A1 Symmetry Research Articles

Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry.

We report the syntheses, structures, and luminescence properties of a series of copper-iodo cuboids supported by L-type antimony ligands. The cuboids are of general formula [(SbR3)4Cu4(I)4] (1-4, 8), where SbR3 is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, iPr, tBu, Ph); triphenyl, iPr2Ph, and Me2Ph stibines resulted in the formation of dimers of type [(SbR3)4(Cu)2(I)2] (5-7). While similar luminescent copper-halide cubes have been studied, the corresponding "heavy-atom" congeners have not been studied, and ligation of such heavy-atom moieties is often associated with long-lived triplet states and low-energy absorption and emission profiles. Overall, two obligate parameters are found to imbue NIR emission: (i) short Cu-Cu bonds and (ii) high crystallographic symmetry; both of these properties are found only in [(SbiPr3)4Cu4(I)4] (1, in I23; λem = 711 nm). The correlation between NIR emission and high crystallographic symmetry (which intrinsically includes high molecular symmetry)-versus only molecular symmetry-is confirmed by the counterexample of the molecularly symmetric tBu-substituted cuboid [(SbtBu3)4Cu4(I)4] (3, λem = 588 nm, in R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of the SbtBu3 ligand should red-shift emission beyond that of the SbiPr3-supported cuboid, the emission of 3 is limited to the visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for 1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S0) and excited triplet state (T1) demonstrate two key factors necessary for low-energy NIR emission: (i) a significant contraction of the interconnected Cu4 intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A1 symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu4 core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T1, thereby inhibiting low-energy NIR emission.

Квазиупругое и низкочастотное комбинационное рассеяние света в кристаллах ниобата лития с дефектами стехиометрии

The paper investigates quasielastic light scattering spectra at a temperature of 296 K in lithium niobate samples of various degrees of imperfection as measured by means of the acoustic quality factor. We performed a quantitative spectrum analysis in the 0--70 cm-1 frequency range for samples with different Q-factor values in a model accounting for the connection between a low-frequency optical mode of the A1(TO) symmetry type and the acoustic density of states observed in a spectrum as a result of violating the wavevector selection rule in a stoichiometrically defective crystal. The results of comparing these simulations to experimental data show that stoichiometrical defects significantly contribute to the quasielastic light scattering intensity in congruent lithium niobate crystals

Full-Dimensional Ab Initio Potential Energy Surface and Vibrational Energy Levels of Li₂H.

We built a full-dimensional analytical potential energy surface of the ground electronic state of Li2H from ca. 20,000 ab initio multi-reference configuration interaction calculations, including core–valence correlation effects. The surface is flexible enough to accurately describe the three dissociation channels: Li (2s 2S) + LiH (1Σ+), Li2 (1Σg+) + H (1s 2S) and 2Li (2s 2S) + H (1s 2S). Using a local fit of this surface, we calculated pure (J = 0) vibrational states of Li2H up to the barrier to linearity (ca. 3400 cm−1 above the global minimum) using a vibrational self-consistent field/virtual state configuration interaction method. We found 18 vibrational states below this barrier, with a maximum of 6 quanta in the bending mode, which indicates that Li2H could be spectroscopically observable. Moreover, we show that some of these vibrational states are highly correlated already ca. 1000 cm−1 below the height of the barrier. We hope these calculations can help the assignment of experimental spectra. In addition, the first low-lying excited states of each B1, B2 and A2 symmetry of Li2H were characterized.

Broadband phonon-polariton dispersion relation of ferroelectric LiTaO3 crystals

Lithium tantalate is a well-known optical crystal. The dispersion relation of phonon-polaritons was studied in ferroelectric lithium tantalate crystals. The reflectivity spectra of E//x and E//z polarizations, which are related to E(x,y) and A1(z) symmetry modes respectively, were measured by the FT-IR measurement in the far- and mid-IR regions. The dispersion relation of phonon-polariton frequency against real and imaginary parts of polariton wavevector was determined. E(x,y) and A1(z) symmetry polariton modes were also studied by forward Raman scattering. By the combination with the observed polariton dispersion relation determined by forward Raman scattering and terahertz time-domain spectroscopy, the broadband polariton frequency dispersion was determined between 10 cm−1 and 1200 cm−1.

