A1 Symmetry Research Articles

Nonlinear optical properties of glassy TeO[formula omitted]: ab initio modeling of [formula omitted] and Hyper Raman spectra

The nonlinear optical properties of amorphous tellurium oxide is studied in a framework of computational technique based on the ab initio quantum-chemical calculations. The calculated values of refractive index and third-order nonlinear susceptibility are in a very good agreement with experimental data. The origin of the high nonlinear properties of TeO2 is attributed, in general, to the hyperpolarizability of Te atoms electron lone pairs and to the hyperpolarizability of electronic density localized on bonds of the asymmetric Te-O-Te bridges in part. The computational technique is extended on Hyper Raman spectroscopy including scattering on longitudinal modes. The spectra was calculated for both crystalline and glassy TeO2 with a very good reproduction of experimental data. An extra bands inactive in classical Raman spectroscopy is studied using this aproach. The most intense high-frequency band in the spectra of paratellurite is attributed to anti-symetric stretching of long-short bonds in TeO4 disphenoid with A2 symmetry. The influence of the structural unit configuration on the band position in Hyper Raman spectra is established. The best agreement between calculated phonon spectrum and experimental data is obtained using hybrid functional approach, which explains strong correlation on valence electron states of Te atoms. The strong dependence of LO modes intensity in Hyper Raman spectra on third order nonlinear susceptibility is obtained and explained. The reported technique can be easily extended on a big family of binary and ternary compounds.

Modeling ultrafast anharmonic vibrational coupling in gas-phase fluorobenzene molecules.

In this work, we study the energy flow through anharmonic coupling of vibrational modes after excitation of gas-phase fluorobenzene with a multi-THz pump. We show that to predict the efficiency of anharmonic energy transfer, simple models that only include the anharmonic coupling coefficients and motion of modes at their resonant frequency are not adequate. The full motion of each mode is needed, including the time while the mode is being driven by the pump pulse, because all the frequencies present in the multi-THz pump contribute to the excitation of the non-resonantly excited vibrational modes. Additionally, the model gives us the insight that modes with either A1 or B2 symmetry are more actively involved in anharmonic coupling because these modes have more symmetry-allowed energy transfer pathways.

Polarized Raman, infrared and dielectric spectra of lead-free K0.5Na0.5NbO3 piezoelectric system: insights from ab-initio theoretical and experimental studies

The K0.5Na0.5NbO3 (KNN) system has emerged as one of the most promising lead-free piezoelectric over the years. In this work, we perform a comprehensive investigation of electronic structure, lattice dynamics and dielectric properties of room temperature phase of KNN by combining ab-initio DFT based theoretical analysis and experimental characterization. We assign the symmetry labels to KNN vibrational modes and obtain ab-initio polarized Raman spectra, Infrared reflectivity, Born-effective charge tensors, oscillator strengths etc. The KNN ceramic samples are prepared using conventional solid-state method and Raman and UV–Vis diffuse reflectance spectra are obtained. The computed Raman spectrum is found to agree well with the experimental spectrum. In particular, the results suggest that the mode in range ∼840–870 cm−1 reported in the experimental studies is longitudinal optical with A1 symmetry. The Raman mode intensities are calculated for different light polarization set-ups that suggests the observation of different symmetry modes in different polarization set-ups. The electronic structure of KNN is investigated and optical absorption spectrum is obtained. Further, the performances of DFT semi-local, meta-GGA and hybrid exchange-correlations functionals, in the estimation of KNN band gaps are investigated. The KNN bandgap computed using GGA-1/2 and HSE06 hybrid functional schemes are found to be in excellent agreement with the experimental value. The COHP, electron localization function and Bader charge analysis is also performed to deduce the nature of chemical bonding in the KNN. Overall, our study provides several bench-mark important results on KNN that have not been reported so far.

Optimal control of N-H photodissociation of pyridinyl.

