Optical investigations of the metal-insulator transition in a low-dimensional organic conductor (EDT-TTF)4[Hg3I8]

The polarized reflectance spectra of single crystals of the low-dimensional organic conductor (EDT-TTF)₄[Hg₃I₈] undergoing a metal-insulator phase transition at a temperature T < 35 K have been presented. The spectral region of the study is 700–6000 cm^?1 (0.087–0.74 eV), and the temperature range is 300–9 K. It has been shown that the reflectance spectra are determined by a system of quasi-free electrons of the upper half-occupied molecular π-orbitals, which form a half-filled metallic band in the crystals. A high anisotropy of the spectra and their temperature dependences have been found. For two polarizations, the quantitative analysis of the spectra at 100 and 25 K has been performed in the framework of the phenomenological Drude model, the effective mass and the width of the initial metallic π-electron band have been deter-mined, and it has been found that the conducting system in the crystals has a quasi-one-dimensional character. As temperature decreases, the spectra demonstrate substantial changes indicating the formation of the energy gap (or pseudogap) in the spectrum of electronic states in the range of ～1500–2500 cm^?1. In the low-frequency region (700–1600 cm^?1), a vibrational structure has been observed, and the most intense feature of the structure (ω = 1340 cm^?1) is caused by the interaction of electrons with intramolecular vibrations of the C=C bonds of the EDT-TTF molecule. For temperatures of 15 and 9 K, the analysis of the spectra has been performed in the framework of the theoretical “phase phonon” model taking into account the interaction of electrons with the intramolecular vibrations. It has been concluded that the metal-insulator transition observed in the reflectance spectra of the crystals is similar to the Peierls dielectric transition that occurs in a system of electrons coupled with the intramolecular vibrations of the molecules forming the crystal.     

Optical properties of new organic conductors based on the BEDT-TSeF molecule (the κ-(BETS)4Hg2.84Br8 superconductor and κ-(BETS)4Hg3Cl8 Metal) in the range 300–15 K

Polarized reflectance and optical conductivity spectra of single crystals of two new isostructural organic conductors based on the BEDT-TSeF molecule, namely, the κ-(BETS)₄Hg_2.84Br₈ superconductor (T _c =2 K) and the κ-(BETS)₄Hg₃Cl₈ metal, which undergoes a smooth transition to the dielectric state near 35 K, have been obtained in the spectral region 700–6500 cm⁻¹ at temperatures of 300–15 K. At 300 K, the spectra of both compounds are nearly identical and differ from the Drude spectrum characteristic of metals. The nature of the observed difference is discussed, and the spectra are described in terms of a cluster approach with inclusion of electron-electron correlations in the Hubbard approximation combined with the Drude model. The parameters of the theory were determined, including the electron transfer integrals between molecules in a cluster. The spectra in the conducting plane of the crystals were found to be essentially anisotropic, which should be assigned to specific features of in-plane interaction between molecules. The spectra of the superconductor and the metal become increasingly different as the temperature is lowered. The spectra of the metal obtained for T<150 K exhibit splitting of the broad electronic maximum in the mid-IR region into two bands, which is accompanied by a splitting of a vibronic feature deriving from electron interaction with intramolecular BETS vibrations of ν₃(A _g ) symmetry. No such splitting is observed in the superconductor spectra with decreasing temperature. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 11, 2004, pp. 1921–1929. Original Russian Text Copyright ? 2004 by Vlasova, Drichko, Petrov, Semkin, Zhilyaeva, Lyubovskaya, Olejniczak, A. Kobayashi, H. Kobayashi.     

