case the r_m structure is in good agreement wich the r_e structure.     

Oxygen transport properties of dense and porous (La0.8Sr0.2)0.99Co0.8Ni0.2O3-δ

We have determined k_ex and D_chem for (La_0.8Sr_0.2)_0.99Co_0.8Ni_0.2O_3-δ by the use of electrical conductivity relaxation on a dense sample and by applying the ALS model to measured AC impedance spectrum on a porous electrode. Extracting k_ex and D_chem from the methods resulted in comparable values. k_ex and D_chem also agreed well with literature values on La_0.8Sr_0.2CoO_3-δ, indicating that nickel substitution does not change the oxygen transport properties. k_ex of the porous sample was further found to decrease with a five times higher rate than D_chem when measured by using an Electrochemical Impedance Spectroscopy (EIS) over several days.     

Investigation of proton diffusion in Nafion®117 membrane by electrical conductivity and NMR

The proton dynamics in Nafion ^®117 is investigated by comparison of the diffusion coefficient D_NMR estimated from PFG-NMR with that of D_σ estimated from electrical conductivity. At high water content region, D_σ is about two times higher than D_NMR as a result of Grotthuss mechanism. At low water content region, D_σ and D_NMR are in good agreement with each other. Both of the diffusion coefficients decrease steeply at low water content region. It can be explained as a result of the percolation transition due to the isolation of ion clusters, which is suggested by the recent small angle X-ray scattering data.     

Development of mechanoluminescence technique for impact studies

A new technique called, mechanoluminescence technique, is developed for measuring the parameters of impact. This technique is based on the phenomenon of mechanoluminescence (ML), in which light emission takes place during any mechanical action on solids. When a small solid ball makes an impact on the mechanoluminescent thin film coated on a solid, then initially the elastico ML (EML) intensity increases with time, attains a maximum value I_m at a particular time t_m, and later on it decreases with time. The contact time T_c of ball, can be determined from the relation T_c=2t_c, where t_c is the time at which the EML emission due to compression of the sample becomes negligible. The area from where the EML emission occurs can be taken as the contact area A_c. The maximum compression h is given by h=A_c/(πr), where r is the radius of the impacting ball, and thus, h can be determined from the known values of A_c and r. The maximum force at contact is given by F_m=(2mU₀)/T_c, where m is the mass of the impacting ball and U₀ is the velocity of the ball at impact. The maximum impact stress σ_m can be obtained from the relation, σ_m=F_m/A_c=(2mU₀)/(T_cA_c). Thus, ML provides a real-time technique for determining the impact parameters such as T_c, A_c, h, F_m and σ_m. Using the ML technique, the impact parameters of the SrAl₂O₄:Eu film and ZnS:Mn coating are determined. The ML technique can be used to determine the impact parameters in the elastic region and plastic region as well as fracture. ML can also be used to determine the impact parameters for the collision between solid and liquid, if the mechanoluminescent material is coated on the surface of the solid. The measurement of fracto ML in microsecond and nanosecond range may provide a tool for studying the fragmentations in solids by the impact. Using the fast camera the contact area and the depth of compression can be determined for different intervals of time.     

Electron spin-lattice relaxation of Yb3+ and Gd3+ ions in glasses

The electron spin-lattice relaxation rate (T ₁ ^?1) was measured in two glass samples: (i) a phosphate glass doped with 1 wt% Yb₂O₃ and (ii) a Li₂Si₄O₉ glass sample doped with 0.2 wt% Gd₂O₃. In the Yb³⁺-doped glass sample,T ₁ was measured by an electron-spin-echo technique from 4.2 to 6 K, by the modulation method from 10 to 26 K and by the EPR linewidth from 30 to 100 K. It was found thatT ₁ ^?1 αT ⁿ withn=9 in the range 4.2–6 K.n decreased gradually as the temperature was increased and tended towards 2 above 40 K. Over the entire temperature range 4.2–100 K,T ₁ ^?1 was fitted toAT+BT ⁹ J ₈ (Θ _D/T) (whereA andB are two temperature-independent constants,J ₈ is the well-known Van Vleck integral andΘ _D is the Debye temperature). The value ofΘ _D (=46.3±0.9 K) so determined is in good agreement with that of Stevens and Stapleton from theirT ₁ measurements in the range 1.5 to 7 K. In the Gd³⁺-doped glass, it was observed thatT ₁ ^?1 αT over the range 50–150 K. The data for Ye³⁺-doped glass sample were accounted for by assuming that the phonon modulation of the ligand field is the dominant mechanism, associated with a low Debye temperature in accordance with the published data obtained by using other techniques to study lattice dynamics. On the other hand, the data on the Gd³⁺-doped glass sample were explained to be predominantly due to a mechanism involving Two-Level-Systems (TLS)     

“Crystal fields” for magnetic ions in metals from itinerant electrons

The interaction of conduction electrons with host ions are shown to give rise to “crystal fields”, V_C, which act on the magnetic ion in some dilute magnetic alloys. Crystal fields, V_N, arising directly from ions neighbouring the magnetic ion have been compared with V_C to show that the overall crystal field splitting from all 3d magnetic ions in axial symmetries is enhanced by the p-like character of V_C. A⁰₄〈r⁴〉 from V_N is also increased by the d-component of V_C. The angular properties of itinerant electron states give rise to the above results. For all rare-earth ions in fcc symmetry the radial and angular components of conduction-electron states with mainly d, but also f character, are shown to give rise to a reversal in sign of A⁰₄〈r⁴〉 from V_N. A⁰₆〈r⁶〉 is enhanced by the f component of the conduction electrons.     