Determining the Parameters of Tetrahedral Splittings in the A1- and A2-Type States of Molecules with Spherical Symmetry (T d or О h )

A method of analytical description of one of the most complex problems in the study of high symmetry quantum-mechanical objects, namely, tetrahedral splittings, is considered. Results are presented that allow tetrahedral splittings of the vibrational states of A1 and A2 symmetries for polyad N ≤ 4 to be determined in an analytical form.

Negative ion formation through dissociative electron attachment to the group IV tetrachlorides: Carbon tetrachloride, silicon tetrachloride and germanium tetrachloride

The current contribution constitutes the third and final part of our trilogy of papers on electron attachment reactions of the group IV tetrahalides; XY4 (X=C, Si, Ge and Y=F, Cl, Br). In this context we extend our previous studies on XF4 and XBr4 and report results for electron attachment to the tetrachlorides: CCl4, SiCl4 and GeCl4 in the incident electron energy range from about 0 to 10eV. At the same time we give a summary of the currently available literature on electron interactions with those latter compounds. Upon electron attachment the formation of Cl−, XCl3−, XCl2− and Cl2− is observed from all the tetrachlorides, and additionally the molecular anion SiCl4− is observed from SiCl4. The main DEA contributions are observed through narrow, threshold peaks (at 0eV) and we attribute these features to single particle resonances associated with the a1 symmetry LUMOs of those compounds. Contributions from another low-lying resonance, which we assign as a 2T2 shape resonance associated with the t2 symmetry LUMO+1, is also observed in the ion yield curves for all the tetrachlorides. The energy of the peak position of those contributions varies in the range from about 1 to 2eV, depending on the compound and the fragment formed. In addition to these low energy contributions, higher energy, fairly broad, features are observed for all the tetrachlorides. These contributions exhibit a peak in the energy range between 5 and 8eV, again depending on the compound and the fragment formed. Further to the experimental data, we report DFT and coupled cluster calculations on the thermochemical thresholds for the individual fragments as well as the respective bond dissociation energies and electron affinities. These calculated values are compared with the experimental appearance energies and literature values, where they are available.

Structural phase transition, impedance spectroscopy and narrow optical band Gap in [formula omitted

In the progress of exploring lead-free ferroelectric perovskites, a new solid solution of 1-x KNbO3- x BaSc12 Nb12O3 is synthesized using solid state method. The effect of Ba and Sc codoping on structural phase transition, dielectric, ferroelectric, electrical, and optical properties is systematically studied. A narrow band gap of 1.98eV is observed at x = 0.05. On further increase in x, the optical band gap increases due to increased strain. The crystal symmetry changes from orthorhombic at x = 0.00 to tetragonal phase at x ≤ 0.15, remains in pseudocubic phase for 0.15 < x < 0.35, and finally transforms to the cubic symmetry at x ≥ 0.35. A new Raman active mode evolves at 180 cm-1 at x = 0.15, which could be the TO or LO phonon of A1 symmetry. The electrical microstructure of the prepared electroceramics at room temperature has been investigated using impedance spectroscopy. This newly synthesized ferroelectric perovskite material has promising potential applications for photocatalysis and photovoltaics, especially under the visible light spectrum.

Low-energy-electron scattering by CH3CN

We report integral and differential cross sections for the elastic scattering of low-energy electrons by methyl cyanide (CH3CN), also known as acetonitrile. The cross sections were computed using the Schwinger multichannel method implemented with pseudopotentials. The fixed-nuclei scattering calculations were performed in the static-exchange and static-exchange plus polarization approximations for energies up to 15 eV. In our calculations with polarization effects, we found a π * shape resonance at around 2.22 eV and a broad structure associated to a σ * shape resonance at around 7 eV. The low-lying resonance was assigned to the electron capture by the two-fold degenerate π * orbital of the E symmetry of C 3v group; the second was assigned to a σ * shape resonance in the A 1 symmetry. We compared our cross sections with theoretical results and experimental data available in the literature, and in general we found good agreement for the positions of the two resonances.

Ultrafast dynamics of coherent optical phonon correlated with the antiferromagnetic transition in a hexagonal YMnO3 epitaxial film

We report on the observation of the coherent optical phonon in a hexagonal YMnO3 epitaxial film using a reflection-type pump-probe technique at various temperatures, excitation powers, and energies. We detected an oscillatory structure with a frequency of ∼5.1 THz, which is assigned to the coherent optical phonon with A1 symmetry, in a time-domain signal. It was found that the coherent optical phonon can be observed at temperatures from 10 K to room temperature, while the oscillation amplitude markedly decreases with an increase in temperature around ∼70 K corresponding to the Néel temperature. The temperature dependence of the oscillation amplitude indicates that the coherent optical phonon is sensitive to the spin-lattice coupling connected with the antiferromagnetic transition.