Spectroscopic effects of conical intersections of molecular potential energy surfaces

A simple vibronic coupling model involving two electronic states and two vibrational modes is considered. The model is based on harmonic diabatic potentials and linear coupling of the diabatic electronic states. It is shown that the adiabatic electronic potential energy surfaces exhibit, in general, a conical intersection. The well known E × E and E × B Jahn-Teller problems are contained as special cases. Using numerical methods the optical absorption spectrum is calculated exactly. Extremely complex vibronic spectra are obtained when the conical intersection occurs within the Franck-Condon (FC) zone. The exact vibronic spectra are compared with spectra calculated in the adiabatic and FC approximation. The genuine spectroscopic effects of conical intersections are revealed by a comparison with the results of standard one-dimensional vibronic coupling calculations. The presence of a conical intersection limits the applicability of the adiabatic and FC approximations much more strongly than in the one-dimensional case. The upper adiabatic electronic state is strongly affected by non-adiabatic coupling even when the point of intersection lies outside the FC zone. The relevance of these results for the calculation of molecular electronic spectra is briefly discussed.     

Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-based organic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different electrical properties, namely, superconductors with different transition temperatures and conductors, which transfer to insulating state with decreasing temperature, have been studied by micro-optic spectroscopy. Polarized reflectance spectra of microcrystals have been measured for the three principal crystallographic directions within the 700–40000 cm⁻¹ region, and the corresponding spectra of the optical functions obtained. The anisotropy of the electronic system in the crystals has been established as two-dimensional. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. It is concluded that the conductors have smaller vibronic coupling constants, more narrow allowed electronic bands, and stronger electron-electron interaction compared to those of the superconductors, and that vibronic coupling is the necessary condition for the onset of superconductivity in the superconductors studied. Fiz. Tverd. Tela (St. Petersburg) 41, 897–899 (May 1999)     

Cooperative emission of photons by weakly coupled chromium ions in Al2O3

Under strongcw-excitation ruby crystals exhibit complex anti-Stokes fluorescence spectra in the near ultraviolet. Based on ring dye laser excitation spectra, fluorescence decay and concentration dependence, we show that these spectra can be decomposed into single ion emissions and a cooperative emission at the double energy of the² E⁴ A ₂ transition. The unusual vibronic sideband of the latter suggests a general mechanism for the coupling of distant ions in polar insulators involving the electric field of longitudinal optical phonons.Work supported by Sonderforschungsbereich 65 Frankfurt/Darmstadt     

Luminescence study of the optically active uranyl crystals: NaUO2(CH3COO)3

Luminescence spectra of NaUO₂(CH₃COO)₃ single crystals at 4.2 K have been investigated by means of site selective excitation technique, circularly polarized luminescence and time-resolved luminescence spectroscopy. It has been found that the splittings in the pure electronic and their vibronic transition lines are ascribed to the transitions from two different emitting sites.     

Optical investigations of intermolecular electronic interactions and “free” charge carriers in quasi-two-dimensional organic conductors and superconductors of the group κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1 ? x

The reflectance spectra and optical conductivity spectra of quasi-two-dimensional organic conductor crystals of the group of compounds κ-(BEDT-TTF)₂Cu[N(CN)₂]Br _x Cl_{1 − x} with x = 0, 0.40, 0.73, 0.85, and 0.90 in the spectral region from 6 meV to 0.74 eV at temperatures T = 90-20 K have been quantitatively analyzed in the framework of a combined semiempirical model including the cluster (tetramer) theory for strongly correlated electrons coupling with intramolecular vibrations and the Drude theory of free electrons with the aim of studying intermolecular electronic interactions and their influence on the formation of the ground state in these crystals. It has been established that the parameters characterizing the charge transfer between molecules in dimers and tetramers and the vibronic coupling constants are almost identical for the compounds under investigation. A large value of the effective Coulomb repulsion (U/t = 1.3–1.5) indicates that strong electron correlations occur in the compounds both with a metal-insulator phase transition (x = 0, 0.40) and with a metal-superconductor phase transition (x = 0.85, 0.90). The conclusion has been drawn that, at x = 0, electron correlations favor an antiferromagnetic spin ordering and a metal-insulator phase transition, whereas for superconductors (x = 0.85, 0.90), the metal-insulator phase transition is hindered as a result of the structural disorder due to the difference in orientations of the terminal CH₂ groups of the BEDT-TTF molecule. It has been shown that quasi-free charge carriers interact with electrons localized on the clusters and do not interact with intramolecular vibrations. It has been noted that the oscillator strength of the observed electronic transitions in the initial metallic band (N _eff = 0.38–0.31 per dimer) is considerably less than the corresponding value for free (noninteracting) charge carriers (N = 1). This suggests that the Coulomb correlations and vibronic coupling play a decisive role in the kinetic phenomena observed in the molecular conductors and superconductors under investigation.     

