Mode selectivity in ion–molecule reactions of NH3+

Ammonia ions (NH₃ ⁺) are prepared in two internal states of nearly the same energy, a state with five quanta in the umbrella bending mode (E_int=0.60 eV) and another state with one quantum in the all-symmetric stretch and two quanta in the umbrella bending mode (E_int=0.63 eV). These ions are allowed to react with different neutral reagents, and the product ions are mass analyzed and detected. For each reaction, the product branching ratios are measured as a function of center-of-mass collision energy. Whereas reactions with D₂O, D₂, and CD₄ are found to be uninfluenced by the state preparation of the NH₃ ⁺ reagent, reactions with ND₃, partially deuterated methylamine (CD₃NH₂), and tetrahydrofuran (c-(CH₂)₄O)show varying degrees of mode selectivity, the reaction with tetrahydrofuran to the least extent. We suggest that mode selectivity in these ion–molecule reactions should be a general feature when the charge transfer channel is energetically open, the geometry of the reagent ion differs markedly from the corresponding neutral, and different reaction pathways compete for product production. Received: 14 December 1999 / Published online: 16 August 2000     

Temperature dependence of the spectral weight in p- and n-type cuprates: A study of normal state partial gaps and electronic kinetic energy

The optical conductivity of CuO₂ (copper-oxygen) planes in p- and n-type cuprates thin films at various doping levels is deduced from highly accurate reflectivity data. The temperature dependence of the real part σ₁ (ω) of this optical conductivity and the corresponding spectral weight allow to track the opening of a partial gap in the normal state of n-type Pr_2−xCe_xCuO₄ (PCCO) but not of p-type Bi₂Sr₂CaCu₂O_8+δ (BSCCO) cuprates. This is a clear difference between these two families of cuprates, which we briefly discuss. In BSCCO, the change of the electronic kinetic energy E_kin—deduced from the spectral weight—at the superconducting transition is found to cross over from a conventional BCS behavior (increase of E_kin below T_c) to an unconventional behavior (decrease of E_kin below T_c) as the free carrier density decreases. This behavior appears to be linked to the energy scale over which spectral weight is lost and goes into the superfluid condensate, hence may be related to Mott physics.     

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO₃, congruent Cr:LiNbO₃ and congruent Cr,Mg:LiNbO₃ were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr³⁺ impurity ions that preferentially occupy Li⁺ sites (Cr_Li) in the Cr:LiNbO₃ crystals. Low-field centers related to Cr³⁺ substitution of Nb⁵⁺ (Cr_Nb) occur in addition to Cr_Li in co-doped Cr,Mg:LiNbO₃ samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr³⁺ centers from low- to high-field type due to strong pressure-induced blue shift of the ⁴ T ₂ state, resulting in its crossing with the ² E state of Cr³⁺. This level-crossing effect was observed for the dominant Cr³⁺ _Li and Cr³⁺ _Nb centers at pressures that correlate well with estimations based on the ⁴ T ₂-² Eenergy gap (230 cm^-1 and 1160 cm^-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^-1/kbar, respectively). We also studied inhomogeneous broadeningof the ² E?⁴ A ₂transitions at ambient pressure for the minor high-field “defect” Cr³⁺ _Li centers in congruent LiNbO₃. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr³⁺ ions in the LiNbO₃ lattice. Received: 25 June 2001 / Published online: 2 November 2001     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

Dielectric spectra of LiTFSI-doped chitosan/PEO blends

Solid-polymer-blend electrolyte consisting of chitosan and polyethylene oxide (PEO) in a 1:1 weight ratio and doped with lithium trifluoromethanesulfonimide (LiTFSI) salt was prepared by solution cast technique. The highest conducting film with conductivity value of 1.40 × 10^-6 S cm⁻¹ at room temperature consists of 30 wt% LiTFSI. The temperature dependence for the highest conducting film obeyed Arrhenius relationship. From loss tangent–frequency plots at different temperatures, the frequency f _max at which the plot is a maximum was obtained. From this, ln f _max vs 10³/T was plotted. The activation energy value obtained from the log σ vs 10³/T plot and ln f _max vs 10³/T plot is about the same, suggesting that the processes of conductivity and relaxation for the charge carriers are the same. This paper was presented at the International Conference on Solid State Science and Technology 2006, Kuala Terengganu, Malaysia, Sept. 4–6, 2006.     