Elastic electron scattering from nitrobenzene.

We present integral, momentum transfer, and differential cross sections for an elastic scattering of electrons by nitrobenzene. Our calculations employed the Schwinger multichannel method with pseudopotentials and were performed in the static-exchange and static-exchange plus polarization approximations. The cross sections were computed for impact energies up to 10 eV. We observed four resonances in the static-exchange calculations, and three when polarization effects were included. This result indicates that the low-lying resonance in the B1 symmetry of C2v group, observed in the static-exchange calculation, became a bound state when polarization was taken into account. Our calculations including polarization effects assigned the low-lying resonance located at 0.92 eV to the A2 symmetry and the other two resonances, located at 2.07 eV and 6 eV, to the B1 symmetry. These results compare well with the attachment energies obtained through electron transmission spectroscopy data and with dissociative electron attachment results for the NO2- sub-product, suggesting for the latter that the π* resonances mediate the dissociation process.

Raman scattering in right angle configuration on Cu2ZnSiSe4 single crystals

Polarized Raman scattering and resonance Raman scattering spectra of Cu2ZnSiSe4 crystals measured at temperature 300 and 10 K were investigated. Nine vibrational modes of A2 symmetry, seven modes of B2 symmetry and nine modes of B1 symmetry were determined in Raman spectra taken at right angle configuration from the (2 1 0) crystal plane. A resonance Raman scattering with participation of 2LO, 3LO and more phonons was observed at photon energies higher than the ground state of exciton transition at low temperature.

Surface optical phonon modes in hexagonal shaped Al0.97Ga0.03N nanostructures

Raman modes of plasma assisted molecular beam epitaxy grown, c-plane oriented and hexagonal shaped cylindrical Al0.97Ga0.03N nanostructures are reported. Apart from the group theoretically allowed optical phonons at zone center (wave vector, q = 0), two additional phonon modes around 763 and 846 cm−1 are observed in between the reported values of longitudinal optical (LO q=0) and transverse optical (TO q=0) phonon modes. The observed phonon mode frequencies are matched with the simulated dispersion curve for the surface optical (SO) phonon modes and subsequently assigned as SO(A 1) and SO(E 1) with A 1 and E 1 symmetries, respectively. A change in the dielectric constant of the surrounding medium using CCl4 is demonstrated to show a significant red shift of the additional modes with respect to those in air, and they show good agreement with the calculated SO phonon frequencies. The origin of SO phonon modes are attributed to the relaxation of Raman selection rules away from the Brillouin zone centre because of the presence of periodic surface modulations of sharp edges at the corners of well-faceted Al0.97Ga0.03N hexagonal crystallites in the sample. An appropriate quantification is also made in this regard to find out the Fourier component of the surface potential responsible for activating the SO mode.

Vibration and vibration-torsion levels of the S1 state of para-fluorotoluene in the 580-830 cm-1 range: Interactions and coincidences.

A study of the vibration and vibration-torsion levels of para-fluorotoluene in the 580-830 cm-1 region is presented, where a number of features are located whose identity is complicated by interactions and overlap. We examine this region with a view to ascertaining the assignments of the bands; in particular, identifying those that arise from interactions involving various zero-order states (ZOSs) involving both vibrations and torsions. Resonance-enhanced multiphoton ionization (REMPI) is employed to identify the wavenumbers of the relevant transitions, and subsequently zero-kinetic-energy (ZEKE) spectra are recorded to assign the various eigenstates. In some cases, a set of ZEKE spectra are recorded across the wavenumber range of a REMPI feature, and we construct what we term a two-dimensional ZEKE (2D-ZEKE) spectrum, which allows the changing ZOS contributions to the eigenstates to be ascertained. Assignment of the observed bands is aided by quantum chemical calculations and all b1 and a2 symmetry vibrational wavenumbers are now determined in the S1 state and cation, as well as those of the D10 vibration. We also compare to the activity seen in the corresponding S1 ← S0 spectrum of para-difluorobenzene.