The N-H photodissociation dynamics of the pyridinyl radical upon continuous excitation to the optically bright, first excited ππ* electronic state by an ultra-violet (UV) laser pulse has been investigated within the mathematical framework of optimal control theory. The genetic algorithm (GA) is employed as the optimization protocol. We considered a three-state and three-mode model Hamiltonian, which includes the reaction coordinate, R (a1 symmetry); the coupling coordinates (namely, out-of-plane bending coordinate of the hydrogen atom of azine group), Θ (b1 symmetry); and the wagging mode, Q9 (a2 symmetry). The three electronic states are the ground, ππ*, and πσ* states. The πσ* state crosses both the ground state and the ππ* state, and it is a repulsive state on which N-H dissociation occurs upon photoexcitation. Different vibrational wave functions along the coupling coordinates, Θ and Q9, of the ground electronic state are used as the initial condition for solving the time-dependent Schrödinger equation. The optimal UV laser pulse is designed by applying the GA, which maximizes the dissociation yield. We obtained over 95% dissociation yield through the πσ* asymptote using the optimal pulse of a time duration of ∼30 000a.u. (∼725.66fs).

Significant phonon assignments in multiferroic BiFeO3 single crystal

In the present work, the technique of Back Scattering Micro-Raman was employed at low and room temperatures in order to examine all the possible phonon vibration modes present in a crystal of BiFeO3(BFO). The observation & identification of all E (TO), E (LO), A1 (TO), and A1 (LO) normal modes was sucessfully accomplished. The low temperature Raman spectroscopy measurements showed two one-magnon branches, σ-magnon at 20.06 cm−1 and γ-magnon at 26.93 cm−1. The spectral characteristics of the three A1 (LO) modes were observed at roughly 176.63 cm−1, 225.93 cm−1 and 505.53 cm−1, exhibiting low intensity longitudinal optical modes. The observed peaks at 145.34 cm−1, 223.92 cm−1, 292.54 cm−1, and 552.18 cm−1 were identified and categorized as A1 (TO) modes. E (TO) peaks were observed at 73.30 cm-1, 131.94 cm-1, 239.35 cm-1, 265.04 cm-1, 278.45 cm-1, 349.95 cm-1, 374.69 cm-1, 441.04 cm-1, 522.99 cm-1. In addition, E(LO) peaks were observed at 80.76 cm-1, 175.03 cm-1, 241.59 cm-1, 275.42 cm-1, 346.92 cm-1, 371.34 cm-1, 419.38 cm-1, and 469.65 cm-1.

BaTiO3 solid solutions co-doped with Gd3+ and Eu3+: Synthesis, structural evolution and dielectric properties

In this paper, the synthesis, structural evolution, and dielectric properties of BaTiO3 solid solutions co-doped with rare earth elements (Gd3+ and Eu3+) prepared using the solid-state reaction method are presented. Chemically pure precursor powders of BaCO3, TiO2, Gd2O3, and Eu2O3 were mixed in stoichiometric proportions, ground in an agate mortar before being uniaxially pressed at 250 MPa and sintered at 1300 °C in air atmosphere for 6 h to obtain Ba1–3xGd2xTi1–3xEu4xO3 (x = 0, 0.0015, 0.01 and 0.1). X-ray diffraction (XRD) and Rietveld's refinement results reveal the crystal phase ferroelectric tetragonal BaTiO3 for the samples with x = 0, 0.0015 and 0.01, as well as a consistent increment in the lattice parameters caused by the doping. The solubility limit of Gd3+ and Eu3+ in the BaTiO3 structure is reached for the sample with x > 0.01, and the orthorhombic Eu2TiO5 and monoclinic BaTi2O5 secondary phases were identified. The Raman results show the characteristic peaks of ferroelectric tetragonal BaTiO3 around 716 cm−1 (LO of A1 symmetry), 515 cm−1 (TO of A1 symmetry), and 305 cm−1 (B1). The maximum relative permittivity measured at 1 kHz was recorded to be 6151.8 for the sample with x = 0.0015, and a decrease in the Curie temperature to 105 °C with respect to the undoped sample was observed. The punctual microanalysis for the samples with x = 0.0015, 0.01, and 0.1 reveals grains composed of Ba, Ti, O, Gd, and Eu homogeneously distributed in the BaTiO3 structure. The Ba/Ti ratios for samples with x = 0.0015, 0.01 and 0.1 are 2.86, 2.91, and 0.68, respectively, indicating substitution at the Ti site for samples with x = 0.0015 and 0.01 and a substitution at the Ba site for the sample with x = 0.1.

