Electron paramagnetic resonance of deep boron acceptors in 4H-SiC and 3C-SiC crystals

EPR spectra of deep boron in 4H-SiC and 3C-SiC crystals have been observed and studied. Two sites in 4H-SiC produced deep-boron EPR signals, quasi-cubic k and hexagonal h. In both cases the deep-boron center symmetry is close to axial along the c crystal axis, and the g factor anisotropy is about an order of magnitude larger than that for shallow boron centers. In the 3C-SiC crystal, the deep-boron symmetry is also close to axial along one of the four 〈111〉 directions. The model proposed for the deep boron center with acceptor properties is B_Si-v _C, where B_Si is the boron substituting for silicon, and v _C is the carbon vacancy, with the B_Si-v _C direction coinciding in 4HSiC with the hexagonal axis of the crystal for both k and h positions. In the cubic 3C-SiC crystal, there are four equivalent deep boron centers, which represent B_Si-v _C pairs with the bond directed along one of the four 〈111〉 crystal directions. Fiz. Tverd. Tela (St. Petersburg) 40, 36–40 (January 1998)     

Electronic structures of trithiapentalene analogues

The electronic structures of trithiapentalene analogues are studied by ab initio molecular orbital methods and density functional theory. Calculation of the normal vibration frequencies for substituted trithiapentalenes with C_2v symmetry indicates that σ-substituted trithiapentalene structures with C_2v symmetry are more destabilized than by substitution with the π-type group. CiLC-IRC analysis of the electronic structures for trithiapentalene analogues is performed for the pathway between the C_2v and C_s structures for 1,6,6aλ⁴-trithiapentalene and 1,6-dithia-6aλ⁴-oxapentalene, and it is found that the diradical states in both compounds are a critical factor in determining the stability of the structures with C_2v symmetry. Furthermore, the electronic state of 1,6,6aλ⁴-trithiapentalene with C_2v symmetry has less diradical character than that of 1,6-dithia-6a-oxapentalene.     

Symmetry and structure of N–O shallow donor complexes in silicon

Shallow donors in silicon related to nitrogen–oxygen complexes have been investigated by piezospectroscopy of their hydrogenic transitions in the far infrared. Complete stress dependences up to 0.25 GPa were obtained for the 1s→2p₀ and 1s→2p_± transitions of the most prominent members of the (N, O)-family, N–O-3 and N–O-5. Very unusual for shallow donors in silicon, the symmetry of the ground state wave function is T₂-like. The lifting of orientational degeneracy for stress in the 〈1 0 0〉, 〈1 1 1〉, and 〈1 1 0〉 directions is compatible with a C_2v defect symmetry. Data from the other species of the (N, O)-family are indicative for the same symmetry. The microscopic structure of these centers, in part contradictory to present theoretical models, is discussed.     

Theoretical study on the Cope rearrangement mechanisms and the homoaromaticity of semibullvalene,barbaralane, and 1,5‐methanosemibullvalene

Cope rearrangement mechanisms and the homoaromaticity of semibullvalene, barbaralane, and 1,5‐methanosemibullvalene in the ground and lowest excited states were studied by ab initio methods. In the ground state, the rearrangement reactions of semibullvalene and barbaralane occurred concertedly through the transition states with C_2v symmetry, and the transition states had a homoaromatic nature. In particular, the transition state of barbaralane exhibited the strongest homoaromaticity among the three systems treated here. On the other hand, for 7,8‐methanosemibullvalene, the structure with C_2v symmetry was not a transition state but one with a stable energy minimum. The energy minimum structure with C_2v symmetry had a biradical character. The lowest excited states of semibullvalene and barbaralane were the excitation to the σ* anti‐orbital, ¹B₂ and ¹B₁ states, and led to near di‐allyl states. The lowest excitation state of 1,5‐methanosemibullvalene had C_s symmetry and was the A″ state excitation in one side of two allyl parts. Copyright © 2011 John Wiley & Sons, Ltd.     

Electron affinities of the bromine oxides BrOn,n = 1-4

Bromine oxides are of significant interest due to their importance in atmospheric chemistry. Density functional theory (DFT) methods have been used in conjunction with a DZP++ (double-ζ plus polarization with diffuse functions) basis set to study the molecular geometries and total energies of BrOn and BrO^- _n (n = 1-4). The adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vert) of the bromine oxide and the vertical detachment energy (VDE) of each anion are reported. Harmonic vibrational frequencies and zero point energies are also reported. Five different DFT methods were employed for comparison. Among these, the BHLYP method predicts the geometries and the vibrational frequencies in best agreement with available experimental data, while the the other methods do better in predicting the limited number of energetic quantities determined observationally. The predicted adiabatic electron affinities are 2.38 eV (BrO, experiment 2.35 eV), 2.36 eV (BrO₂), 3.35 eV (BrOO), 4.32 eV (BrO₃), 2.91 eV (BrOOO) and 5.28 eV (BrO₄). The electronic ground state of BrOO^- is a triplet (³A^") state. Predicted Br–;O bond distances range from 1.61 (BrO₃) to more than 2 Å for Br...O₂ for the neutral molecules; and from 1.61 Å for BrO^- to 1.82 Å for BrO^- to more than 2 Å for Br^-...O₃ among the anions. The BrOO isomer (C_s symmetry) is predicted to lie 17–18 kcal mol^-1 below the C_2v symmetry OBrO structure. The asymmetric ³A^" anion BrOO^- analogously lies below OBrO^-, in this case by 40–41 kcalmol_-1. BrOOO (C_s symmetry) is predicted to lie 42-45 kcalmol-1 below the symmetric C_3v BrO structure. Finally the asymmetric BrOOO- anion (C₁ symmetry) is predicted to lie 10-13kcal mol-1 below symmetric C_3v BrO^- ₃.     

Competition mechanism between singlet and triplet superconductivity in the tight-binding model with anisotropic attractive potential

Based upon the tight-binding formalism a model of a high-T_c superconductor with isotropic and anisotropic attractive interactions is considered analytically. Symmetry facets of the group C_4v are included within a method of successive transformations of the reciprocal space. Complete sets of basis functions of C_4v irreducible representations are given. Plausible spin-singlet and spin-triplet superconducting states are classified with regard to the chosen basis functions. It is displayed that pairing interaction coefficients and the dispersion relation, which can be characterized by the parameter η= 2t₁/t₀, have a diverse and mutually competing influence on the value of the transition temperature. It is also shown that in the case of a nearly half-filled conduction band and an anisotropic pairing interaction the spin-singlet d-wave symmetry superconducting state is realized for small values of the parameter η, whereas in the opposite limit, for sufficiently large values, the spin-triplet p-wave symmetry superconducting state has to be formed. This result cannot be obtained within the Van Hove scenario or BCS-type approaches, where the p-wave symmetry superconducting state absolutely dominates. The specific heat jump and the isotope shift as functions of the parameter η are assessed and discussed for the d-wave symmetry singlet and the p-wave symmetry triplet states.     

Spin-spin and spin-other-orbit effects of optical spectra and the spin-Hamiltonian parameters for Cr ions in MgO crystals

An extended complete diagonalization method/microscopic spin-Hamiltonian (CDM/MSH) program has been developed, which is applicable for d³ ions at sites of tetragonal symmetry type I (C_4v, D_2d, D₄, D_4h) and trigonal symmetry type I (C_3v, D₃, D_3d). The Hamiltonian includes the spin-spin (SS) and spin-other-orbit (SOO) magnetic interactions besides the spin-orbit (SO) magnetic interaction usually taken into account. Utilizing the extended CDM/MSH program, the optical spectra, the spin-Hamiltonian (SH) parameters of the ground state ⁴B₁, and the splitting δ(²E) of the first excited ²E state for Cr³⁺ (3d³) ions at C_4v symmetry sites in MgO crystals have been successfully investigated. It is found that although the SO magnetic interaction is the most important one, the contributions to the SH parameters and the optical spectra from the SS and SOO magnetic interactions for Cr³⁺:MgO crystals are appreciable and should not be omitted, especially reaching 27.8% for the zero field splitting parameter D.     

An incoherent inelastic neutron scattering study of NH4TaWO6

The incoherent inelastic neutron scattering spectrum of the defect pyrochlore, NH₄TaWO₆, has been determined at 20 K. The appearance of the spectrum indicates that the NH ₄ ⁺ ion is distorted from its free ion tetrahedral geometry. A profile refinement method has been used to fit a calculated spectrum to the data based on an ion of C _3v symmetry.     

Molecular picture of excited states and fragmentation paths of the Na 5 F 4 cluster

The stability against fragmentation and possible relaxation of the lowest excited states of the Na₅F₄ cluster (representative of cubic non stoechiometric clusters with an excess sodium atom, also called sodium-tail) is investigated by means of one-electron pseudopotential calculations with particular reference to photoabsorption processes from the ground state. Whereas the equilibrium configuration of the ground state has C_3v symmetry, the doubly degenerate 1²E excited state is affected by a conical intersection and a Jahn-Teller effect associated with the rotation of the sodium tail around the C₃-axis. This yields a “Mexican hat" topology for the lowest sheet with three equivalent C_s minima. Alternatively the 2²A₁ state has a minimum retaining the C_3v symmetry. The dissociation paths of the cluster along the C₃-axis into respectively Na₄F_{4 +} Na and Na₄F_{3 +} NaF are also investigated. Among the former paths, the excited states are found adiabatically stable with respect to the products. However in the A₁ symmetry, fragmentation into NaF exhibits an interesting avoided crossing between configurations correlated respectively with Na₄F₃ ^{+ +} NaF^- and Na₄F_{3 +} NaF. Such interaction, similar to the well-known charge exchange processes in elementary molecules might induce non adiabatic predissociation of the 2²A₁ state. This mechanism is invoked to explain the differences between R2PI and depletion spectra, correlated with the dissociation or relaxation of the excited states. Received 24 March 2000 and Received in final form 11 July 2000     

High-resolution FTIR spectroscopy of the v11 and v2 + v7 bands of ethylene-d4

The Fourier transform infrared spectrum of the v₁₁ band of ethylene-d₄ (C₂D₄) has been recorded with an unapodized resolution of 0.006 cm^?1 in the frequency range 2150 to 2250cm^?1. The v₁₁ band, with a band centre of about 2201 cm^?1, was found to be perturbed by the nearby v₂ + v₇ band centred at about 2235 cm^?1 by a b-type Coriolis interaction. By fitting a total of 772 infrared transitions of v₁₁ using a Watson's A-reduced Hamiltonian in the I^r representation with the inclusion of b-type Coriolis interaction term, two sets of constants, up to quartic distortion constants for the v₁₁ = 1 state, and principal rotational constants for the v₂ + v₇ = 1 dark state, were derived. The inertia defect of the v₁₁ state was found to be 0.0693 ± 0.0004u Ų.     

C 60 and the icosahedral group Ih: Simplification methods and group-theoretical applications

Group-theoretical methods are used to induce “global” distortions of C ₆₀ from basis function(s) of a single “local” distortion in two cases: (1) The 60 vibrational modes arising from radial displacements of carbon atoms are found in terms of basis functions of A′, the one-dimensional symmetric representation of C_s; (2) The 24 vibrational modes arising from tangential displacements of hypothetical atoms placed at pentagon centers are found in terms of basis functions of E ₁, the vector irreducible representation of C_5v . The induction process is simplified by an icon notation (v) or (vμ) which uniquely labels the operations in I_h if μ,v = 0, 1, 2, 3, 4 and under/over lining of a digit is included. These icons describe the transformations IUS^vTS^μ of Felix Kline referred to a five-fold z-axis and two-fold x-axis and serve to label and “distinguish” symmetrically equivalent points of the truncated icosahedron.     

Manifestations of the dynamic Jahn-Teller effect in trigonally distorted defect centres

General effects resulting from the dynamic Jahn-Teller interaction on the single cubic symmetry T ₂ orbital state, which in C_3v forms a basis for the A ₁ + E symmetry representations, are investigated. As a result of the discussion of the types of centre in various host lattices which are likely to exhibit these effects, we are led to consider the centre responsible for the ‘self-activated emission’ in II–VI semiconductors. It is shown that a combination of theory with the experimental E.S.R. and optical polarization data strongly suggests that a dynamic Jahn-Teller interaction is occurring in these centres.     

l-type doubling of the kl = −1 levels in the microwave spectrum of the C4v molecule BrF5

Measurements are reported for the J 3 → 4, 4 → 5, and 5 → 6 transitions of BrF₅ in the excited vibrational states v₅(B₁) = 1 and v₉(E) = 1. These two states are nearly degenerate and an unusually strong Coriolis interaction perturbs both excited state spectra. New expressions are obtained for the E-species absorption frequencies which are valid in a strong Coriolis resonance situation. The analysis of the E-state spectrum has provided the first experimental observation of the doubling of the kl = −1 levels predicted for molecules with C_4v symmetry.     

Dissymmetrization in X-irradiated RbTiOPO4 crystal

Electron paramagnetic resonance has been used to study the hole and electron paramagnetic centers formed in X-irradiated RbTiOPO₄, the crystals of the KTP family. X-irradiation of RbTiOPO₄ crystals at 77 K produced an oxygen hole center and four different trivalent titanium electron centers I₁, II, III and IV. Theg-tensors, their principal values and axes for the defects were calculated and compared with those for KTiOPO₄ centers. X-irradiation at 300 K produced another two oxygen hole centers and three electron centers I₁, I₂ and II. EPR spectra of the center II revealed dissymmetrization, i.e., irregular distribution of growth defects, between the physically equivalent sites lowering the point group symmetry of the local environment of paramagnetic centers Ti³⁺.     

SiX2(X=H，F)分子的结构与势能函数

应用QCISD/6-311++G(3df,3pd)和B3P86/6-311++G(3d2f)对SiH₂,SiF₂的结构进行了优化,优化出SiH₂分子的稳定构型为C_2v,电子态为¹A₁,其平衡核间距R_e=0.15149nm、键角∠HSiH=92.5025°、离解能为3.7098eV. SiF _{关键词： 2}')" href="#">SiH₂ 2')" href="#">SiF₂ 多体项展式理论 势能函数     

Tropolone monomer: Vibrational spectrum and proton tunneling

Infrared matrix-isolation spectra are reported for the tropolone monomers C₇H₅O₂H and C₇H₅O₂D. Tropolone possesses an intramolecular hydrogen bond and the possibility for proton tunneling from one oxygen atom to the other. Clearcut multiplets attributed to tunneling are observed for the OD stretching mode and for heavy atom modes of both C₇H₅O₂D and C₇H₅O₂H. The (mixed) carbonyl stretching mode particularly seems to facilitate tunneling from one conformation to the other. The tunneling phenomena suggests that tropolone monomer has nearly C_2v symmetry. As a crude estimate, the tunneling potential energy barrier is calculated to be less than 5600 cm^?1 in the ground electronic state of C₇H₅O₂D. The barrier is lower in the $\text{Π}{}^1$ electronic state than in the ground electronic state. A vibrational assignment that encompasses most of the 39 fundamental modes is proposed. The vibrations are classified using C_2v symmetry species and a parallel vibrational analysis is presented for tropone, C₇H₆O, which is a true C_2v molecule.     

Infrared spectra of solid germane

An IR spectroscopic study has shown that GeH₄ molecules are situated on one set of sites of C, symmetry in phase IV. The III–IV transition at 63 K is apparently a second-order phase transition. In phases II and III the molecules are on sites of C_3v. or C₃ symmetry. The II–III phase transition was observed at 67 K. In solid GeD₄, phase transitions were observed at 68.5 and 77 K. In phases II and III the site symmetry is C_I. The II–III phase transition in GeD₄ is apparently second order. There is evidence that the ν₁, vibration of GeD₄ is IR active in the solid state.     

BeH2(X1Σ+g)与H2S(X1A1)分子的结构与解析势能函数

运用单双取代二次组态相关(QCISD)方法,在6-311++G(3df,3pd)基组水平上,对BeH₂和H₂S分子的结构进行了优化计算,得到基态BeH₂分子的稳定结构为D_∞h构型,电子态为X¹Σ⁺_g,平衡核间距R_BeH=0.13268nm,R_{关键词： 2}')" href="#">BeH₂ 2S')" href="#">H₂S Murrell-Sorbie函数 多体项展式理论 解析势能函数     

Ferromagnetic and antiferromagnetic ordering in the wurtzite-type diluted magnetic semiconductors

We determined the magnetic symmetry groups (Shubnikov groups) of the wurtzite-type crystals doped with magnetic atoms periodically distributed at cation sites. The magnetic groups of doped crystals with ferro- or antiferromagnetic properties arise from the P6₃ mc (C _6v ⁴), P3m1 (C _3v ¹), C _s ³ (Cm) or C ₁¹ (P1) ordinary space groups. Those arising from the P6₃ mc or P3m1 group can present macroscopic ferromagnetic polarization only along the wurtzite symmetry axis whereas those arising from the C _s ³ (Cm) group can present ferromagnetic polarization only in the symmetry plane and those arising from the C ₁¹ (P1) group can present polarization along any direction. Actual structures do not keep in general long-range order for magnetic atom distribution but the samples with macroscopic polarization should present local symmetries close to those listed above. Therefore, orientation of easy magnetization can provide information about the most frequent local symmetry.     

Theoretical investigation of a single erbium center in hexagonal gallium nitride

A theoretical energy-level analysis was based on parametric Hamiltonian for the 4f¹¹ electronic configuration of Er³⁺ ions in C_3v center, and it permitted a phenomenological characterization of crystal-field (CF) Hamiltonian parameters. This characterization allowed us to calculate the energy of the missing stark levels of the ⁴I_15/2 and ⁴I_13/2 states and to confirm the presence of a single emission center with C_3v symmetry. The calculated CF and strength parameters are compared with those obtained for Sm³⁺, Pr³⁺ and Yb³⁺ ions in the same host and with Er³⁺ in other isostructural hosts.