Mechanical,electronic and optical properties of SeZnO3: a GGA+U study

ABSTRACT

Based on first principles computations, the structural, mechanical, electronic band structure, and optical properties of SeZnO₃ compound have been predicted. The dependence of selected observables of SeZnO₃ compound on the effective U (the Hubbard on-site Coulomb repulsion) parameter has been investigated in detail. The elastic constant, Young’s modulus, bulk modulus, shear modulus, Poisson ratio, anisotropic factor, acoustic velocity, and Debye temperature have been computed. The calculated electronic band structure and density of states indicate that SeZnO₃ is a semiconductor material and has indirect band gap. The computations of the optical spectra, as a function of the incident photon radiation in 0–35?eV energy range has also been performed and the interband transitions are examined. The results indicate that Hubbard parameter plays a crucial role in explaining mechanical, electronic, and optical properties of SeZnO₃.     

Investigations of the optical and EPR spectra for (NiX6) (X=Cl, Br, I) clusters

In this paper, the formulae of optical spectral levels and electron paramagnetic resonance (EPR) spectra in trigonal symmetry of 3d⁸ ions are established on the basis of strong field mechanism and a two spin-coupling (SO) parameters model. Unlike the classical crystal-field approach which has only taken the SO coupling of the central metal ions into account, the contribution of the SO coupling of the ligand ions to the optical and EPR spectra has been included in these formulae. When the optical and EPR spectra of the strong covalent crystals are calculated, the reasonable results can be obtained if the two SO parameters model has been put into action. As an application, the optical and EPR spectra of the (NiX₆)⁴⁻ clusters in CsMgX₃:Ni²⁺ (X=Cl, Br, I) crystals have been studied by the complete diagonalization (of energy matrix) method (CDM). The calculated results agree well with experimental findings. From the investigations, a more valid method to calculate the optical and EPR spectra for 3d⁸ ions clusters is provided.     

Reflectivity spectra of new organic metal (BEDO-TTF)5[CsHg(SCN)4]2

The polarized reflectivity and optical conductivity spectra of microcrystals of the new organic conductor (BEDO-TTF)₅[CsHg(SCN)₄]₂ based on the donor molecule bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) have been studied in the spectral ranges 600–6500 and 9000–40000 cm^?1 at 300 K for three principal lattice directions. The optical evidence for the quasi-two-dimensional character of the conducting electronic system is obtained. The conclusion is made that the studied crystal is the quasi-two-dimensional semimetal with overlapping electron energy bands. The basic parameters of the electronic system of the crystal are determined in the framework of the Drude model. It is found that the allowed electron energy bands of the crystal are somewhat narrower than those of the previously studied structurally allied superconductor based on the same molecule. The features of vibrational structure are identified in the σ(ω) spectra for the specified three polarizations.     

Effect of deuteration on the electronic and electron-vibrational properties of the organic conductor k-(D8-BEDT-TTF)2[Hg(SCN)2Br]

We present here for the first time polarized reflection spectra and optical conductivity spectra of single crystals of the newly deuterated organic conductor k-(D₈-BEDT-TTF)₂[Hg(SCN)₂Br] at room temperature. The spectral region investigated is 700–40 000 cm⁻¹. We examined the effect of deuteration on electronic and electron-vibrational transitions observed in the spectra. The observed shift of the electron “dimer” transition in the infrared toward lower frequencies upon deuteration is linked with an increase in the interaction between neighboring, mutually perpendicular dimers in the structure of the deuterated crystal. A lowering of the symmetry of the BEDT-TTF molecule is demonstrated in crystals similar to k-(BEDT-TTF)₂[Hg(SCN)₂Br], relative to the symmetry D _2h of the free molecule. We refine the assignment of the spectral features determined by the interaction of electrons with the fully-symmetric intramolecular vibrations of the C=C, C-S, and C-C-H bonds of the BEDT-TTF molecule. Fiz. Tverd. Tela (St. Petersburg) 40, 1595–1598 (September 1998)     