Żabińskiite, ideally Ca(Al0.5Ta0.5)(SiO4)O, a new mineral of the titanite group from the Piława Górna pegmatite, the Góry Sowie Block, southwestern Poland

AbstractŻabińskiite, ideally Ca(Al0.5Ta0.5)(SiO4)O, was found in a Variscan granitic pegmatite at Piława Górna, Lower Silesia, SW Poland. The mineral occurs along with (Al,Ta,Nb)- and (Al,F)-bearing titanites, a pyrochlore-supergroupmineral and a K-mica in compositionally inhomogeneous aggregates, ∼120 μm × 70 μm in size, in a fractured crystal of zircon intergrown with polycrase-(Y) and euxenite-(Y). Żabińskiite is transparent, brittle, brownish, with a white streak, vitreous lustre and a Mohs hardness of ∼5. The calculated density for the refined crystal is equal to 3.897 g cm–3, but depends strongly on composition. The mineral is non-pleochroic, biaxial (–), with mean refractive indices ≥1.89. The (Al,Ta,Nb)-richest żabińskiite crystal,(Ca0.980Na0.015)∑=0.995(Al0.340Fe3+0.029Ti0.298V0.001Zr0.001Sn0.005Ta0.251Nb0.081)∑=1.005[(Si0.988Al0.012)O4.946F0.047(OH)0.007)∑=5.000];60.7 mol.% Ca[Al0.5(Ta,Nb)0.5](SiO4)O; is close in composition to previously described synthetic material. Żabińskiite is triclinic (space group symmetryA1) and has unit-cell parametersa= 7.031(2) Å,b= 8.692(2) Å,c= 6.561(2) Å, α = 89.712(11)°, β = 113.830(13)°, γ = 90.352(12)° andV= 366.77 (11) Å3. It is isostructural with triclinic titanite and bond-topologically identical with titanite and other minerals of the titanite group.Żabińskiite crystallized along with (Al,Ta,Nb)-bearing titanites at increasing Ti and Nb, and decreasing Ta activities, almost coevally with polycrase-(Y) and euxenite-(Y) from Ca-contaminated fluxed melts or early hydrothermal fluids.

Infrared dielectric functions, phonon modes, and free-charge carrier properties of high-Al-content AlxGa1−xN alloys determined by mid infrared spectroscopic ellipsometry and optical Hall effect

We report on the analysis of a combined mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect investigation of wurtzite structure c-plane oriented, crack-free, single crystalline, and high-Al-content AlxGa1−xN layers on 4H-SiC. For high-Al-content AlxGa1−xN, a two mode behavior is observed for both transverse and longitudinal branches of the infrared-active modes with E1 symmetry, while a single mode behavior is found for the longitudinal modes with A1(LO) symmetry. We report their mode dependencies on the Al content. We determine and discuss static and high frequency dielectric constants depending on x. From the analysis of the optical Hall effect data, we determine the effective mass parameter in high-Al-content AlxGa1−xN alloys and its composition dependence. Within the experimental uncertainty limits, the effective mass parameters are found isotropic, which depend linearly on the Al content. The combination of all data permits the quantification of the free electron density N and mobility parameters μ.

Optical properties of AlGaN nanowires synthesized via ion beam techniques

AlGaN plays a vital role in hetero-structure high electron mobility transistors by employing a two-dimensional electron gas as an electron blocking layer in multi-quantum well light emitting diodes. Nevertheless, the incorporation of Al into GaN for the formation of the AlGaN alloy is limited by the diffusion barrier formed by instant nitridation of Al adatoms by reactive atomic N. The incorporation of Al above the miscibility limit, however, can be achieved by the ion beam technique. The well known ion beam mixing (IBM) technique was carried out with the help of Ar+ irradiation for different fluences. A novel approach was also adopted for the synthesis of AlGaN by the process of post-irradiation diffusion (PID) as a comparative study with the IBM technique. The optical investigations of AlGaN nanowires, synthesized via two different methods of ion beam processing, are reported. The effect of irradiation fluence and post-irradiation annealing temperature on the random alloy formation was studied by the vibrational and photoluminescence (PL) spectroscopic studies. Vibrational studies show one-mode phonon behavior corresponding to the longitudinal optical (LO) mode of A1 symmetry [A1(LO)] for the wurtzite phase of AlGaN nanowires in the random alloy model. A maximum Al atomic percentage of ∼6.3%–6.7% was calculated with the help of band bowing formalism from the Raman spectral analysis for samples synthesized in IBM and PID processes. PL studies show the extent of defects present in these samples.

Development and Implementation of Advanced Fitting Methods for the Calculation of Accurate Molecular Structures.