Ferroelectric phase stabilization, phase transformations and?thermal properties in (1? x )PbZrO3– x Pb(Co1/3Nb2/3)O3 solid solution

The solid solution between the antiferroelectric PbZrO₃ (PZ) and relaxor ferroelectric Pb(Co_1/3Nb_2/3)O₃ (PCoN) was synthesized by the columbite method. The phase structure and thermal properties of (1−x)PZ–xPCoN, where x=0.0–0.3, were investigated. With these data, the ferroelectric phase diagram between PZ and PCoN has been established. The crystal structure data obtained from XRD indicates that the solid solution PZ–PCoN, where x=0.0–0.3, successively transforms from orthorhombic to rhombohedral symmetry with an increase in PCoN concentration. The AFE→FE phase transition was found in the compositions of 0.0≤x≤0.10. The AFE→FE phase transition shift to lower temperatures with higher compositions of x. The width of the temperature range of FE phase was increased with increasing amount of PCoN. It is apparent that the replacement of the Zr⁴⁺ ion by (Co_1/3Nb_2/3)⁴⁺ ions would decrease the driving force for antiparallel shift of Pb²⁺ ions, because they interrupt the translational symmetry. This interruption caused the appearance of a rhombohedral ferroelectric phase when the amount of PCoN was more than 10 mol%.     

Proton conduction in (NH4)3 H(SO4)2 single crystals

D.C. electrical conductivity, DTA and coulometric studies on (NH₄)₃ H(SO₄)₂ single crystals are made. Conductivity is markedly anisotropic with maximum along c¹ direction. A sudden jump in the conductivity plot along c¹ direction at 413 K is supported by a large endothermic peak in DTA, confirming the presence of transition at this temperature. The values of activation energy calculated from conductivity measurements indicated that the charge carriers are protons. This was further confirmed by coulometric experiment where the gas evolved was hydrogen, as established by a gas chromatograph and the volume of H₂ released agreed with that expected from electrolysis. The mechanism of protonic conduction in this crystal is discussed.     

Femtosecond photodissociation dynamics of HNO3 after exitation of the S3-state at 200 nm

The photodissociation dynamics of HNO₃ in the electronic S₃ (2 ¹ A ^′ ) state leading to the fragments OH and NO₂ was investigated in real time. HNO₃ was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO₂ by three-photon ionization). A 2 ¹ A ^′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO₂ ^* leading to products NO(X ² Π) and O(³ P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO₂ from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO₂. Received: 12 November 1999 / Published online: 13 July 2000     

Enhanced conductivity in ionic conductor-insulator composites: numerical models in two and three dimensions

We describe a two-dimensional (2D) and a three-dimensional (3D) percolation model for ionic conductor-insulator composites such as copper(I) bromide-titanium dioxide (CuBr-TiO₂) or lithium iodide-alumina (LiI-Al₂O₃). These composites present an enhanced conductivity closely related to the insulator concentration. This effect is explained by the formation of highly conducting space charge regions near the phase boundaries which are represented by good conductor bonds. Our numerical model takes into account grain size and correlation effects. The dimension has a leading role for the conduction properties. In the 2D case, the good conductor bonds do not percolate, whatever the insulator concentration, and the maximum conductivity of the composite samples is of the same order as that of the ionic conductor grains. The behavior of the system is very different in the 3D case where, for a large domain of composition, the good conductors percolate through the regions between the conductor grains. For the CuBr-TiO₂ composites the conductivity versus composition curve is bell-shaped. Conversely, in the LiI-Al₂O₃ system, a linear relation between the conductivity and the insulator volume fraction is obtained in the experiments. Our model gives a plausible interpretation of the conductivity in both systems. Received 10 April 2001     

On the visibility of electron-electron interaction effects in field emission spectra

One of the most convenient methods to obtain information about the energy distribution function of electrons in conducting materials is the measurement of the energy resolved current j(ω) in field emission (FE) experiments. Its high energy tail j_>(ω) (above the Fermi edge) contains invaluable information about the nature of the electron-electron interactions inside the emitter. Thus far, j_>(ω) has been calculated to second order in the tunnelling probability, and it turns out to be divergent toward the Fermi edge for a wide variety of emitters. The extraction of the correlation properties from real experiments can potentially be obscured by the eventually more divergent contributions of higher orders as well as by thermal smearing around E_F. We present an analysis of both factors and make predictions for the energy window where only the second order tunnelling events dominate the behaviour of j_>(ω). We apply our results to the FE from Luttinger liquids and single-wall carbon nanotubes.     