Engineering the electro-optic effect in HfO2 and ZrO2 through strain and polarization control

The ability to control the optical properties of a material with an electric field has led to optical memory devices, communication systems, optical signal processing, or quantum cryptography. Understanding electro-optic effects, especially in thin films, would improve the efficiency of these applications. In particular, the influence of epitaxial strains is of prime importance. In addition, the active control of these effects would be of great interest to tailor the material to the desired performance. Here, we demonstrate through first-principle calculations that the linear electro-optic response (Pockels effect) of two silicon-compatible ferroelectrics is stable with respect to bi-axial strain and that the electro-optic response can be strongly enhanced through the electrical control of the polarization. We attribute the former to the lack of optical phonon softening and a weak elasto-optic response and the latter to the externally induced softening of a phonon of symmetry A1. Our results are readily applicable to other polar materials and show that the electro-optic effect can be efficiently engineered to meet the performance criteria of future technologies.

Formula omitted]-type doubling in the rotational spectrum of CH[formula omitted]–He: A group theoretical explanation

In a previous paper, we reported the pure rotational spectrum of the CH3+–He ion observed in a 22-pole ion trap by IR-MW double resonance spectroscopy [Töpfer et al. (2018)]. In that work the K=1 rotational levels in the ground state of the ion were observed to be split, i.e. the K-type doubling was observed. This finding would be consistent with a slightly asymmetric rotor of Cs symmetry, although ab-initio calculations predict that the equilibrium structure of the ion is of C3v symmetry. In the present study we inspect the energy-level correlation-diagram between the semi-rigid symmetric-top (C3v), the free internal-rotor (C3v(M)), and the semi-rigid asymmetric-top models (Cs), by using group theory, especially the correlation and reverse correlation in the symmetry representations. We find that the lowest K=1 rotational levels of the CH3+–He ion can be of A1⊕A2 symmetry rather than of E symmetry, and thus they can be split depending on the nature of the C–He bending vibration.

Anomalous LO-TO splitting observed by combined IR reflectance and Raman scattering in KTiOPO4 (KTP) single crystal

A comprehensive study of vibrational spectra of a KTP single crystal is presented using both Raman scattering and Infrared (IR) reflectance. The 196 mode frequencies are listed according to a revised group theoretical analysis, which addresses the different atomic clusters comprising the crystal unit cell and their symmetry. Inverse LO-TO splitting is suggested to interpret the ν3 vibrational spectra of the PO43- clusters of the A1 symmetry. Theoretical account invokes a proximity between strong and weak polar vibrations and Lyddane-Sachs-Teller’s model of LO-TO splitting in a dielectric medium [1]. Many other mode frequencies appear too close to allow unequivocal interpretation of their LO-TO splitting sign.

The different recanalization rates of posterior communicating artery aneurysms with a fetal posterior communicating artery and anterior communicating artery aneurysms with a variation of the unilateral A1 segment.