The determination of accurate equilibrium molecular structures plays a fundamental role for understanding many physical-chemical properties of molecules, ranging from the precise evaluation of the electronic structure to the analysis of dynamical and environmental effects in tuning their overall behavior. For this purpose the so-called semiexperimental approach, based on a nonlinear least-squares fit of the moments of inertia associated with a set of available isotopologues, allows one to obtain very accurate results, without the unfavorable computational cost characterizing high-level quantum chemical methods. In the present work the MSR (Molecular Structure Refinement) software for the determination of equilibrium structures by means of the semiexperimental approach is presented, and its implementation is discussed in some detail. The software, which is interfaced with a powerful graphical user interface, includes different optimization algorithms, an extended error analysis, and a number of advanced features, the most remarkable ones concerning the choice of internal coordinates and the method of predicate observations. In particular, a new black-box scheme for defining automatically a suitable set of nonredundant internal coordinates of A1 symmetry in place of the customary Z-matrix has been designed and tested. Finally, the implementation of the method of the predicate observations is discussed and validated for a set of test molecules. As an original application, the method is employed for the determination of the semiexperimental structure for the most stable conformer of glycine.

A combined theoretical and experimental study of the valence and Rydberg states of iodopentafluorobenzene.

A new ultraviolet (UV) and vacuum ultraviolet (VUV) spectrum for iodopentafluorobenzene (C6F5I) using synchrotron radiation is reported. The measurements have been combined with those from a recent high-resolution photoelectron spectroscopic study. A major theoretical study, which includes both Franck-Condon (FC) and Herzberg-Teller (HT) analyses, leads to conclusions, which are compatible with both experimental studies. Our observation that the VUV multiplet at 7.926 eV in the VUV spectrum is a Rydberg state rather than a valence state leads to a fundamental reassignment of the VUV Rydberg spectrum over previous studies and removes an anomaly where some previously assigned Rydberg states were to optically forbidden states. Adiabatic excitation energies (AEEs) were determined from equations-of-motion coupled cluster with singles and doubles excitation; these were combined with time dependent density functional theoretical methods. Frequencies from these two methods are very similar, and this enabled the evaluation of both FC and HT contributions in the lower valence states. Multi-reference multi-root configuration interaction gave a satisfactory account of the principal UV+VUV spectral profile of C6F5I, with vertical band positions and intensities. The UV spectral onset consists of two very weak transitions assigned to 11B1 (πσ*) and 11B2 (σσ*) symmetries. The lowest unoccupied molecular orbital of a σ*(a1) symmetry has a significant C-I* antibonding character. This results in considerable lengthening of the C-I bond for both these excited states. The vibrational intensity of the lowest 11B1 state is dominated by HT contributions; the 11B2 state contains both HT and FC contributions; the third band, which contains three states, two ππ*(11A1, 21B2) and one πσ*(21B1), is dominated by FC contributions in the 1A1 state. In this 1A1 state, and the spectrally dominant bands near 6.7 (1A1) and 7.3 eV (1A1 + 1B2), the C-I bond length is in the normal range, and FC components dominate.

Effect of Gd3+ doping on structural and optical properties of ZnO nanocrystals

To investigate the effect of rare earth ions on the excitation energy redistribution between the UV and visible photoluminescence, the Gd3+ doped ZnO nanocrystals prepared through Sol-Gel method were studied. Different structural characterizations and spectroscopic studies were carried out to establish a correlation between the structural effect and the photoluminescence. Our study indicates that the incorporation of Gd3+ in ZnO nanocrystals prepared through this synthesis takes place predominantly on the surface up to 0.18 mole fraction of Gd3+. This is manifested by the observed lattice contraction due to the presence of Gd3+ on the surface. This structural defect introduces various defects that modifies the photoluminescence properties. For these modifications in crystal system, the A1 symmetry phonon modes of wurtzite structure are found to be gradually more Raman active than the other modes as the Gd3+ concentration is increased while the E modes are suppressed. These doping induced modification of phonon spectrum strongly influences the exciton phonon coupling and thereby the photophysical properties. Resulting crystal imperfections strongly enhance the defect band luminescence. It is shown that energy transfer from selectively excited Gd3+ centers to surface states can be effectively exploited for the sensitization of defect band luminescence in various fields such as bio imaging, light emitting devices etc.

Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory.

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1+ν3 symmetric plus antisymmetric C-H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1+ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0(2ν1) > S0(ν1 + ν3) > S0(2ν3). The 2ν3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry.