Polarized IR Reflectance Spectra Research Articles

Optical Properties of LiGdF4 Single Crystal in the Terahertz and Infrared Ranges

The basic mechanisms of absorption of THz range radiation in optically perfect LiGdF4 single crystals were studied using the broadband experimental data and the dielectric response function analysis within the harmonic oscillator model. The polarized IR reflection spectra have allowed one to determine the phonon contribution in the absorption coefficient in the THz range, while transmission spectra in the THz range were used to obtain the birefringence value and the effects of various mulitparticle processes. Additionally, we established the optical and electrodynamic parameters of the LiGdF4 single crystal, which are necessary for the design of nonlinear optical devices.

Analysis of the polarized IR reflectance spectra of the monoclinic α-oxalic acid dihydrate.

In contrast to the extensive molecular and crystal structure investigations on oxalic acid dihydrate (C2H2O4·2H2O, α-POX) and its deuterated analogues (α-DOX), stands the absence of a complete vibrational spectra analysis. Such analysis is desirable in view of the proton dynamics in α-POX crystals. In this communication we report the room temperature polarized IR reflectance spectra of a single crystal of α-POX recorded from the ac crystal plane, and from the plane containing the b-crystallographic axis, with polarization along the axis. Dispersion analysis of the reflectance spectra of both Bu and Au symmetry type modes, using model dielectric and reflectance function valid for the monoclinic case, have been performed and the results are discussed. Some aspects of the obtained fit results for some of the spectral regions and the peculiar change of the reflectance function with polarization angle are also discussed in this work. A correlation between the crystal structure and measured spectra, together with the results of the performed dispersion analysis, gave answers to some of the problems concerned with the orientation of the transition dipole moments of the IR active modes. The assignment of modes is assisted by DFT calculations. Another aspect covered in this work is the model reflectance functions using different averaging theories that have been applied in obtaining the reflectance spectrum of a polycrystalline sample. The results of the comparison between these spectra and the recorded reflectance spectra from a polycrystalline sample were further discussed.

Optical Constants of an Apatite Single Crystal in the IR Range of 6–28 μm

On the basis of polarized IR-reflection spectra in the range of 5000–350 cm–1 measured from the natural face of an optically transparent fluorapatite single crystal, components of a complex refractive index (optical constants) for radiation-vector orientations E ‖ c and E ⊥ c have been calculated by the Kramers–Kronig method. The fluorapatite single crystal has been chosen from several samples: it contains a minimum amount of impurities and has a high degree of crystallinity in accordance with the criteria of IR spectroscopy and Raman spectroscopy. Tabular data on optical constants for ordinary and extraordinary rays are given for the IR range of 6–28 μm. The obtained absorption spectra are compared with the results of quantum-chemical ab initio calculations within the B3LYP simulation.

Study of wide band-gap crystal LiCaAlF6by IR-reflection spectroscopy andab initiocalculations

Polarized IR-reflection spectra and results of ab initio calculations of vibrational and electronic properties of LiCaAlF6 single crystal are presented. It is shown that the crystal band gap is direct. Experimental and theoretical parameters are obtained for dipole-active and all phonons, respectively, including silent modes. Experimental IR-reflection and Raman spectra are well described in the frame of results obtained by ab initio calculations. The peculiarities are discussed concerning the structure of electronic bands, the interatomic interactions, the character of lattice vibrations, and the phonon dispersion.

Dispersion analysis of triclinic K2Cr2O7

Polarized IR-reflection spectra of a single crystal of K2Cr2O7 were recorded at near normal incidence. Dispersion analysis was applied, taking fully into account the triclinic symmetry of the crystal and the measuring conditions, to determine the oscillator parameters. The oscillator parameters and the dielectric background tensor were proven valid by comparison between measured and based on these parameters modeled spectra of a randomly oriented polycrystalline sample.

Effects of structure distortion on optical phonon properties of crystalline beta-BaTeMo2O9—A novel nonlinear optical material: Infrared and Raman spectra as well as first-principles calculations

The infrared dielectric property of monoclinic BaTeMo2O9 single crystals is studied by polarized IR reflectance spectra from 20 to 1800 cm−1. Based on the modified Lorentz model, the frequencies, strengths, and dampings of TO modes as well as the orientations of the dipole momenta are determined, agreeing well with Raman spectra and results from First-principles calculation. The observed modes are visually assigned to the specific atoms' motions in the primitive cell based on the theory calculations. A large shift of the internal modes of the anion groups relative to free anion co-ordination polyhedra is observed, which can be used to indicate the distortions of co-ordination polyhedra related to the nonlinear optical properties. Further, the experimental results of the strengths of the oscillators support the elimination and splitting of degenerate modes in free regular polyhedrons. These results offer a way to evaluate the nonlinear optical properties by use of traditional IR reflectivity spectra.

Vibrational spectra and dispersion analysis of K2Ni(SeO4)2·6H2O Tutton salt single crystal doped with K2Ni(SO4)2·6H2O

Dispersion analysis of the polarized IR reflectance spectra of K2Ni(SeO4)2·6H2O doped with K2Ni(SO4)2·6H2O has been performed. Vibrational parameters like oscillator strength, attenuation constant and frequency of the transversal phonons for the modes of Au symmetry type plus the orientation of the transition dipole moments for the modes of Bu symmetry type in the ac crystal plane have been obtained. The spectra-structure correlation of the H2O stretching vibrations show that bands appearing in the spectra for polarization of the external radiation oriented along the b axis are mainly due to the H2O stretching vibrations of one of the three crystallographically distinct sets of water molecules. The orientation of the transition dipoles of stretching vibrations of the selenate ion differ from the characteristic spectra of the sulfate analog in that no mutually perpendicular transition dipoles are found in the ac crystal plane. Water librational bands masked with the bands of the ν4(SO42-) mode in the sulfate analog have now been unveiled and assigned. The ratio between the oscillator strength and the attenuation constant appears to be a helpful tool in the assignment of the sulfate stretching vibrations and water librations. The vibrational and orientational characteristics of the ν4(SeO42-) modes were obtained. Тhe ν3(SO42-) frequency region of the isomorphously isolated SO42- ion in the K2Ni(SeO4)2·6H2O matrix was investigated in some detail. Contrary to the expected three, four bands can be identified. Three of them were assigned to ν3(SO42-) based on the orientation of the transition dipole moments. On the basis of the IR, but also Raman spectra of the pure and mixed crystals, a discussion of the influence of the potential field and the hydrogen bonds with the change in the volume of the unit cell is given.

Infrared and Raman studies of quasi-one-dimensional conductors ( o-DMTTF) 2X (X=Br, I)

Polarized IR reflectance spectra (700–16 000 cm −1) and Raman spectra for red excitation (λ=632.8 nm) of ( o-DMTTF) 2X (X=Br, I) single crystals were measured as a function of temperature ( T=10–300 K). Raman data provide evidence of uniform charge distribution in o-DMTTF stacks down to the lowest temperatures. Despite metallic behavior from dc conductivity measurements the optical conductivity data suggest the presence of an optical energy gap. The energy gaps of both compounds have been determined. Temperature dependence of both Raman and IR spectra indicates two phase transitions at about 50 and 200 K.

Dispersion analysis of polarized IR reflectance spectra of Tutton salts: The ν3(SO 42−) frequency region

Polarized IR reflectance spectra of eight Tutton salts single crystals: M′ 2M″(SO 4) 2·6H 2O; M′: K + or NH 4 +; M″: Ni 2+, Co 2+, Zn 2+ or Mg 2+) were recorded at near-normal incidence. Dispersion analysis was performed for the whole mid-IR region. The results on ν 3(SO 4 2−) frequency region are reported in this paper. Reflectance spectra were recorded using radiation with polarization along the b crystallographic axis and in the ac crystal plane. Results of the fit for the transversal frequency, oscillator strength, attenuation constant and the orientation of the transition moments in the ac crystal plane for the ν 3(SO 4 2−) frequency region are presented. The longitudinal frequencies for the A u vibrations were obtained from the energy-loss function spectra, while for the B u vibrations a particular approximation was used. In addition to frequency values, the directions of the longitudinal transition dipole moments in the ac crystal plane were also found. The dependence of the principal axes of the real and the imaginary parts of the dielectric tensor, on the radiation frequency, was established. An attempt was made to estimate the transition moment directions of the ν 3(SO 4 2−) mode components by correlation of the spectroscopic and structural findings.