Posterior communicating artery (PcomA) aneurysms are more likely to recanalize than anterior communicating artery (AcomA) aneurysms. However, it is still unclear whether the recanalization rate of these aneurysms is a result of involvement from the fetal posterior cerebral artery (fPCA) in PcomA aneurysms and variation of the unilateral A1 segment in AcomA aneurysms. The purpose of this study is to retrospectively evaluate the different recanalization rates between PcomA aneurysms with fPCA and AcomA aneurysms with a variation of the unilateral A1 segment. We retrospectively collected information regarding 214 patients, each with communicating segment aneurysms between January 2013 and January 2020. Follow-up documentation on clinical and imaging data was comparatively analyzed between variant types, and recanalization rates of the variant and normal types were analyzed by stratification. Of the 84 variant-type aneurysms (PcomA with fPCA and AcomA with a variation of the unilateral A1 segment, 41/43), complete recanalization occurred in 23 patients (27.4%), and it was significantly more likely to occur in PcomA aneurysms with fPCA (39.1%) than in AcomA aneurysms with a variation of the unilateral A1 segment (16.3%). Stent-assisted coil embolization (SACE) has been shown to reduce recanalization (OR =0.092, 95% CI: 0.011 to 0.790, P=0.03). Additionally, variant types and the normal type (non-fetal, 106, and bilateral A1 symmetry, 24) have different odds ratios (OR) of recanalization (P=0.04), and the OR of the variant subtypes was significant, unlike the normal type (P=0.49). This study suggests that PcomA aneurysms with fPCA are more likely to recanalize than AcomA aneurysms with a variation of the unilateral A1 segment.

Impact of subcallosal artery origin and A1 asymmetry on surgical outcomes of anterior communicating artery aneurysms.

Surgical clipping of anterior communicating artery (ACoA) aneurysms remains challenging due to their complex anatomy. Anatomical risk factors for ACoA aneurysm surgery require further elucidation. The aim of this study is to investigate whether proximity of the midline perforating artery, subcallosal artery (SubCA), and associated anomaly of the ACoA complex affect functional outcomes of ACoA aneurysm surgery. A total of 92 patients with both unruptured and ruptured ACoA aneurysms, who underwent surgical clipping, were retrospectively analyzed from a multicenter, observational cohort database. Association of ACoA anatomy with SubCA origin at the aneurysmal neck under microsurgical observation was analyzed in the interhemispheric approach subgroup (n = 56). Then, we evaluated whether anatomical factors associated with SubCA neck origin affected surgical outcomes in the entire cohort (both interhemispheric and pterional approaches, n = 92). In the interhemispheric approach cohort, combination of A1 asymmetry and aneurysmal size ≥ 5.0mm was stratified to have the highest probability of the SubCA neck origin by a decision tree analysis. Then, among the entire cohort using either interhemispheric or pterional approach, combination of A1 asymmetry and aneurysmal size ≥ 5.0mm was significantly associated with poor functional outcomes by multivariable logistic regression analysis (OR 6.76; 95% CI 1.19-38.5; p = 0.03) as compared with A1 symmetry group in the acute subarachnoid hemorrhage settings. Combination of A1 asymmetry and larger aneurysmal size was significantly associated with SubCA aneurysmal neck origin and poor functional outcomes in ACoA aneurysm surgery. Interhemispheric approach may be proposed to provide a wider and unobstructed view of SubCA for ACoA aneurysms with this high-risk anatomical variant.

Rotational and vibrational spectroscopy of 1-cyanoadamantane and 1-isocyanoadamantane

Because of their high stability, the presence of diamond-type molecules has long been suspected in the interstellar medium, a hypothesis supported by the extraction of diamond nanocrystal from some meteorites. We report the rotational and vibrational investigation of two polar derivatives of adamantane (C10H16), 1-cyanoadamantane (C10H15–CN) and 1-isocyanoadamantane (C10H15–NC), using room temperature gas phase absorption spectroscopy. Pure rotational spectra have been recorded at millimeter wavelengths (75–220 GHz) while vibrational spectra were obtained in the far- and mid-infrared domains (50–3500 cm−1). Quantum chemical calculations have been performed on these two C3v rotors to support the spectral analysis enabling the assignment, for both species, of more than 7000 pure rotational transitions in the ground (A1 symmetry) and first vibrationally excited (E symmetry) states, and of most of the infrared active bands. The pure rotational lines were fit to their experimental accuracy using a symmetric-top Hamiltonian. Our study provides all necessary information for an active search of these species in space.