顺式聚乙炔中双极化子的新电子定域态

本文用自洽迭代的方法研究了顺式聚乙炔cis-(CH)_x中双极化子的电子能谱。结果发现,除了禁带中两个在连续模型中存在的深能级电子束缚态外,还存在一些浅能级的电子束缚态。它们形成分立的能级,并分别位于导带的顶部和底部以及价带的顶部和底部。这些电子态的定域程度随着电子晶格耦合参数λ及电子相互作用U的不同而变化。 关键词：     

Spectroscopic characterization of structural isomers of naphthalene: 1-Phenyl-1-butyn-3-ene

Laser induced fluorescence (LIF), single vibronic level dispersed fluorescence (DFL) spectra, and high resolution rotationally resolved scans of the S₀–S₁ transition of the C₁₀H₈ isomer 1-phenyl-1-butyn-3-ene have been recorded under jet-cooled conditions. The S₀–S₁ origin of PAV at 34 922 cm⁻¹ is very weak. A vibronic band located 464.0 above the origin, assigned as 30¹₀, dominates the LIF excitation spectrum, with intensity arising from vibronic coupling with the S₂ state. High resolution scans of the S₀–S₁ origin and 30¹₀ vibronic bands determine that the former is a 65:35 a:b hybrid band, while 30¹₀ is a pure a-type band, confirming the role for vibronic coupling and identifying the coupled state as the S₂ state. DFL spectra of all vibronic bands in the first 800 cm⁻¹ of the spectrum were recorded. A near-complete assignment of the vibronic structure in both S₀ and S₁ states is obtained. Herzberg–Teller vibronic coupling is carried by two vibrations, ν₂₈ and ν₃₀, involving in-plane deformations of the vinylacetylene side chain, leading to Duschinsky mixing evident in the intensities of transitions in excitation and DFL spectra. Extensive Duschinsky mixing is also present among the lowest five out-of-plane vibrational modes, involving motion of the side chain. Comparison with the results of DFT B3LYP and TDDFT calculations with a 6-311+G(d,p) basis set confirm and strengthen the assignments.     

On the theory of vibronic superradiance

The Dicke superradiance on vibronic transitions of impurity crystals is considered. It is shown that parameters of the superradiance (duration and intensity of the superradiance pulse and delay times) on each vibronic transition depend on the strength of coupling of electronic states with the intramolecular impurity vibration (responsible for the vibronic structure of the optical spectrum in the form of vibrational replicas of the pure electronic line) and on the crystal temperature through the Debye-Waller factor of the lattice vibrations. Theoretical estimates of the ratios of the time delays, as well as of the superradiance pulse intensities for different vibronic transitions well agree with the results of experimental observations of two-color superradiance in the polar dielectric KCl:O ₂ ^? . In addition, the theory describes qualitatively correctly the critical temperature dependence of the superradiance effect.     

Orientation effects on the bandgap and dispersion behavior of 0.91Pb（Zn_（1/3）Nb_（2/3））O_3-0.09PbTiO_3 single crystals

0.91Pb(Zn_1/3Nb_2/3)O₃--0.09PbTiO₃ (PZN--9%PT) single crystals with different orientations are investigated by using a spectroscopic ellipsometer, and the refractive indices and the extinction coefficients are obtained. The Sellmeier dispersion equations for the refractive indices are obtained by the least square fitting, which can be used to calculate the refractive indices in a low absorption wavelength range. Average Sellmeier oscillator parameters E_o, $\lambda$_o, S_o, and E_d are calculated by fitting with the single-term oscillator equation, which are related directly to the electronic energy band structure. The optical energy bandgaps are obtained from the absorption coefficient spectra. Our results show that the optical properties of [001] and [111] poled crystals are very similar, but quite different from those of the [011] poled crystal.     

