Absorption Spectra Of Single Crystals Research Articles

Modelling of vibrational Davydov splittings in α-T6 single crystal

The polarized infrared absorption spectra of α-sexithienyl single crystal show the Davydov splitting of some intramolecular ungerade phonons. They are analyzed into details considering the perturbation on the molecular force constants due to the solid state interactions within the unit cell. The proposed model allows to identify the effects of the pertubation within the molecular crystal and to estimate the new force constants of the unit cell.

Optical absorption spectra of 10-MeV electron-irradiated paratellurite single crystals

Optical absorption spectra of TeO2 crystals, irradiated at room temperature by up to 2·1018 cm−2 10-MeV electrons and subsequently annealed to 575K, are studied. The dependence of Urbach absorption edge parameters in TeO2 on the electron beam fluence is discussed. The irradiation-induced near-edge broad (2.5–3.5eV) absorption band is shown to be related to oxygen vacancies, annealing at 475–525K.

One-and two-magnon contributions in optical spectra of KNiF3 single crystal

The one-magnon (1M) and two-magnon (2M) contributions were studied in optical absorption spectra of KNiF3 single crystal, measured in the temperature range from 5 K to 300 K. The three absorption bands were considered in details. The first band is due to the magnetic-dipole transition3 A2g(F) → 3T2g(F), centred at 7700 cm−1, and contains the Brillouin zone-centre one-magnon contribution with the energy ω1M = 25 ± 5 cm−1. The other two bands are due to the electric-dipole transitions 3A2g(F) → 1Eg(D) and 3A2g(F) → 1Eg(G), centred at 16000 cm−1 and 31200 cm−1, respectively. Both bands contain the Brillouin zone-boundary two-magnon contribution with the energy ω2M = 813 ± 10 cm−1.

Absorption and electroabsorption spectra of an α-sexithiophene single crystal at 20 K

Absorption and electroabsorption spectra of two Davydov components of the same molecular state reveal significant differences in line-width and response to an electric field. The difference is caused by a particular packing of four molecules in the unit cell which results in a very large transition dipole of one component. The weak component, on the other hand, reflects the properties of the molecular state. The electroabsorption spectra give no hints to a charge transfer state or of mixing of Davydov components by the field.

Excited-State Exchange Coupling in Bent Mn(III)−O−Mn(III) Complexes: Dominance of the π/σ Superexchange Pathway and Its Possible Contributions to the Reactivities of Binuclear Metalloproteins

Spectroscopic and computational studies of the mono-oxo-bridged complex [Mn(III)2O(OAc)2(Me3tacn)2]2+ (μ-O,Me3tacn, where Me3tacn denotes 1,4,7-trimethyl-1,4,7-triazacyclononane) are presented and discussed. The polarized single-crystal absorption spectra exhibit marked changes with increasing temperature between 10 and 300 K, in particular a significant red-shift and a broadening of the dominant feature in the visible spectral region. These data serve as the basis for evaluating density functional theory calculations to obtain quantitative molecular orbital descriptions of the dominant Mn−O−Mn superexchange pathways in μ-O,Me3tacn. Both the spectroscopic and computational data indicate that the mixed π/σ Mn(xz)−O−Mn(z2) superexchange pathway (where x and z are in the Mn−O−Mn plane and z is oriented along the Mn−O vector) produces the key contribution to exchange coupling. The weak ferromagnetic coupling in the ground state (J = +9 cm-1, ℋ = −2JS1S2) results from a near cancellation of ferromagnetic and a...

Electric field effects on the Davydov components of a strong intramolecular transition:α-sexithiophene single crystals

Low-temperature absorption and electroabsorption spectra of a thin single crystal of $\ensuremath{\alpha}$-sexithiophene reveal a distinctly different line shape and sensitivity to electric fields of the Daydov components of the first intramolecular $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ transition. The weak component of very small linewidth responds by a quadratic Stark shift which is attributed to a molecular polarizability of $900{\AA{}}^{3}.$ The strong Davydov component of much larger linewidth responds by an additional effect which dominates its spectrum and results in an electroabsorption spectrum of second derivative line shape. This signal is attributed to coupling of the field to the large transition dipole of the four molecules of the unit cell. The spectra show no response of a charge-transfer state and no Franz-Keldysh effect of a band continuum. The data indicate that Davydov components of the same molecular state are not mixed by the field.

Absorption spectra of anatase TiO2 single crystals heat-treated under oxygen atmosphere

Anatase TiO2 single crystals large enough to measure the optical properties have been grown by chemical vapor transport reaction. As-grown crystals having blue color were heat-treated under oxygen atmosphere. The change in crystal color from blue through yellow to colorless depending on oxygen annealing was detected by optical absorption spectra. The blue color is due to free carrier absorption. The yellow color is originated from an oxygen vacancy which is a color center. Models are discussed for these change during the oxygen annealing.

Spectroscopy and magnetism of polymeric Ln(CCl3COO)3·2 H2O and their heteronuclear Ln2Cu(CCl3COO)8·6 H2O analogues (Ln=Sm, Gd)

In order to study the exchange interactions between f- and d-electron ions two series of compounds, heteronuclear Ln2Cu(CCl3COO)8· 6 H2O (1, Sm; 2, Gd) and polymeric Ln(CCl3COO)3·2 H2O (3, Sm; 4, Gd) trichloroacetates, were synthesised. Polymeric trichloroacetates 3 and 4 are isomorphous with erbium chloroacetate, whose structure contains dimeric units linked by carboxyl bridges and water molecules into endless chains. Heteronuclear 1 and 2 are isomorphous with Nd2Cu(CCl3COO)8·6 H2O reported by us previously. Absorption spectra of single crystals of 1–4 for two orientations of the crystals down to 4 K were investigated. Magnetic susceptibility measurements in the range 300–1.8 K were carried out. Optical and magnetic properties are related to the X-ray data. Electron transition probabilities were calculated and the effect of coupled ion pairs in the polymeric structure on the optical and magnetic properties was analysed. Changes in magnetic properties in relation to the mononuclear systems are discussed.

Optical spectroscopy and density functional calculations of chromium(V)-doped YVO4 and YPO4: influence of the second coordination sphere.

Low temperature polarized single-crystal absorption and luminescence spectra of Cr(V)-doped YVO4 and the powder luminescence spectrum of Cr(V)-doped YPO4 are reported and discussed. A rich fine structure and strong polarization effects are observed in the near-infrared. Due to a strong interaction of the Cr(V) ion with two Y3+ ions in the second coordination sphere of the Cr(V) ion, the electronic ground state is different from the one expected on the basis of an angular overlap calculation in which only the four oxygen ligands are taken into account. This effect of the Y3+ ions on the ground state of Cr(V) is confirmed by a density functional calculation and by literature EPR data. CrO4(3-) bending modes are responsible for the fine structure in the d-d transition and the resulting distortion in the emitting excited state.

Effect of neutron irradiation on the IR absorption spectra of cupric oxide single crystals

A strong increase of the absorption coefficient with photon energy increasing from 0.1 to 1.0 eV is observed in the spectra of CuO single crystals irradiated with neutrons to a fluence of 5×1018 cm−2. The difference of the absorption coefficients before and after irradiation depends on the wavelength as λ−2. The effect of neutron irradiation on CuO is qualitatively similar to that of neutrons on other semiconductors (for example, GaAs) and differs from that obtained by irradiating CuO with charged particles.

Influence of temperature and phase transitions on the Urbach’s tails of absorption spectra for TlInS2 single crystals

Optical absorption spectra of TlInS2 single crystals were investigated in the temperature range of 10–340 K. The exponentially increasing absorption tails were explained as Urbach tails. TlInS2 single crystals exhibited five different convergent groups of lines ln α(E,T) having different convergence points of E0 and α0 between 120 and 340 K. Characteristic Urbach’s parameters such as steepness parameter [σ(T)], Urbach’s energy [Eu(T)], and effective phonon energy (hνp) were obtained as a function of temperature. σ(T) and Eu(T) exhibited a number of jump-like trends at the identified temperature regions. Jump-like changes of σ(T) and Eu(T) were explained to be due to the phase transitions (PTs) and contributions of electron–(exciton)–phonon interaction caused by exciton dissociation. The temperature dependence of the band gap (BG) exhibited six clearly distinct regions and differs from the temperature dependent BG behavior of known semiconductors. Absorption edge and band gap anomalies in TlInS2 samples at PT were interpreted using the theory proposed by Zametin [Phys. Status Solidi B 124, 625 (1984)]. Measured BG jumping values, ΔEgα, are greater than those calculated from this theory in the 340–200 and 175–120 K regions, while one of them is smaller than that of the calculated in the 198–180 K region. These differences were explained as due to PTs and simultaneous influence of phonon coupling with electrons (excitons) induced by exciton dissociation, and the existence of the chaotic states caused by the coexistence of different phases which have opposite sign contributions below the ferroelectric phase transition, respectively.

Electronic structures of highly symmetrical compounds of f elements XXXI: Simulation of the crystal field splitting pattern of (THF)3Li(μ-Cl)Nd[N(SiMe3)2

The absorption spectra of an unoriented single crystal of the title compound (1) have been measured at room and at low temperatures. Assuming a similar sequence of crystal field (CF) levels as for the previously analyzed Nd[N(SiMe3)2]3 (2) a truncated CF splitting pattern is derived from the spectra obtained. The parameters of an empirical Hamiltonian are fitted to the energies of 71 levels to give an r.m.s. deviation of 24.2 cm−1. The parameters obtained are compared with those of 2 and Nd[N(SiMe3)2]3(CNC6H11)2 (3). The observed trends of CF parameters are consistent with the results of simple model calculations in the framework of the angular overlap model.

Electronic absorption spectroscopy of neodymium acetate single crystals

Electronic absorption spectra of neodymium acetate single crystals were measured at 293 and 4 K. The intensities of 4f–4f transitions were calculated and the Judd–Ofelt Ωλ parameters evaluated. These results differ in comparison with other lanthanide carboxylate compounds by relatively high `hypersensitive' transition intensity and the Ω2 parameter value. The vibronic mechanism of the 4f–4f transitions is discussed and the role of low-energy phonons is shown.

Optical absorption of vanadium doped ZnGa2Se4 single crystals

Vanadium doped ZnGa2Se4 single crystals were grown by the chemical vapor transport method. Optical absorption peaks in the measurement of the vanadium doped ZnGa2Se4 single crystal absorption spectrum at 6020, 9319, 12716, and 15290 cm−1 bands are found to be the electron transition of V3+ ion with Td symmetry from ground state A32(F3) to excited states T32(3F),3T1(3F),3T1(3P), and E1(G1). Crystal-field parameter and Racah parameters, which were obtained from the optical-absorption peaks, are found to be `Dq=602 cm−1, B=265 cm−1, and C=5294 cm−1, respectively. We also found the value of B is reduced 70% compared to the V3+ free ion. The V3+ ion splits into three states: Γ3+Γ5, Γ4, Γ1 by the first-order spin-orbit coupling effect. Hence, the spin-orbit coupling constant is λ=−312 cm−1.

Polarized electronic absorption spectra of Cr 2 SiO 4 single crystals

Polarized electronic absorption spectra, E∥a(∥X), E∥b(∥Y) and E∥c(∥Z), in the energy range 3000–5000 cm–1 were obtained for the orthorhombic thenardite-type phase Cr2SiO4, unique in its Cr2+-allocation suggesting some metal-metal bonding in Cr2+Cr2+ pairs with Cr-Cr distance 2.75 A along [001]. The spectra were scanned at 273 and 120 K on single crystal platelets ∥(100), containing optical Y and Z, and ∥(010), containing optical X and Z, with thicknesses 12.3 and 15.6 μm, respectively. Microscope-spectrometric techniques with a spatial resolution of 20 μm and 1 nm spectral resolution were used. The orientations were obtained by means of X-ray precession photographs. The xenomorphic, strongly pleochroic crystal fragments (X deeply greenish-blue, Y faint blue almost colourless, Z deeply purple almost opaque) were extracted from polycrystalline Cr2SiO4, synthesized at 35 kbar, above 1440 °C from high purity Cr2O3, Cr (10% excess) and SiO2 in chromium capsules. The Cr2SiO4-phase was identified by X-ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm–1, in the absorption spectra of Cr2SiO4 single crystals show properties (temperature behaviour of linear and integral absorption coefficients, polarization behaviour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr2+ in a distorted square planar coordination of point symmetry C2. The crystal field parameter of Cr2+ is estimated to be 10 Dq≃10700 cm–1. A relatively intense, sharp band at 18400 cm–1 and three other minor features can, from their small half widths, be assigned to spin-forbidden dd-transitions of Cr2+. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000 cm–1 of band III are indicative of some Cr-Cr interactions, i.e. δ-δ* transitions of Cr24+, whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Therefore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr2+ as well as δ-δ* transitions of Cr24+ pairs with metal-metal interaction. To explain this, a dynamic exchange process 2 Crloc2+⇔Cr2, cpl4+ is suggested wherein the half life times of the ground states of both exchanging species are significantly longer than those of the respective optically excited states, such that the spectra show both dd- and δ-δ*-transitions.