Parametrization of nonstoichiometric lithium niobate crystals with different states of defectivity

Abstract Quasi-elastic light scattering (QELS) and Raman scattering have been studied at 296 K in congruent lithium niobate (CLN) crystals with a different degree of defectiveness. The defectiveness degree has been detected due to acoustic Q-factor. A quantitative analysis of spectra has been carried out in the frequency range 0–70 cm−1 in samples with different Q-factor. The applied model considers a connection between low-frequency optical mode A1(TO) symmetry type with relaxing intrinsic energy. The model describes stoichiometry defects. Results of model calculations and experiments allows us to conclude that different types of defects play a substantial role in formation of dynamic central peak at a structure phase transition in CLN crystals.

Excitation wavelength-dependent SERS and DFT study to probe Herzberg-Teller selection rules on charge-transfer effect.

In the present work, Herzberg-Teller selection rules on the charge-transfer (CT) effect in surface-enhanced Raman spectroscopy (SERS) are explored for the 3,4,9,10-perylene tetracarboxylic diimide (PTCDI) adsorbed on the Ag nano-island film (AgNIF) using several Raman excitation wavelengths. The UV/VIS/NIR spectrum of PTCDI adsorbed on the AgNIF indicates that excitation wavelengths of 514.5 nm, 633 nm, and 785 nm are in resonance with CT states of the complex, PTCDI adsorbed on the AgNIF. This CT resonance results in intensity enhancement of non-totally symmetric vibrational modes with b1 symmetry in SERS. The three resonances (molecular, localized surface plasmon, and charge transfer) are observed for SERS with 514.5 nm. The totally symmetric Raman bands with symmetry a1 exhibit maximum enhancement in SERS with a 514.5 nm wavelength and suggest the maximum electromagnetic mechanism in SERS with 514.5 nm. Few Raman-forbidden modes and silent modes are allowed in the SERS spectra due to the reduced symmetry of PTCDI in the proximity of the AgNIF. Moreover, density functional theory computation is also carried out to calculate vibrational modes and electronic transitions.

Explicitly correlated ab initio potential energy surface and predicted rovibrational spectra for H2O–N2 and D2O–N2 complexes

An ab initio intermolecular potential energy surface (PES) for the van der Waals complex of H2O-N2 that explicitly incorporates the intramolecular Q2 bending normal mode of the H2O monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] with an augmented correlation-consistent triple zeta basis set and an additional bond function. Analytic five-dimensional intermolecular PESs for ν2(H2O) = 0 and 1 are obtained by fitting to the multi-dimensional Morse/long-range potential function form. These fits to 40 890 points have the root-mean-square (rms) discrepancy of 0.88 cm-1 for interaction energies less than 2000.0 cm-1. The resulting vibrationally averaged PESs provide good representations of the experimental microwave and infrared data: for microwave transitions of H2O-N2, the rms discrepancy is only 0.0003 cm-1, and for infrared transitions of the A1 symmetry of the H2O(ν2 = 1 ← 0)-N2, the rms discrepancy is 0.001 cm-1. The calculated infrared band origin shifts associated with the ν2 bending vibration of water are 2.210 cm-1 and 1.323 cm-1 for H2O-N2 and D2O-N2, respectively, in good agreement with the experimental values of 2.254 cm-1 and 1.266 cm-1. The benchmark tests and comparisons of the predicted spectral properties are carried out between CCSD(T)-F12a and CCSD(T)-F12b approaches.

Optical properties in the infrared range of the birefringent α-GeO2 single crystal

The components of the frequency-dependent complex refractive index were determined indirectly for the new non-centrosymmetric α-GeO2 crystal using polarized Fourier transform infrared reflectivity spectra measured in the far- and mid-infrared spectral region at room temperature. All the longitudinal- and transverse-optical infrared active modes with E and A2 symmetry, according to the D3 point group, were identified and localized within the 100−1000 cm−1 range in very good agreement with a previous first-principles based calculation. For the A2- and E-type modes, both the longitudinal- and transverse-optical splitting were detected. The refractive indices no (E→ ⊥c) and ne (E→ //c) in the infrared domain present considerably higher values than the ones observed in the visible light range, and the high birefringence would find application in many optical devices.

Shape resonance of sulphur dioxide anion excited states using the CAP-CIP-FSMRCCSD method

We have studied the shape resonance of excited states of sulphur dioxide (SO2) anion by using the correlated independent particle Fock space multi-reference coupled cluster (CAP-CIP-FSMRCCSD) method augmented by complex absorption potential. These resonant states have been trapped experimentally in recent years by electron collision. In particular, we have investigated e–-SO2 scattering and computed the negative-ion resonance states of the anion responsible for the two resonances around 4.45 and 6.56 eV and compared the results with the existing experimental observations. From the computational results using the CAP-CIP-FSMRCCSD method, it has been observed that both the resonances near 4.45 and 6.56 eV result from A1 symmetries.

A remarkable enhancement in photocatalytic activity of facilely synthesized Terbium@Zinc oxide nanoparticles by flash combustion route for optoelectronic applications

The facile synthesis of pristine and Terbium@Zinc oxide (Tb@ZnO) samples was produced by fast combustion route. The roughly spherical shaped morphology of Tb@ZnO was investigated by field emission scanning electron microscopy (FESEM) with agglomerated images of small size of particles. The dimension of particle shrinkages with increased the doping contents of Tb into ZnO lattice. The optical band gap of Tb@ZnO sample varies with the concentration of Tb and found to be decreased as 3.276, 3.267, 3.261 and 3.260 eV on increasing concentration of Tb from 1 to 5 wt%. The non-polar phonon mode with E2 symmetry was observed at 99 and 437 cm−1 while polar mode with A1 and E1 symmetry was reported at 330 and 580 cm−1, respectively, for pristine and Tb@ZnO samples. The photocatalytic activity of Tb@ZnO NPs has been investigated by the decolorization of methylene green (MG) dye under black light irradiation (367 nm). A remarkable photocatalytic performance of Tb@ZnO was noticed compared to the pure one and the highest percentage of activity was obtained for 1.0 wt% Tb@ZnO. Current outputs proposed the use of synthesized Tb@ZnO NPs in optoelectronic and photocatalyst applications.

Raman spectra of interface phonons in long-period AlN/GaN superlattices as a tool for determination of the structure period

AlN/GaN superlattices (SL) grown by metalorganic vapor-phase epitaxy with the period of SLs varied from 20 nm to 140 nm, and the thickness of the structures ranged from 0.7 to 1 µm were studied by polarized Raman spectroscopy technique. The peaks within a complex spectral feature of the symmetry A1(TO) observed at about 580 cm−1, were assigned to the interface and quasi-confined phonons. Frequency splitting between the peaks was found monotonously increasing along with the SL period. This dependence was explained using the dielectric continuum model. A method based on this finding was proposed for estimating the period SL using Raman spectroscopy. This method extends the traditional approach based on the studies of folded acoustic phonons in the short-period SLs.

Raman Scattering in Non-Stoichiometric Lithium Niobate Crystals with a Low Photorefractive Effect

Raman spectra of lithium niobate single crystals strongly doped by zinc and magnesium, it has been established, contain low-intense bands with frequencies 209, 230, 298, 694, and 880 cm−1. Ab ignition calculations fail to attribute these bands to fundamental vibrations of A2 symmetry type unambiguously. Such vibrations are prohibited by the selection rules in the space group C3V6 (R3c). Ab initio calculations also proved that low-intense “extra” bands with frequencies 104 and 119 cm−1 definitely do not correspond to vibrations of A2 symmetry type. We have paid special attention to these extra bands that appear in LiNbO3 single crystals Raman spectra despite the fact that they are prohibited by the selection rules. In order to do so, we have studied a number of lithium niobate single crystals, both nominally pure and doped, by Raman spectroscopy. We have assumed that some “extra” bands correspond to two-particle states of acoustic phonons with a total wave vector equal to zero. We have also detected a Zn concentration area (0.05–0.94 mol.% ZnO in a crystal) where doped crystal structure is more ordered: The order of alternation of the main, doping cations, and vacancies along the polar axis is increased, and oxygen octahedra are less distorted.