Search

We have characterized amorphous In2O3:H (H : ∼4 at.%) transparent conducting films by Rutherford backscattering spectrometry (RBS), thermal desorption spectroscopy, spectroscopic ellipsometry, and Hall measurements. The amorphous In2O3:H films have been fabricated at room temperature by sputtering of an In2O3 ceramic target under Ar, O2, and H2O vapor with variation of a flow ratio r(O2) = O2/(O2+Ar). We observe (i) signals originating from Ar in RBS spectra for all the films and (ii) desorption of H2O and Ar gases during post thermal annealing of the films. Furthermore, O2 desorption together with H2O and Ar is observed for the films grown at r(O2) > 0.375%, whereas In desorption together with H2O and Ar is observed for the films grown at r(O2) < 0.375%. These results suggest that the films have void and/or multi-vacancy rich structures inside the amorphous network, and the variety of atoms, such as Ar, H2O, and weakly bonded O and In, is present in the void structures for the films grown at O2-rich and O2-poor conditions, respectively. Corresponding to the structural changes, optical and electrical properties also change at r(O2) = 0.375%. For the films grown at r(O2) < 0.375%, we observe a broad absorption in the visible wavelengths that cannot be explained by free carrier absorption. In this film, the carrier mobility reduces rapidly with increasing carrier density. Analysis of spectroscopic ellipsometry and Hall measurements reveals that a large decrease in mobility is due to a large increase in carrier effective mass, in addition to the effect of ionized impurity scattering. In this article, we discuss the optical and transport properties with the variation of oxygen stoichiometry and microscopic structures in the amorphous In2O3:H films. [ABSTRACT FROM AUTHOR]

The composition and structure of an ultra-thin tin oxide film on Rh(100), prepared by the deposition of a submonolayer of tin followed by annealing in an O2 atmosphere, were examined by a combination of low-energy electron diffraction (LEED), Auger electron spectroscopy, X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), and ab initio calculations based on density functional theory (DFT). Although the LEED pattern exhibited c(2 × 8) spots clearly, a uniform periodicity of the c(2 × 8) unit cell was not observed in the STM images. The bright dots that were observed periodically in the STM image were similar to those of the ultra-thin Sn2O3 film on Rh(111) and formed a zigzag arrangement with the numerous point and line defects. The XPS study revealed that the Sn 3d5/2 peak of the tin oxide film on Rh(100) showed a metallic state as well as an oxide state that was between the SnO2 and SnO states. The structural models, which were based on the Sn2O3 structure on Rh(111), were determined using DFT total energy calculations. The simulated STM images of the two slightly different honeycomb-chain models well reproduced the zigzag arrangement in the STM image. The STM image and XPS spectrum were interpreted using a combination of the two models. [ABSTRACT FROM AUTHOR]

Understanding of the origin of some of the intermediate frequency modes (IFMs) in the Raman spectrum of carbon nanotubes has remained controversial in the literature. In this work, through controlled introduction and elimination of defects in carbon nanotubes, we attempt to isolate the IFMs due to structural defects from that of the combination modes in single walled carbon nanotubes (SWCNTs). Our investigations on pristine and defect engineered SWCNTs using ion-irradiation, thermal annealing, and laser processing show systematic changes in the IFMs in the range 400-1200 cm-1 and its manipulation with the processing parameters. In particular, we found that the intensity of IFM at 929 cm-1 scale up with the increasing defect concentration, while that at 668 cm-1 follows opposite behavior. New IFM peaks were observed upon the creation of a controlled amount of structural defects through 30 keV N+ ion irradiation. Elimination of defects through vacuum annealing results into reduction of intensity of some IFMs identified as defect related, while the intensity of characteristic combination modes correspondingly increases. Our results show that the IFMs observed at 709, 805, 868, 926, and 1189 cm-1 are due to structural defects in the SWCNTs, while those in the range 400-550 cm-1 and at 669 cm-1 are due to the combination modes. Our x-ray photoelectron spectroscopy analysis on ion irradiated SWCNTs supports the Raman results. [ABSTRACT FROM AUTHOR]

We report on spectroscopic analyses of plasmas produced by laser irradiation of nitrogen-free and nitrogen-containing polymer materials. Ultraviolet laser pulses of 5 ns duration and 4 mJ energy were focused onto the samples with a fluence of about 20 Jcm-2. The plasma emission was analyzed with an Echelle spectrometer equipped with a gated detector. Comparing the spectra recorded during ablation in air and argon, it is shown that the spectral line emission of atomic nitrogen originates from the excitation of the ambient air, whereas the CN molecular bands are essentially emitted from the ablation plume. Furthermore, the measurements demonstrate an additional contribution of nitrogen emission from the air molecules accumulated in the polymer. Storage under vacuum over a duration of the order of one day leads to the release of the absorbed air. As a consequence of the air absorption, the measurement of elemental composition of polymers via laser-induced breakdown spectroscopy is particularly difficult. Here, we quantify the atmospheric contribution to the plume emission during polymer analysis. [ABSTRACT FROM AUTHOR]

Three-dimensional integration with through-silicon vias (TSVs) has emerged as an effective solution to overcome the wiring limit imposed on device density and performance. However, thermal stresses induced in TSV structures raise serious thermomechanical reliability concerns. In this paper, we analyze the near-surface stress distribution in a TSV structure based on a semi-analytic approach and finite element method, in comparison with micro-Raman measurements. In particular, the depth dependence of the stress distribution and the effect of elastic anisotropy of Si are illustrated to properly interpret the Raman data. The effects of the surface oxide layer and metal plasticity of the via material on the stress and Raman measurements are discussed. The near-surface stress characteristics revealed by the modeling and Raman measurements are important for design of TSV structures and device integration. [ABSTRACT FROM AUTHOR]

Fully relativistic, four-component density functional theory electronic structure calculations were performed for the MAu dimers of the 7p elements, 113 through 118, and their 6p homologs, Tl through Rn. It was shown that the M-Au bond strength should decrease from the 6p to 7p homologs in groups 13 and 14, while it should stay about the same in groups 15 through 17 and even increase in group 18. This is in contrast with the decreasing trend in the M-M bond strength in groups 15 through 17. The reason for these trends is increasingly important relativistic effects on the np AOs of these elements, particularly their large spin-orbit splitting. Trends in the adsorption energies of the heaviest elements and their homologs on gold are expected to be related to those in the binding energies of MAu, while sublimation enthalpies are closely connected to the binding energies of the MM dimers. Lack of a correlation between the MAu and MM binding energies means that no correlation can also be expected between adsorption enthalpies on gold and sublimation enthalpies in groups 15 through 17. No linear correlation between these quantities is established in the row of the 6p elements, as well as no one is expected in the row of the 7p elements. [ABSTRACT FROM AUTHOR]

In para-H2-{molecule} interactions, the common assumption that para-H2 may be treated as a spherical particle is often substantially in error. For example, quantum mechanical eigenvalues on a full four-dimensional (4D) potential energy surface for para H2-{linear molecule} species often differ substantially from those calculated from the corresponding two-dimensional (2D) surface obtained by performing a simple spherical average over the relative orientations of the H2 moiety. However, use of an 'adiabatic-hindered-rotor' approximation can yield an effective 2D surface whose spectroscopic properties are an order of magnitude closer to those yielded by a full 4D treatment. [ABSTRACT FROM AUTHOR]

We present a two-dimensional potential surface for the isomerization in the hydroperoxyl radical HO2 and calculate the vibrational spectrum. We then show that a simple effective spectroscopic fitting Hamiltonian is capable of reproducing large scale vibrational spectral structure above the isomerization barrier. Polyad breaking with multiple resonances is necessary to adequately describe the spectral features of the system. Insight into the dynamical nature of isomerization related to the effective Hamiltonian is gained through classical trajectories on the model potential. Contrary to physical intuition, the bend mode is not a 'reaction mode,' but rather isomerization requires excitation in both stretch and bend. The dynamics reveals a Farey tree formed from the 2:1 and 3:1 resonances, corresponding to the resonance coupling terms in the effective Hamiltonian, with the prominent 5:2 (2:1+3:1) feature dividing the tree into parts that we call the 3:1 and 2:1 portions. [ABSTRACT FROM AUTHOR]

We present a two-dimensional potential surface for the isomerization in the hydroperoxyl radical HO2 and calculate the vibrational spectrum. We then show that a simple effective spectroscopic fitting Hamiltonian is capable of reproducing large scale vibrational spectral structure above the isomerization barrier. Polyad breaking with multiple resonances is necessary to adequately describe the spectral features of the system. Insight into the dynamical nature of isomerization related to the effective Hamiltonian is gained through classical trajectories on the model potential. Contrary to physical intuition, the bend mode is not a 'reaction mode,' but rather isomerization requires excitation in both stretch and bend. The dynamics reveals a Farey tree formed from the 2:1 and 3:1 resonances, corresponding to the resonance coupling terms in the effective Hamiltonian, with the prominent 5:2 (2:1+3:1) feature dividing the tree into parts that we call the 3:1 and 2:1 portions. [ABSTRACT FROM AUTHOR]

Using full-dimensional EOM-IP-CCSD/aug-cc-pVTZ potential energy surfaces, the photoelectron spectrum, vibrational structure, and ionization dynamics of the water dimer radical cation, (H2O)2+, were computed. We also report an experimental photoelectron spectrum which is derived from photoionization efficiency measurements and compares favorably with the theoretical spectrum. The vibrational structure is also compared to the recent experimental work of Gardenier et al. [J. Phys. Chem. A 113, 4772 (2009)] and the recent theoretical calculations by Cheng et al. [J. Phys. Chem. A 113, 13779 (2009)]. A reduced-dimensionality nuclear Hamiltonian was used to compute the ionization dynamics for both the ground state and first excited state of the cation. The dynamics show markedly different behavior and spectroscopic signatures depending on which state of the cation is accessed by the ionization. Ionization to the ground state cation surface induces a hydrogen transfer which is complete within 50 fs, whereas ionization to the first excited state results in a much slower process. [ABSTRACT FROM AUTHOR]

The optical response of the AlxGa1-xN alloy has been analyzed using a simplified model of interband transitions (E≤10 eV). The present model reveals distinct structures at energies of the E0, E1, F1, and E0′ critical points. The Cauchy-Lorentz expression popularly used for the modeling of the three-dimensional M0 and saddle-point excitonic transitions does not satisfy the Kramers-Krönig requirements but does so if its form is properly modified. As a result, excellent agreement is achieved between the experimental and calculated [variant_greek_epsilon](E) spectra over the entire range of photon energies. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of the AlxGa1-xN alloy are also presented. The high-frequency and static dielectric constants of the AlxGa1-xN alloy are determined to be [variant_greek_epsilon]∞(x)=5.27-1.07x and [variant_greek_epsilon]s(x)=9.28-1.45x, respectively. [ABSTRACT FROM AUTHOR]

We have theoretically investigated the tunneling current induced by a terahertz (THz) field applied to an asymmetric double quantum well. The excitation couples an initially localized state to a nearby continuum of extended states. We have shown that the calculated current has similar features as those present in the optical spectra, such as interference effects due to the interaction between the continuum and the localized states, in addition to many-photon transition effects. The induced current is calculated as a function of the intensity of the THz field. A second THz field is used to yield nonlinear processes, useful to control the interference effects. We believe that part of the issues studied here can be useful for the integration of novel switching mechanisms based on optics (THz) and electronic current. [ABSTRACT FROM AUTHOR]

Observables in linear and nonlinear infrared spectroscopy may be computed from vibrational response functions describing nuclear dynamics on a single electronic surface. We demonstrate that the Herman–Kluk (HK) semiclassical approximation to the quantum propagator yields an accurate representation of quantum coherence effects in linear and nonlinear response functions for coupled anharmonic oscillators. A considerable numerical price is paid for this accuracy; the calculation requires a multidimensional integral over a highly oscillatory integrand that also grows without bound as a function of evolution times. The interference among classical trajectories in the HK approximation produces quantization of good action variables. By treating this interference analytically, we develop a mean-trajectory (MT) approximation that requires only the propagation of classical trajectories linked by transitions in action. The MT approximation accurately reproduces coherence effects in response functions of coupled anharmonic oscillators in a regime in which the observables are strongly influenced by these interactions among vibrations. [ABSTRACT FROM AUTHOR]

Misfit defects in a 3C-SiC/Si (001) interface were investigated using a 200 kV high-resolution electron microscope with a point resolution of 0.194 nm. The [110] high-resolution electron microscopic images that do not directly reflect the crystal structure were transformed into the structure map through image deconvolution. Based on this analysis, four types of misfit dislocations at the 3C-SiC/Si (001) interface were determined. In turn, the strain relaxation mechanism was clarified through the generation of grow-in perfect misfit dislocations (including 90° Lomer dislocations and 60° shuffle dislocations) and 90° partial dislocations associated with stacking faults. [ABSTRACT FROM AUTHOR]

The surface structure of 1-butyl-3-methylimidazolium hexafluorophosphate is studied by high-resolution Rutherford backscattering spectroscopy (HRBS) at room temperature. Elemental depth profiles are derived from the observed HRBS spectrum through spectrum simulation. While the obtained carbon profile has a sharp peak at the surface, the nitrogen profile shows a broader peak at a depth ∼0.3 nm. These observations indicate that the butyl chain protrudes from the bulk liquid to the vacuum at the surface. The profiles of phosphorous and fluorine also have a broad peak at almost the same depth as the nitrogen profile, indicating that the anions are located near the imidazolium rings. These results are in good agreement with recent molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Spin networks, namely, the 3nj symbols of quantum angular momentum theory and their generalizations to groups other than SU(2) and to quantum groups, permeate many areas of pure and applied science. The issues of their computation and characterization for large values of their entries are a challenge for diverse fields, such as spectroscopy and quantum chemistry, molecular and condensed matter physics, quantum computing, and the geometry of space time. Here we record progress both in their efficient calculation and in the study of the large j asymptotics. For the 9j symbol, a prototypical entangled network, we present and extensively check numerically formulas that illustrate the passage to the semiclassical limit, manifesting both the occurrence of disentangling and the discrete-continuum transition. [ABSTRACT FROM AUTHOR]

We study the effect of non-Markovian reservoirs on the heat conduction properties of short to intermediate size molecular chains. Using classical molecular dynamics simulations, we show that the distance dependence of the heat current is determined not only by the molecular properties, rather it is also critically influenced by the spectral properties of the heat baths, for both harmonic and anharmonic molecular chains. For highly correlated reservoirs the current of an anharmonic chain may exceed the flux of the corresponding harmonic system. Our numerical results are accompanied by a simple single-mode heat conduction model that can capture the intricate distance dependence obtained numerically. [ABSTRACT FROM AUTHOR]

Fast field cycling 1H NMR relaxometry is applied to determine the dispersion of spin-lattice relaxation time T1(ω) of the glass former glycerol in broad temperature (75–360 K) and frequency (10 kHz–30 MHz) ranges. The relaxation data are analyzed in terms of a susceptibility χ″(ω)∝ω/T1(ω), related to the second rank (l=2) molecular orientational correlation function. Broadband dielectric spectroscopic results suggest the validity of frequency temperature superposition above the glass transition temperature Tg. This allows to combine NMR data of different temperatures into a single master curve χ″(ωτα) that extends over 15 decades in reduced frequency ωτα, where τα is the structural α-relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy (l=1) and depolarized light scattering (l=2). At ωτα<1, NMR susceptibility is significantly different from both the dielectric and light scattering results. At ωτα>1, there rather appears a difference between the susceptibilities of rank l=1 and l=2. Specifically, at ωτα>1, where the susceptibility is dominated by the so-called excess wing, the NMR and light scattering spectra (both l=2) rather coincide with each other and are about three times more intense than the dielectric (l=1) spectrum. This is explained by assuming that the high frequency dynamics correspond to only small-angle excursions. Below Tg, dielectric and NMR susceptibility compare well and exhibit an exponential temperature dependence. [ABSTRACT FROM AUTHOR]

Calculations of Franck–Condon factors are crucial for interpreting vibronic spectra of molecules and studying nonradiative processes. We have developed a new method for calculating Franck–Condon factors of multidimensional harmonic oscillators including the Duschinsky effect. Closed-form formulas of two-, three-, and four-dimensional Franck–Condon factors were derived straightforwardly by using the properties of Hermite polynomials and Gaussian integrals. This new method was applied to study the photoelectron spectra of H2O+(B 2B2) and D2O+(B 2B2), whose equilibrium geometries and harmonic vibrational frequencies were calculated by using the coupled cluster singles and doubles with perturbative triples [CCSD(T)] method together with the basis sets of 6-311++G(3df,2pd) and aug-cc-pVTZ. The adiabatic ionization energies were computed by using the CCSD(T) method extrapolated to the complete basis set limit with aug-cc-pVXZ (X=D,T,Q,5). It was found that the simulated photoelectron spectra were mainly composed of ν2 progressions and the combination bands of ν1 and ν2, whereas pure ν1 transitions should be too weak to be observable, contrary to the literature reports. It was also found that the first discernible peak in the experimental photoelectron spectra did not correspond to the adiabatic transition. The adiabatic ionization energies of H2O+(B 2B2) and D2O+(B 2B2) are proposed to be 16.78 and 16.83 eV, about 0.40 and 0.58 eV lower than the best experimental values, respectively. Conversely, the calculated ionization energies are in agreement with the proposed values within 0.02 eV. [ABSTRACT FROM AUTHOR]

The structure of hydroxymethyl hydroperoxide (HOCH2OOH) (HMHP) has been examined using coupled cluster and multireference configuration interaction methods to study the excited states and probable photodissociation products. The results are compared to experiments. The vertical excitation energies for several excited states of HOCH2OOH are presented as well as the excited state energies along the O–O, O–H, C–O, and C–H dissociation pathways. The results help in the interpretation of experimental UV absorption spectra and elucidate the photodissociation mechanism of HMHP under tropospheric conditions. [ABSTRACT FROM AUTHOR]

To understand the nonexponential relaxation associated with solvation dynamics in the ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate, we study power spectra of the fluctuating Franck–Condon energy gap of a diatomic probe solute via molecular dynamics simulations. Results show 1/f dependence in a wide frequency range over 2–3 decades, indicating distributed relaxation times. We analyze the memory function and solvation time in the framework of the generalized Langevin equation using a simple model description for the power spectrum. It is found that the crossover frequency toward the white-noise plateau is directly related to the time scale for the memory function and thus the solvation time. Specifically, the low crossover frequency observed in the ionic liquid leads to a slowly decaying tail in its memory function and long solvation time. By contrast, acetonitrile characterized by a high crossover frequency and (near) absence of 1/f behavior in its power spectra shows fast relaxation of the memory function and single exponential decay of solvation dynamics in the long-time regime. [ABSTRACT FROM AUTHOR]

A new mechanism for the formation of doped polyaniline is presented. Besides providing suitable structural and spectroscopic parameters, the new mechanism allows for the rationalization of the experimentally observed equilibrium between polaron and bipolaron defects in emeraldine salt. The magnetic behavior and the “metallic island” model for conduction in doped polyaniline are also theoretically supported by the new proposal. [ABSTRACT FROM AUTHOR]

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal. The frequency of the ultraviolet probe beam is selected to be in electronic resonance with rotational transitions in the A 2Σ+←X 2Π (1,0) band of NO. This choice results in a resonance between the frequency of the ERE-CARS signal and transitions in the (0,0) band. The theoretical model for ERE-CARS NO spectra has been developed in the perturbative limit. Comparisons to experimental spectra are presented where either the probe laser was scanned with fixed Stokes frequency or the Stokes laser was scanned with fixed probe frequency. At atmospheric pressure and an NO concentration of 100 ppm, good agreement is found between theoretical and experimental spectral peak locations and relative intensities for both types of spectra. Factors relating to saturation in the experiments are discussed, including implications for the theoretical predictions. [ABSTRACT FROM AUTHOR]

The plasma etching damage in p-type GaN has been characterized. From current-voltage and capacitance-voltage characteristics of Schottky diodes, it was revealed that inductively coupled plasma (ICP) etching causes an increase in series resistance of the Schottky diodes and compensation of acceptors in p-type GaN. We investigated deep levels near the valence band of p-type GaN using current deep level transient spectroscopy (DLTS), and no deep level originating from the ICP etching damage was observed. On the other hand, by capacitance DLTS measurements for n-type GaN, we observed an increase in concentration of a donor-type defect with an activation energy of 0.25 eV after the ICP etching. The origin of this defect would be due to nitrogen vacancies. We also observed this defect by photocapacitance measurements for ICP-etched p-type GaN. For both n- and p-type GaN, we found that the low bias power ICP etching is effective to reduce the concentration of this defect introduced by the high bias power ICP etching. [ABSTRACT FROM AUTHOR]

This paper presents an optical characterization of three Bridgman-grown Zn1-x-yBexMgySe mixed crystals in the near-band-edge interband transitions using temperature-dependent photoluminescence (PL) in the temperature range of 10–300 K and surface photovoltage spectroscopy (SPS) at room temperature. PL spectra at low temperatures of the investigated samples consist of an excitonic line, an edge emission due to radiative recombination of shallow donor-acceptor pairs, and a broad band related to recombination through deep level defects. The anomalous S-shape temperature dependence of the exciton emission peak for Zn1-x-yBexMgySe crystals with high Mg content (y=0.26) can be explained as due to localization of excited carriers caused by statistical fluctuations of local composition. The peak positions of the excitonic emission lines in PL spectra correspond quite well to the energies of the fundamental transitions determined from SPS measurements. The parameters that describe the temperature dependence of the transition energy and broadening parameter of the band-edge excitonic emission are evaluated and discussed. [ABSTRACT FROM AUTHOR]

BiFeO3 (BFO) films deposited on SrTiO3 (001) substrates and on LaNiO3-coated SrTiO3 (001) substrates with different annealing ambiences of oxygen and nitrogen were studied by using micro-Raman spectroscopy and x-ray diffraction (XRD). XRD showed that the films are in single-phase with rhombohedral structure. According to the analysis of the group theory, 13 Raman-active modes, which can be classified as 4A1 and 9E modes, have been observed in the BiFeO3 films. Raman spectra along the growth direction of the BFO films in the side-view scattering geometry were performed by the Raman mapping technique. The variations of Raman shift and Raman bandwidth in different depths of the films imply the existence of residual strain along the growth direction of the BFO films. These results are very useful for the understanding of the depth dependence of the physical properties including the interface and surface structure of the BFO films. [ABSTRACT FROM AUTHOR]

Multireference Mo\ller–Plesset (MRMP) perturbation theory [K. Hirao, Chem. Phys. Lett. 190, 374 (1992)] is modified to use improved virtual orbitals (IVOs) and is applied to study ground state potential energy curves for isomerization and dissociation of the N2H2 and C2H4 molecules. In contrast to traditional MRMP or multistate multiconfiguration quasidegenerate perturbation theory where the reference functions are obtained from (often difficult to converge) state averaged multiconfiguration self-consistent field methods, our reference functions are represented in terms of computationally efficient IVOs. For convenience in comparisons with other methods, a first order complete active space configuration interaction (CASCI) calculation with the IVOs is followed by the use of the IVOs in MRMP to incorporate residual electron correlation effects. The potential energy curves calculated from the IVO-MRMP method are compared with computations using state-of-the-art coupled cluster singles and doubles (CCSD) methods and variants thereof to assess the efficacy of the IVO-MRMP scheme. The present study clearly demonstrates that unlike the CCSD and its variants, the IVO-MRMP approach provides smooth and reliable ground state potential energy curves for isomerization of these systems. Although the rigorously size-extensive completely renormalized CC theory with noniterative triples corrections (CR-CC(2,3)) likewise provides relatively smooth curves, the CR-CC(2,3) calculations overestimate the cis-trans barrier height for N2H2. The ground state spectroscopic constants predicted by the IVO-CASCI method agree well with experiment and with other highly correlated ab initio methods. [ABSTRACT FROM AUTHOR]

Theoretical studies on the GenSim clusters have been carried out using advanced ab initio approaches. The lowest energy isomers were determined for the clusters with compositions n+m=2–5. All possible isomers arising due to permutations of Ge and Si atoms were investigated. The L-shaped structure for the trimers, tetragonal with diagonal bond for tetramers, and a trigonal bipyramid for pentamers represent the energy optimized ground state geometries. The bonding analyses revealed that the trimers and tetramers are stabilized through multicenter π bonding. In pentamers, this stabilizing factor is eliminated due to the further cluster growth. The ionization of clusters does not change their geometrical characteristics. The agreement of the calculated ionization and atomization energies with those obtained from the mass spectrometric studies (through estimated appearance potential) validated the reported structures of the clusters. The bonding properties of these species are discussed using their molecular orbital characteristics and analysis of natural bond orbital population data. [ABSTRACT FROM AUTHOR]

A new method of calculation of the second-order dispersion energy is proposed. It is based on the Longuet-Higgins formula [Faraday Discuss. Chem. Soc. 40, 7 (1965)], which describes the dispersion interaction in terms of frequency-dependent density susceptibilities of monomers. In this study, the density susceptibilities are obtained from the coupled cluster theory at the singles and doubles level. Density fitting is applied in order to reduce the computational effort for the evaluation of density susceptibilities. It is shown that density fitting improves the scaling of the computational resources with molecular size by one order of magnitude without affecting the accuracy of the resulting dispersion energy. Numerical results are presented for several van der Waals molecules to illustrate the performance of the new approach. [ABSTRACT FROM AUTHOR]

The structural and electronic properties of MAu16- (M=Ag, Li, Na, and K) have been studied by the scalar relativistic all-electron density-functional calculations, in which particular attention is paid to the stability of the endohedral Au16- cage doped by different dopant atoms. It is found that only the smaller atoms, such as Cu, Li, and Na, can be stably encapsulated in the Au16- cage, while the addition of the larger Ag or K atom prefers to locate in the surface or outside of the cage, which is inconsistent with the previous hypothesis that the Au16- cage could act as a container to hold an arbitrary heterometal atom. The stable endohedral Li@Au16- and Na@Au16- have a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital gap, indicating that they are chemically stable and may be used as potential building blocks for designing cluster-assembled materials. [ABSTRACT FROM AUTHOR]

An approach to treat static correlation within a density-functional framework is presented. To that end, a multiconfiguration optimized effective potential (MCOEP) method is derived. In contrast to standard multiconfiguration self-consistent field (MCSCF) methods and previous combinations of MCSCF procedures with density-functional theory, the MCOEP method yields well-defined physically meaningful orbital and eigenvalue spectra. In addition to the electronic ground state also excited electronic states can be described. The MCOEP method is implemented invoking the localized Hartree–Fock approximation, leading to a multiconfiguration localized Hartree–Fock approach. Applications of the new method to the dissociation of the hydrogen molecule and the isomerization of ethene and cyclobutadiene show that it is capable of describing situations that are characterized by strong static correlation [ABSTRACT FROM AUTHOR]

Sulfur difluoride radicals in their ground state have been produced by a “laser-free” pulsed dc discharge of the SF6/Ar gas mixtures in a supersonic molecular beam and detected by mass-selective resonance-enhanced multilphoton ionization (REMPI) spectroscopy in the wavelength range of 408–420 nm. Analyses of the (3+1) REMPI excitation spectrum have enabled identification of three hitherto unknown Rydberg states of this radical. Following the Rydberg state labeling in our previous work [see J. Phys. Chem. A 102, 7233 (1998)], these we label the K(5p1) [ν0-0=71 837 cm-1, ω1′(a1 sym str)=915 cm-1], L(5p2) [ν0-0=72 134 cm-1, ω1′(a1 sym str)=912 cm-1], and M(5p3) [ν0-0=72 336 cm-1, ω1′(a1 sym str)=926 cm-1] Rydberg states, respectively. [Origins, relative to the lowest vibrational level of the X 1A1 ground state, and vibrational frequencies of the symmetric S–F stretching mode are suggested by the numbers in brackets.] Photofragmentation process of SF2+→SF++F that relates to the REMPI spectrum was discussed. [ABSTRACT FROM AUTHOR]

The electronic excited states of the [COH2]+ system have been studied in order to establish their role in the dynamics of the C++H2O→[COH]++H reaction, which is a prototypical ion-molecule reaction. The most relevant minima and saddle points of the lowest excited state have been determined and energy profiles for the lowest excited doublet and quartet electronic states have been computed along the fragmentation and isomerization coordinates. Also, nonadiabatic coupling strengths between the ground and the first excited state have been computed where they can be large. Our analysis suggests that the first excited state could play an important role in the generation of the formyl isomer, which has been detected in crossed beam experiments [D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985)], but could not be explained in quasiclassical trajectory computations [Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003); J. R. Flores, J. Chem. Phys. 125, 164309 (2006)]. [ABSTRACT FROM AUTHOR]

The quasiclassical absorption spectrum of the water dimer in the A band was calculated taking into account motion in all degrees of freedom of the system. The ab initio excited state potentials employed were interpolated by the modified Shepard interpolation method using QMRCI energies and state-averaged MCSCF gradients and Hessians. The ground state vibrational wavefunction was variationally calculated using an adiabatic separation between the high and low frequency normal modes of the system. The calculated spectrum of water dimer shows a clear blueshift with respect to the monomer, but also a small red tail, in agreement with the prediction by Harvey et al. [J. Chem. Phys. 109, 8747 (1998)]. Previous three-dimensional model studies of the photodissociation of the water dimer by Valenzano et al. [J. Chem. Phys. 123, 034303 (2005)] did not show this red tail. A thorough analysis of the dependence of the spectrum on the modes coupled explicitly in the calculation of the spectrum shows that the red tail is due to coupling between the intramolecular stretch vibrations on different monomers. [ABSTRACT FROM AUTHOR]

Polarized femtosecond pump-probe spectroscopy is used to observe electronic wavepacket motion for vibrational wavepackets centered on a conical intersection. After excitation of a doubly degenerate electronic state in a square symmetric silicon naphthalocyanine molecule, electronic motions cause a ∼100 fs drop in the polarization anisotropy that can be quantitatively predicted from vibrational quantum beat modulations of the pump-probe signal. Vibrational symmetries are determined from the polarization anisotropy of the vibrational quantum beats. The polarization anisotropy of the totally symmetric vibrational quantum beats shows that the electronic wavepackets equilibrate via the conical intersection within ∼200 fs. The relationship used to predict the initial electronic polarization anisotropy decay from the asymmetric vibrational quantum beat amplitudes indicates that the initial width of the vibrational wavepacket determines the initial speed of electronic wavepacket motion. For chemically reactive conical intersections, which can have 1000 times greater stabilization energies than the one observed here, the same theory predicts electronic equilibration within 2 fs. Such electronic movements would be the fastest known chemical processes. [ABSTRACT FROM AUTHOR]

In this work we present the structural and electrical characterization of sputter-deposited CoFe(B)/MgO/Si metal-insulator-semiconductor tunneling junctions for injection and detection of spin polarized current in silicon. The multilayers have been deposited in 700 nm deep trenches, patterned in thick SiO2 dielectric, on n- and p-doped wafers. The films inside the trenches are continuous with a correlated and low roughness. The MgO barrier grows amorphous without indication of pinholes. The dc and ac transport properties of the junctions were studied as a function of temperature and frequency. A relatively high interface trap density at the MgO/Si-interface is extracted from admittance spectra measurements. Transport is dominated by majority carriers in the case of n-doped and by minority carriers for the p-doped wafers. This leads to distinct rectification characteristics for the two wafer types, which would significantly influence the spin injection efficiency of the tunneling junctions. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and Raman spectroscopy have been used to study the incorporation of C in strained Si1-x-yGexCy epilayers lattice matched to Si(001). The samples were characterized by both secondary ion mass spectrometry and Auger emission spectroscopy to obtain the total C concentration and x-ray diffraction data were used to obtain the substitutional C concentration. The difference between the total and substitutional C concentrations, i.e., the nonsubstitutional carbon fraction, was found to be directly correlated with specific spectral lines in both the room temperature Raman and low temperature PL spectra. These variations are discussed and related to C related defects in the epilayers. [ABSTRACT FROM AUTHOR]

The effects of P doping on the nanocrystalline formation in mixed-phase Si:H thin films were investigated using secondary-ion mass spectrometry, Raman spectroscopy, atomic force microscopy, cross-sectional transmission electron microscopy, and scanning Kelvin probe microscopy. We found that Si nanocrystallites in the intrinsic and weakly P-doped materials aggregate to form cone-shaped structures. The local workfunction of the nanocrystalline aggregation areas is larger than the surrounding amorphous areas. Increasing the P-doping level requires an increased hydrogen dilution to reach the similar Raman crystallinity. The nanocrystalline aggregation disappears in the heavily P-doped materials, but isolated nancrystallites appear. The effect of P-doping on the nanostructure is explained with the coverage of P-related radicals on the existing nanocrystalline surface during the deposition and the P segregation in grain boundaries, which prevent new nucleation on the surface of existing nanocrystallites. [ABSTRACT FROM AUTHOR]

In the present work an experimental investigation on the electrohydrodynamic (EHD) interaction effect, induced by means of a dielectric barrier discharge on a 1 atm subsonic air flow, is described. The air flow is obtained in an open circuit blowing wind tunnel at different air speeds. A plane plasma panel equipped with a dielectric discharge barrier system is immersed in the flow. A single phase sinusoidal power supply system, a superposition of a single phase sinusoidal system with a dc voltage supply, and a three phase symmetrical power supply configuration are considered. Electric, fluid dynamic, and spectroscopic diagnostic techniques are utilized. Pitot probe measurements are performed in the boundary layer on the plasma panel surface. Schlieren imaging is done to visualize the plasma sheath. Vibrational and rotational temperatures are evaluated by means of spectroscopic techniques. The force induced on the flow by the dielectric barrier discharge acts on the ions of the non-neutral sheath ahead of the plasma filament when travelling on the panel surface only. This causes a reduction of the time interval during which the force is active. The EHD effect and the momentum transfer to the gas flow in the boundary layer are reduced when the flow speed grows. An increase of the supply frequency and of the supply voltage causes an increase of the momentum transfer. In these experiments no significant increase of the efficiency of EHD effect is observed when utilizing the superposition of a dc voltage to a single phase power supply or a three phase power supply. [ABSTRACT FROM AUTHOR]

One-dimensional acoustic waveguide containing subwavelength-sized Helmholtz resonators is known to exhibit novel physical phenomena. However, no systematic theoretical study on this system has been carried out so far except on a few limited cases. We present a thorough theoretical calculation on the acoustic wave propagation in phononic crystals containing Helmholtz resonators without any geometrical size restrictions. The band structures, transmission spectra, and defect states are studied for diverse geometries using the interface response theory. It is shown that the acoustic band structure of the model is fundamentally different from the conventional acoustic–elastic cases and richer due to the coexistence of the resonant and the Bragg gaps. It is also shown that the presence of a defect resonator in the system can give rise to a localized mode inside the resonance gaps. The results clearly show that the presence of the Helmholtz resonators singly or periodically can play a prominent role in designing any acoustic band gap materials. [ABSTRACT FROM AUTHOR]

Charge carrier traps in Cd0.85Mn0.15Te doped with indium were studied using thermally stimulated current measurements. The investigations were performed in temperatures ranging from 100 to 300 K. Four peaks in the current spectrum were identified. From the initial rise method and the best fit of the spectrum to the theoretical model, the activation energies and the relaxation parameters for the corresponding traps were determined. [ABSTRACT FROM AUTHOR]

The recently introduced phase-modulation mode with constant oscillation amplitude is analyzed and compared with the conventional amplitude-modulation technique. We show that instabilities in the amplitude and phase versus distance curves are prevented by the additional feedback controlling the oscillation amplitude. Moreover, we demonstrate that the phase-modulation technique is capable to measure conservative and dissipative tip-sample forces by dynamic force spectroscopy without any jumps caused by instabilities. This feature is an important advantage compared to a spectroscopy technique based on the conventional tapping mode. [ABSTRACT FROM AUTHOR]

We report time resolved measurements of spin and carrier relaxation in InAs films with carrier densities of 1.3×1016 and 1.6×1016 cm-3 grown on (001) and (111) GaAs, respectively. We used standard pump-probe and magneto-optical Kerr effect spectroscopy at different excitation wavelengths, power densities, and temperatures. We observed sensitivity of carrier and spin relaxation time to the photoinduced carrier density but not to the variation in temperature. We explain our results using the Elliot–Yafet picture of spin relaxation process in narrow gap semiconductors. [ABSTRACT FROM AUTHOR]

An analysis of spectral line profiles is used to calculate the gas temperature and to estimate the upper limit of the electron density in an atmospheric pressure dielectric barrier discharge torch. Two transitions are studied, that of helium (He) at 587.5 nm and that of hydrogen (Hβ) at 486.1 nm, both observed in the spectra of the light emitted from the gap-space region. Relevant broadening mechanisms including the Doppler and Stark effects, as well as the collision processes between an emitter and a neutral particle, are reviewed. It is deduced that the main contribution to the broadened profiles is due to collisions. Through knowledge of the van der Waals interaction potential, a general expression for determining the gas temperature is derived and applied to each transition. The results obtained from both lines are in agreement; i.e., the gas temperature is found to be 460±60 K at the highest voltage applied. This value is consistent with the experimental observation that at these conditions the afterglow plasma cannot ignite paper, whose ignition temperature is 507 K. Since no signature of the Stark effect can be detected either in He or Hβ transition, the upper limit of the electron density, estimated from the uncertainty on the Hβ linewidth, is 4×1012 cm-3. The generality of the method allows one to determine the temperature as a function of other parameters, such as voltage and flow rate. Concerning the applied voltage, the gas temperature increases linearly from 315±30 to 460±60 K, as derived from both lines. Over the same voltage range, a similar behavior is found for the rotational temperature, as deduced from the first negative B(2Σu+,v=0)→X(2Σg+,v=0) transition of the molecular nitrogen ion. However, the temperature varies between 325±30 and 533±15 K, indicating an overestimation of the gas temperature. On the other hand, the gas temperature derived from each of the lines does not show a significant variation with the He flow rate in the range of 5–40 l/min. [ABSTRACT FROM AUTHOR]

Deep level defects in p-type GaN:Mg grown by molecular beam epitaxy were characterized using steady-state photocapacitance and deep level optical spectroscopy (DLOS). Low frequency capacitance measurements were used to alleviate dispersion effects stemming from the deep Mg acceptor. Use of DLOS enabled a quantitative survey of both deep acceptor and deep donor levels, the latter being particularly important due to the limited understanding of minority carrier states for p-type GaN. Simultaneous electron and hole photoemissions resulted in a convoluted deep level spectrum that was decoupled by emphasizing either majority or minority carrier optical emission through control of the thermal filling time conditions. In this manner, DLOS was able to resolve and quantify the properties of deep levels residing near both the conduction and valence bandedges in the same sample. Bandgap states through hole photoemission were observed at Ev+3.05 eV, Ev+3.22 eV and Ev+3.26 eV. Additionally, DLOS revealed levels at Ec-3.24 eV and Ec-2.97 eV through electron emission to the conduction band with the former attributed to the Mg acceptor itself. The detected deep donor concentration is less than 2% of activated [Mg] and demonstrates the excellent quality of the film. [ABSTRACT FROM AUTHOR]

The field-dependent hole mobility of 4,4′,4″-tris(1-naphthylphenylamino)triphenylamine (1-NaphDATA) has been measured on hole-only samples with layer thicknesses between 100 and 500 nm. To determine the mobility we employed three different methods: steady-state space-charge limited currents (SCLC), transient dark injection SCL currents (DI-SCLC), and admittance spectroscopy (AS). The results, especially the field dependence of the mobility, depend on the measuring method. The SCLC mobility increases with the layer thickness and becomes constant above 200 nm, which indicates a transition from contact-limited to bulk-limited conduction. Compared to AS and SCLC we found that DI-SCLC is the most reliable method to measure the mobility on thin samples because it is hardly affected by carrier trapping nor by the layer thickness. Using DI-SCLC an organic layer of 200 nm was sufficient to determine the bulk hole mobility of 1-NaphDATA (μ0=6.57×10-6 cm2/V s and γ=2.74×10-3 (cm/V)1/2, which is very thin compared to time-of-flight samples whose thickness is in the range of micrometers. [ABSTRACT FROM AUTHOR]

Anion photoelectron spectroscopy of C3H- and C3D- is performed using both field-free time-of-flight and slow electron velocity-map imaging. We observe and assign transitions originating from linear/bent (l-C3H) and cyclic (c-C3H) anionic isomers to the corresponding neutral ground states and low-lying excited states. Transitions within the cyclic and linear manifolds are distinguished by their photoelectron angular distributions and their intensity dependence on the neutral precursor. Using calculated values for the energetics of the neutral isomers [Ochsenfeld et al., J. Chem. Phys. 106, 4141 (1997)], which predict c-C3H to lie 74 meV lower than l-C3H, the experimental results establish c-C3H- as the anionic ground state and place it 229 meV below l-C3H-. Electron affinities of 1.999±0.003 and 1.997±0.005 eV are determined for C3H and C3D from the X 2B2←X 1A1 photodetachment transition of c-C3H. Term energies for several low-lying states of c-C3H and l-C3H are also determined. Franck-Condon simulations are used to make vibrational assignments for the bands involving c-C3H. Simulations of the l-C3H bands were more complicated owing to large amplitude bending motion and, in the case of the neutral à 2Π state, strong Renner-Teller coupling. [ABSTRACT FROM AUTHOR]

The satellite transitions acquired in real time by magic angle spinning (STARTMAS) NMR experiment combines a train of pulses with sample rotation at the magic angle to refocus the first- and second-order quadrupolar broadening of spin I=3/2 nuclei in a series of echoes, while allowing the isotropic chemical and quadrupolar shifts to evolve. The result is real-time isotropic NMR spectra at high spinning rates using conventional MAS equipment. In this paper we describe in detail how STARTMAS data can be acquired and processed with ease on commercial equipment. We also discuss the advantages and limitations of the approach and illustrate the discussion with numerical simulations and experimental data from four different powdered solids. [ABSTRACT FROM AUTHOR]

We use time-resolved terahertz-pulse spectroscopy to study the ultrafast carrier dynamics in InGaAs/GaAs (311)A quantum wires. Anisotropy in the photoconductive dynamics is observed when aligning the terahertz probe polarization parallel versus perpendicular to the wire direction. The origin of this anisotropy is the carrier capture into localized quantum-wire states from delocalized wetting layer or barrier regions over time scales from 6 to 30 ps. The capture efficiency is found to be strongly temperature dependent, with thermal emission dominating above 125 K, while state-filling effects within the wires influence the capture rate below 125 K. Transient spectroscopy reveals a Drude-like carrier conductivity. [ABSTRACT FROM AUTHOR]

Tellurium-filled double-walled carbon nanotubes (Te@DWNTs) have been studied by Raman spectroscopy in the temperature interval from 300 to 700 K employing 785 nm excitation wavelength, and their spectra have been compared to those of pristine double-walled carbon nanotubes (DWNTs). The DWNTs were synthesized by catalytic chemical vapor deposition. Assignment of the radial breathing modes and the tangential modes was done based on the one dimensional electronic energy band structure of carbon nanotubes. The tangential mode components of Te@DWNT are downshifted compared to those of pristine DWNT consistent with the proposed weakening of the carbon-carbon bond due to the introduced van der Waals interaction of the Te atoms with the DWNT. It was established that Te@DWNT can be unambiguously identified by the 30% temperature coefficient decrease of the G′-band position. [ABSTRACT FROM AUTHOR]