Electron resonance studies of the renner effect

The gas phase electron resonance spectra of NCO in its ground ²Π_3/2 vibronic state and in two excited vibronic states are described. Theoretical analysis of the spectra yields effective g values for the three states. In additon the ¹⁴N magnetic hyperfine and electric quadrupole coupling constants and the electric dipole moment are determined. The theory of the Renner coupling of electronic and vibrational motion is extended, and shown to account for anomalous contributions to the g values. The theory also shows that these contributions are closely related to the Renner coupling constant.     

Spin-spin relaxation in heteropolynuclear clusters containing iron(III) and copper(II) ions

The magnetic coupling between Fe(III) and Cu(II) and the relaxation behavior of the spin centered at the iron have been studied in di- and trinuclear clusters. The experimental spectra have been analyzed in terms of a simple cross-relaxation model. In the strongly coupled trimer the relaxation rates are determined by a Boltzmann factor and a single field- and temperature-independent scaling parameter¹ w ₀, while the weakly coupled dimer exhibits obviously a more complex behavior.     

Ultraviolet to far infrared optical properties of the one-dimensional conductor K2Pt(CN)4Br0·3. 3H2O

An experimental study of the optical properties of single crystals of the quasi-one-dimensional conductor K₂Pt(CN)₄Br_0·3, 3H₂O in the spectral range from 45,000 to 10 cm^?1 is presented.For light polarized perpendicular to the highly conducting direction the material behaves as a transparent dielectric, as evidenced by reflection and transmission spectra. The reflection spectrum measured with polarization parallel to the chain direction (E∥Z) shows, in addition to the previously reported ‘metal-like’ optical reflectance and plasma edge in the visible, deviations from free carrier type reflectance in the i.r. This was expected because the conductivity is thermally activated at low temperatures. The optical constants for E∥Z polarization have been determined by Kramers-Kronig analysis of the reflection spectrum and, at a few discrete wavelengths, from measurements of polarized reflectance under oblique incidence. The results are qualitatively consistent with various models of the electronic structure but do not distinguish between them.     

The laser-induced fluorescence spectroscopy of gold monoxide: B2Σ−－X2Π3/2 transition

The electronic spectrum of the gaseous gold monoxide molecule has been investigated in the range of 16000－18500?cm^?1 using laser induced fluorescence spectroscopy and single vibronic level emission spectra. Five rotationally resolved vibronic bands are observed and assigned to the transitions B²Σ^? (v'?=?0－4) －X²п_3/2 (v''?=?0), in which the 0－0 transition is observed for the first time. The molecular constants of the excited state B²Σ^? are obtained by a rotational analysis of the spectra. The spin-orbit coupling constant and the vibrational constants of the ground state X²п_i are determined with the accuracy improved by one order of magnitude. The lifetimes of most observed bands are also measured for the first time.     

Low symmetry distortions of hexahalo anions of Re(IV) and Os(IV) investigated by polarized absorption spectra and ligand field calculations

Polarized absorption spectra of intraconfigurational transitions of Re⁴⁺ and Os⁴⁺ in trigonal environments (D _3d site symmetry of host lattices) were measured at low temperature (110 K to 1·9 K). The vibronic side bands and electronic origins arising from electric and magnetic dipole transitions are assigned to appropriate term symbols. The vibrational fine structure is explained by normal vibrations with quanta closely related to those of the ground state. Some of the complex octahedra of doped species do not adopt the type of distortion (stretched or squeezed) to which they are predisposed by the host crystals. In the case where the space provided is not adequate for the doped compound, additional local distortions are present which are different for Re and Os. The degree of splitting due to the trigonal symmetry on octahedral Re⁴⁺ energy levels has been found always larger for the ground state, even though it has only spin degeneracy, than for the excited states. Ligand field theory is able to explain this finding by choosing parameter sets out of a relatively limited range which can be readily scanned.