K_(0.8)Fe_2Se_2晶体c轴向载流子输运特性的研究

成功制备了超导临界温度为27 K的K0.8Fe2Se2晶体,并详细研究了晶体c轴向的载流子输运特性.结合X射线衍射、光学显微镜下的形貌、变温电阻率的测试结果表明,样品存在有"相分离",但是这类层状铁基超导体材料的两个相不是简单沿c轴向层状交替排布的,而应该是沿着c轴向存在弱联系的金属相链接通路,金属相部分形成近3维的空间网状链接模式.热导率测试和复阻抗谱z(ω,T0)的研究表明超导晶体沿着c轴方向存在有大量的相界面,所束缚的极化电荷致使相对介电常数达到106数量级,相应地在10 MHz附近出现负的相位特征.     

Hydrogen-induced-intrinsic transport in undoped microcrystalline silicon

It is demonstrated that microcrystalline silicon (μc-Si:H) of intrinsic character can be produced by post-growth atomic hydrogen treatment. Undoped μc-Si:H films with a dark-conductivity activation energy (E _a) of about 0.20 eV were grown by plasma-enhanced chemical vapor deposition, and then subsequently exposed to an atomic hydrogen plasma. The hydrogen treatment is shown to result in a gradual increase in the E _a with increasing treatment time, followed by saturation at about 0.57 eV, a value observed for truly intrinsic μc-Si:H films. In the saturated state, the dark conductivity is on the order of 10⁻⁷ S/cm. The dark conductivity prefactor is found to follow the Meyer–Neldel rule. It is proposed that charge transport takes place in amorphous-like tissue surrounding the crystalline grains. The results are attributed to the Fermi level shift due to a change in the gap state distribution.     

Electrical and mechanical properties of proton conducting SrCe0.95Yb0.05O3 − α

Yb³⁺-doped ceramic strontium cerate of exactly the composition SrCe_0.95Yb_0.05O_{3 − α} was prepared, having a relative density of 99.0 (± 0.3%). Great care was taken to obtain homogeneous, carbonate free material. Analysis are made of the X-ray powder diffraction pattern of the as-prepared dense ceramic, resulting in the orthorhombic unit cell parameters a = 6.997(2) Å, b = 12.296(3) Å, c = 8.588(2) Å, Z = 8 and d_x = 5.806(2) g cm⁻³. Bending strength values of the ceramic in non-proton and proton conducting state are found to be 177 and 194 MPa respectively. The ceramic kept under proton conducting conditions for 500 h at 300 °C to 800 °C in a N₂ flow containing 155 mbar water vapour and 245 mbar H₂, have shown to remain chemically and structurally stable. Impedance spectroscopy measurements of the bulk conductivity of the proton conducting ceramic revealed an activation energy of 53.2 kJ mol⁻¹ and a preexponential factor of 359.1 (Ω cm)⁻¹ K. In the non-proton conducting state the ceramic is mainly oxygen ion vacancy conducting, which indicates that charge compensation on substituting Yb⁺³ in SrCeO₃ takes place by oxygen ion vacancies.     

Molecular-orbital structure in neutron-rich Be and C isotopes

The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In ¹⁰Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0⁺ state of ¹⁰Be has a large α-α cluster structure, and this is characterized by a (1/2⁺ _σ)² configuration. An enlargement of the α-α distance due to two-valence neutrons along the α-α axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling between the S _z = 0 and the S _z = 1 configurations, is important. In the ground state of ¹²Be, the calculated energy exhibits similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2^- _π)²(1/2⁺ _σ)² configuration properly stronger in comparison with the closed p-shell (3/2^- _π)²(1/2^- _π)² configuration. This is related to the breaking of the N = 8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic α+α+α+n+n+ ^{. . .} model. The combination of the valence neutrons in the π- and the σ-orbit is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of ¹⁶C are the most promising candidates for such structure. Received: 1 May 2001 / Accepted: 4 December 2001     

Temperature variation of electrical conductivity and absorption edge in Cu7GeSe5I advanced superionic conductor

Single crystals of Cu₇GeSe₅I superionic conductor were grown by chemical transport. Their electrical conductivity in the frequency range 1.0×10⁶–1.2×10⁹ Hz and in the temperature range 196–295 K was measured. Cu₇GeSe₅I crystal is shown to exhibit a rather high electrical conductivity (σ₂₉₅=64.0 S/m at 295 K) and a low activation energy (ΔE_a=0.125 eV). Optical absorption edge of Cu₇GeSe₅I crystals in the temperature range 77–300 K was studied, the temperature dependences of the optical pseudogap and Urbach energy being obtained. The effect of different types of disordering on the Urbach absorption edge and electron–phonon interaction parameters was investigated.     