Subjective diffuseness of music signals convolved with binaural impulse responses

The spatial impression of sound in a hall can be quantified using sound field factors such as the interaural cross-correlation coefficient (IACC) calculated from binaural impulse response (BIR), henceforth denoted by IACC_IR. The subjective diffuseness for the listener is a spatial attribute which depends on factors associated both with the source signal and with the actual sound field, and is quantified using the IACC of the signal received by the listener, henceforth denoted by IACC_SR. Therefore, the subjective diffuseness in a given hall may change with the music. The aims of this study are to estimate the IACC_SR from the IACC_IR and the factors, which is obtained from autocorrelation function (ACF) of music signal, and to evaluate the subjective diffuseness by these factors. First, the relationship between the IACC_IR and IACC_SR was investigated. Second, subjective diffuseness was measured by a psycho-acoustical experiment. As a result, the IACC_SR could be estimated from the IACC_IR of the BIR and the effective duration (τ_e) from the ACF of music signal. It was found that the effects of BIRs on subjective diffuseness could be evaluated by IACC_IR for almost all subjects, while the effects of music signals could be evaluated by the τ_e and the width of the peak at τ=0 (W_?(0)) of the ACF.     

Temperature dependence of the diffuse streak system in K2SnCl6

We have investigated the system of diffuse streaks in K₂SnCl₆ single crystals in the temperature interval from 4 to 640 K by the photographic registration of the scattered neutron intensity. The streak intensity decreases from T_{c¹ = 261 K} with increasing as well as with In the region of the phase transition at T_{c¹ = 261 K} and T_{c² = 255 K} the intensity decreases with falling temperature to one third. The high temperature behaviour is explained by a soft A_2g librational mode. Below T_{c² = 255 K} the intensity is proportional to ΔT. The streaks are detectable down to 230 K.     

On the systematic bias in the estimation of black hole masses in active galactic nuclei

In this report,we find the MBH estimated from the formalism of Wang et al.are more consistent with those from the MBH-σ relation than those from previous single-epoch mass estimators,using a large sample of AGNs.Furthermore,we examine the diferences between the line widths of Hβ and Mg Ⅱ in detail by comparing their line profiles.The flux around the line core and that in the wing of both Hβ and Mg Ⅱ show an opposite variation tendency,which indicates the BLR is multi-componential.The contribution of the wing makes the FWHM deviate fromσline,and thus bias the MBH estimated from previous single-epoch mass estimators.Thus the correction on the formalism suggested by Wang et al.is crucial to MBH estimation.     

Stability of the long-period stacking order in Mg3ln AND Mg3(In1−yCdy) alloys

The stability of the stacking ordered structures of close-packed layers in Mg-In alloys near Mg₃In and pseudo-binary alloys Mg₃(In_1?yCd_y) for 0 ? y ? 1 is studied from a standpoint based on the pseudopotential theory. An expression of the structure-dependent energy for an arbitrary type of layer stacking is given by the method described in the previous papers [1,2]. The numerical results explain well the observed trends in the composition and pressure dependences of stacking sequence; the orders of appearance of stacking variants are 3R → 12R → 2H with an increasing Mg-content for Mg-In alloys near Mg₃In, 12R → 18R → 2H with an increasing Cd-content for Mg₃(In_1?yCd_y) and 12R → 18R → 24R with an increasing pressure for Mg₃ln. Speaking in detail, however, the present calculation fails to reproduce a 12R-structure (β₁-phase) at 25% In in the Mg-In system, an 18R-structure of Mg₃(In_0.65Cd_0.35) and an 18R-structure of Mg₃In found under pressure of 20–55 kb, although the energy differences are small in all the cases. From the analysis of components of the structure-dependent energy, it is concluded that the favorable type of layer stacking is determined predominantly by a contribution from the band-structure energy term of a state with the disordered arrangement of constituent atoms and also that an energy required to the alternation of stacking of layers is apparently smaller than the ordering energy. If a specific layer sequence is characterized by a hexagonality, the alloys vary their sequences in such a way that the hexagonality increases from zero (3R) to unity (2H) as the electron-to-atom ratio of the alloys decreases from 2·3 to 2·0.The Ewald and Madelung constants are calculated for various types of layer structure. Both constants change linearly with hexagonality, without regard to period, symmetry and layer sequence.