Polarized IR reflectance spectra of the monoclinic single crystal K 2Ni(SO 4) 2·6H 2O: Dispersion analysis, dielectric and optical properties

Polarized IR reflectance spectra of K 2Ni(SO 4) 2·6H 2O single crystal (belonging to the group of Tutton salts) were recorded at near-normal incidence. From the dispersion analysis performed on the spectra recorded from the ac crystal plane, mode parameters: transversal frequency, oscillator strength, attenuation constant and the orientation of the transition moment were determined. The polarized spectrum along the b crystallographic axis was also recorded and a dispersion analysis performed. Comparison between the spectroscopically obtained transition moment directions with those obtained from the structure data for various modes is discussed. All dielectric tensor component values were obtained for the whole mid-IR frequency range. Also, the real and the imaginary parts of the complex indexes of refraction for the waves with wave vector in the direction of the b crystallographic axis and in the ac plane (when the direction of the electric vector is oriented along the b axis) were found as functions of frequency.

Spectral Studies of DIET, DIEDO and DIETS Organic Donors and Their Charge-Transfer Salts with Paramagnetic Cyano Complex

IR and Raman spectra of neutral iodine substituted electron donors DIET, DIEDO and DIETS were measured. Their fundamental vibrational modes were calculated and compared with the experimental data. Additionally, we studied IR and Raman spectra of 2:1 charge-transfer salts formed by these donors with paramagnetic [Fe(bpca)(CN)3] anions. Polarized IR reflectance spectra of DIET and DIEDO salts were also recorded.

Isosbestic-like point in the polarized reflectance spectra of monoclinic crystals—A quantitative approach

If two overlapping bands originate from transition moments that are perpendicular and lie along the principal axes of the infinity frequency dielectric tensor, an isosbestic-like point (ILP) appears in the overlaid polarized IR reflectance spectra of single crystals. These conditions can also be met in principle in case of the reflectance from the ac-plane of a monoclinic crystal despite of the crystal's low symmetry. In order to determine the transition moment directions, diagonalization of the infinity frequency dielectric tensor should be performed. It is shown that a critical magnitude for the appearance of ILP is the angle of incidence. An increase of this angle leads to a transformation of the ILP to an isosbestic-like region, which eventually vanishes at higher incidence angles. Polarized reflectance spectra of gypsum (CaSO 4·2H 2O), recorded from the ac crystal face, were used to verify the theoretical results.

Infrared reflectance spectra of the organic conductor (BEDT-TTF) 4K[Fe(CN) 5NO] 2

Polarized IR reflectance spectra of the organic metal (BEDT-TTF) 4K[Fe(CN) 5NO] 2 were measured over the range from 700 to 6500 cm −1 at room temperature from various crystal faces parallel and perpendicular to the conducting bis(ethylene dithio)tetrathiafulvalene (BEDT-TTF) layers. Additionally, Raman spectra of single crystals were studied (excitation 514.5 nm). A plasma-edge-like dispersion is analyzed in terms of a simple Drude–Lorentz model and optical transport parameters are determined. The transfer integrals are estimated by assuming a one-dimensional tight-binding model. Electronic and vibrational bands are discussed. On the basis of Raman spectrum, the charge distribution on BEDT-TTF molecule is determined.

Vibrational spectrum of Li2B4O7 crystals

Polarized IR reflection spectra of Li2B4O7 crystals are studied in a spectral range of 80–1600 cm−1 and compared with their Raman spectra. Based on the results of the dispersion analysis of the spectra, the frequencies, damping constants, and oscillator strengths of all vibrations are determined. The inversion of frequencies of the longitudinal and transverse vibrations of the A 1 and E symmetry in a range of 900–1150 cm−1 is found. Based on the data thus obtained, the effective charges are calculated and the types of chemical bonds are analyzed for structural groups of the Li2B4O7 crystal.

A new organic conductor (BEDT-TTF) 5Cl 3(H 2O) 5

A new organic conductor of (BEDT-TTF) 5 Cl 3 (H 2 O) 5 has been synthesized by electrochemical method. The salt exhibits metallic properties at room-temperature (σ=20S cm -1 ) and undergoes a metal-insulator transition at 160K. The crystal structure was determined. Polarized IR reflectance spectra were measured over the frequency range 650∼6500cm -1 for different polarization of the IR light. Raman spectra of single crystal as well as ESR spectra were also studies.

Infrared and Raman studies of the κ-phase charge-transfer salts formed by BEDT-TTF and magnetic anions M(CN)63− (where M = CoIII, FeIII, CrIII)

The isostructural charge-transfer salts κ-[Et4N](BEDT-TTF)4M(CN)6·3H2O (M = CoIII, FeIII, CrIII) are organic semiconductors undergoing two intriguing phase transitions at T = 240 and 150 K. Polarized IR reflectance spectra (from 650 to 6500 cm−1) of these compounds were studied at room temperature. An electronic dispersion in IR spectra was analyzed in terms of a Drude–Lorentz model and optical transport parameters were determined. IR reflectance spectra of FeIII crystals were measured in the temperature range from 190 to 300 K and the influence of the phase transition at 240 K was studied. We report also FT-NIR Raman studies of FeIII salt as a function of temperature from 110 to 300 K. The Raman line related to the ν3(Ag) CC mode of BEDT-TTF splits below 240 K into three components providing evidence of charge redistribution in the BEDT-TTF layers.

Infrared and Raman analysis of plasma CVD boron nitride thin films

We present an optical investigation, by means of polarized IR reflectivity and Raman scattering, of the quality and stability of PECVD grown BN films obtained on the anode and on the cathode of a parallel-plate radio-frequency reactor by glow discharge decomposition of two different gas mixtures, B2H6-H2-NH3 and B2H6-N2. The addition of Ar to B2H6-N2 has also been investigated. It is shown that polarized IR reflection spectra of thin films have a complex optical behaviour, which is a function of the oblique angle of incidence and the substrate material used in the deposit. However, these spectra are predictable when optical theory is applied. The results obtained show the capabilities of polarized IR reflection and Raman scattering to perform detailed investigations on the microstructure and aging of BN thin films, independently of the substrate material.

Polarized infrared reflectance spectra of single crystal α-Bi2O3

Polarized IR reflectance spectra of monoclinic single crystal α-Bi2O3 for different directions of the wave vectork and the electric fieldE of the incident radiation have been measured in a wide temperature range (25–400 K). A significant dependence of the spectrum shape on the direction of the vectorE within the ac-plane was observed. Frequencies, strengths, widths and polarizations of the TO modes were restored using the Lorentz dispersion model extended for the monoclinic crystal lattice. Remarkable distinction between intensities of Au- and Bu-modes is observed.

LO-TO splittings, effective charges and interactions in electro-optic meta-nitroaniline crystal as studied by polarized IR reflection and transmission spectra

The polarized IR reflection spectra of the meta-nitroaniline ( m-NA) single crystal along the a, b and c crystallographic axes as well as the b and c polarized transmission spectra have been measured in the 100–400 cm −1 region. The LO-TO splitting values have been calculated from the reflection spectra by fitting them with the four parameter dielectric function. The dipole moment derivatives, relevant to dynamic effective charges, of the vibrations have also been calculated and used to check the applicability of the oriented gas model (OGM) to reflection spectra. The discrepancies from the OGM have been discussed in terms of vibronic couplings, weak hydrogen bondings (HB) and intramolecular charge transfer.

Infrared reflection spectra of KClO 4 single crystals

The polarized IR reflection spectra of potassium perchlorate single crystals were recorded and analysed for the frequencies of the transversal and longitudinal optical zone centre modes using classical oscillator-fit method and Kramers—Kronig analysis. For comparing the TO and LO mode frequencies obtained with the corresponding Raman data, frequencies (ω 0) unaffected by the long range electrostatic fields were calculated. These frequencies are similar to those obtained from conventional IR absorption spectra of Nujol mulls or KBr discs. From the TO/LO splittings of both the internal and external vibrations Scott and Szigeti dynamical effective ionic charges were determined and discussed in terms of ionicity and bond polarity.