Investigation of auto-ionizing states in xenon by resonantly enhanced multi-photon ionization

We have examined the autoionization spectrum of xenon by resonantly enhanced three-photon ionization (2 + 1 REMPI) involving intermediate states 5p ⁵6p[J = 0, 2]. The properties of the observed autoionization resonances change significantly with the choice of the intermediate state. For ionization via an intermediate state with predominantly 5p ⁵(²P_3/2) core character, a strong continuum with embedded window-type 5p ⁵(²P _1/2)nd'-autoionization resonances is observed. For intermediate states, predominantly with 5p ⁵(²P_1/2) core character, both 5p ⁵(²P _1/2)nd'- as well as 5p ⁵(²P _1/2)ns'-resonances are present in the spectrum as overlapping, nearly symmetric peaks on a rather weak continuum. Calculations confirm the significant dependence of the spectral lineshapes upon the excitation pathway to the autoionizing state. The ionization data are compared with spectra obtained by monitoring third-harmonic generation via autoionizing states without resonant excitation of intermediate states. These spectra also exhibit the signature of both the nd'- and ns'-resonances. Received 30 September 2002 Published online 28 January 2003 RID="a" ID="a"Permanent address: Rostov State University of Transport Communication, 344038, Rostov-on-Don, Russia. RID="b" ID="b"e-mail: halfmann@physik.uni-kl.de     

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron G _E,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q ²= 0.15 (GeV/c)² in a measurement of the recoil polarisation ratio P _x/P_z in the quasifree reaction D(e,e′n)p. At this Q ² the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenh?vel et al.. After the corresponding corrections a value of G _E,n(0.15 (GeV/c)²) = 0.0481±0.0065_stat±0.0053_syst is obtained. Received: 12 April 1999     

Preparation and characterization of a new PEO-PPG blend polymer electrolyte system

This paper reports on preparation and characterization of thin films of a new zinc ion conducting blended polymer electrolyte system containing polyethylene oxide [PEO] and polypropylene glycol [PPG] complexed with zinc triflate [Zn(CF₃SO₃)₂] salt. The room temperature ionic conductivity (σ _298K) data of such PEO-PPG polymer blends prepared by solution casting technique were found to be of the order of 10^?5 S cm^?1, whereas the optimized composition containing 90:10 wt% ratio of PEO and PPG possessed an appreciably high ionic conductivity of 7.5?×?10^?5 S cm^?1. Subsequently, six different weight percentages of zinc triflate viz., 2.5, 5, 7.5, 10, 12.5 and 15, respectively, were added into the above polymer blend and resulting polymer-salt complexes were characterized by means of various analytical tools. Interestingly, the best conducting specimen namely 87.5 wt% (PEO:PPG)-12.5 wt% Zn(CF₃SO₃)₂ exhibited an enhanced room temperature ionic conductivity of 6.9?×?10^?4 S cm^?1 with an activation energy of 0.6 eV for ionic conduction. The present XRD results have indicated the occurrence of characteristic PEO peaks and effects of salt concentration on the observed intensity of these diffraction peaks. Appropriate values of degree of crystallinity for different samples were derived from both XRD and DSC analyses, while an examination of surface morphology of the blended polymer electrolyte system has revealed the formation of homogenous spherulites involving a rough surface and relevant zinc ionic transport number was found to be 0.59 at room temperature for the best conducting polymer electrolyte system thus developed.     

Quasi-simultaneous detection of CH2O and CH by cavity ring-down absorption and laser-induced fluorescence in a methane/air low-pressure flame

The combination of two-dimensional, planar laser-induced fluorescence (PLIF) and cavity ring-down (CRD) absorption spectroscopy is applied to map quantitatively the spatial distributions of CH₂O and CH in a methane/air flame at 25 Torr. Both species are detected in the same spectral region using the overlapping CH₂O A ¹ A ₂ -X ¹ A ₁ 4₁ ⁰ and CH B-X(1,0 )bands. The combination of diagnostic techniques exploits the spatial resolution of LIF and the quantitative CRD absorption measure of column density. The spatially resolved PLIF provides the distribution of absorbers and line-of-sight CRD absorption the absolute number density needed for quantitative concentration images. The peak CH₂O concentration is (3.5±1.4 )×10¹⁴ cm^-3, or 1450±550 ppm at 1000 K. The lack of precise absorption cross-section data produces these large error limits. Although a flame model predicts lower amounts, these large uncertainties limit this measurement’susefulness as a test of the flame chemistry. Received: 24 April 2001 / Revised version: 10 July 2001 / Published online: 10 October 2001