Temperature dependent absorption spectrometry on LiF:Mg,Ti

Optical absorption spectra of irradiated single crystals of LiF:Mg,Ti have been studied while the sample temperature has been raised during the measurement. Nine absorption bands could be distinguished. Careful computerized curve fitting of the spectra allow for the behaviour of the various bands during heating. The absorption spectra have been compared with spectra measured after a pulse anneal. Clear differences have been observed between the continuous and pulse anneal spectra indicating that the defect state of the sample at room temperature after a pulse anneal is not the same as during TL emission. A striking result is the rise and fall during the heating process of the absorption bands at 4.50eV (not noticed before) and 5.54eV (known from the literature). Similarities between the pulse and continuous anneal spectra have been observed for the behaviour of the 4.00 and 5.10eV bands.

Urbach’s tails in the absorption spectra of CuInTe2 single crystals with various deviations from stoichiometry

The optical absorption coefficient α near the band edge is found to vary exponentially with photon energy in several samples of CuInTe2 single crystals. This behavior, where the extrapolation of ln α vs hυ at different temperatures converges to a single point, is consistent with Urbach’s rule. The temperature dependence of Urbach energy KT/σ, calculated from the steepness parameter σ, agrees with the theoretical model that takes into account the contribution of both electron-phonon interaction and structural disorder. It is established that the observed phonon energy higher than the average value calculated from the reported optical modes is due to structural disorder which is caused by the compositional deviation from the ideal stoichiometry.

A Reversible, Isosymmetric, High-Pressure Phase Transition in Na3MnF6

The crystal structure of Na3MnF6 has been investigated at high pressures by means of single-crystal x-ray diffraction, and its Mn(III) coordination environment has been studied by means of single-crystal optical absorption spectroscopy using diamond anvil techniques. Compressibility data (unit cell parameters) were collected in the pressure range from ambient to 4.06 GPa, and structural refinements based on single-crystal diffraction data were performed at 0.12, 0.91, 2.27, and 2.79 GPa. The monoclinic space group symmetry (P21/n) is retained in the entire pressure range, but, at increasing pressure, a discontinuous phase transition is observed at ∼2.2 GPa. This is interpreted as an effect of a reversible, isosymmetric phase transition with a hysteresis width of 0.5 GPa, observed when the pressure is successively lowered. The structure refinements show that the phase transition involves a reorientation of the static prolate distortion of the coordination around manganese(III). The angle between the elongation axis (z) of the MnF63- octahedron with [0 0 1] flips from ∼20° at ambient pressures to ∼70° at 2.79 GPa. Polarized single-crystal absorption spectra of Na3MnF6 show drastic changes of the polarization of bands due to spin-allowed d−d transitions in Mn(III) when passing the transition pressure, which confirm the results of the single-crystal structure refinements. A possible explanation for this transition is discussed in terms of structure packing arguments. The isothermal bulk modulus at ambient pressure and its pressure derivative were determined to B0 = 47.8(1) GPa and B0‘ = 1.2(1), respectively.

Raman scattering in (NH2CH2COOH)2CdBr2 single crystal

The Raman and infrared absorption spectra of (NH2CH2COOH)2CdBr2 single crystal were studied. The observed Raman and infrared lines were assigned to the internal vibrations of glycine molecules. The vibrational spectra show that the glycine molecule takes the form of the zwitterion in the crystal. © 1998 John Wiley & Sons, Ltd.

Manganese valency and the colour of the Mn2AsO4(OH) polymorphs eveite and sarkinite

AbstractPolarized optical absorption spectra of single crystals of the two dimorphic Mn2AsO4(OH)-minerals eveite and sarkinite show that minor concentrations of Mn3+ at the octahedral site determine the colour and cause the distinct pleochroism in green and yellow of eveite. In the sarkinite spectra only absorption bands due to spin-forbidden d-d transitions in Mn2+ at six- and five-coordinated sites are observed, which produce a weak flesh-red mineral colour and a very faint pleochroism.