Polarized Absorption Bands Research Articles

Strain-dependent intersubband absorption in the valence band of SiGe quantum wells

We present intersubband absorption measurements performed on p-type Si1 − xGex/Si(0 0 1) quantum wells (x = 0.35) in the presence of an external strain. We find that the in-plane polarized absorption band is rather insensitive to the strain. For the incident light polarized along the growth direction, the absorption strength increases under the tensile strain. The latter observation is well accounted for by a six-band k · p model and is explained by redistribution of hole population between the heavy- and the light-hole subbands caused by the strain-induced energy shifts of the bands.

Crystal growth and spectroscopic properties of praseodymium and cerium co-doped Y2SiO5

Praseodymium and cerium co-doped yttrium silicate (Pr,Ce:YSO) crystal with high optical quality was grown by the Czochralski method. The segregation coefficient of Pr3+ and Ce3+ is measured to be 0.547 and 0.136, respectively. The polarized absorption bands centered at 268nm, 299nm and 356nm were assigned to 4f (2F5/2)→5d inter-configuration transitions of Ce3+, while bands centered at 220nm and 250nm were assigned to 4fn–4fn−15d transitions of Pr3+. The VUV–UV excitation spectra for monitoring Pr3+ emissions centered at 610nm (1D2→3H4) and 489nm (3P0→3H4), as well as Ce3+ emission centered at 450nm were recorded at T=8K in the range of 100nm–320nm. Polarized excitation spectra for Ce3+ emission centered at 424nm and Pr3+ emission centered at 611nm were recorded at T=300K. Polarized emission spectra were obtained upon excitation at 365nm and 456nm at T=300K. Lifetimes were estimated to be 170μs for Pr3+ (1D2), 3μs for Pr3+ (3P0) and 39ns for Ce3+ (5d1).

Temperature dependence of polarized absorption bands in p-type CdGeAs2

The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2→V1 intervalence band transition, and its intensity taken with E‖c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E‖c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E‖c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems.

Theoretical Study into the Dichroism of the Absorption Bands in the IR Spectra of Oriented Polymers

The IR dichroism of the polarized absorption bands of oriented polymers has been considered. An equation has been obtained that makes it possible to calculate the dichroism of the absorption bands for a different degree of the polymer orientation. This equation has been simplified especially for structurally insensitive absorption bands. In this case it allows one to determine the type of polarization of absorption bands.

Calculation of the Band Intensities of the IR Spectra of Oriented Polymers

An attempt has been made to extend the theory of IR spectra of polymers to oriented polymers. Formulas for the absorption-band intensities in polarization IR spectra of oriented polymers reflecting the dependence of the polarized absorption-band intensity on the structural and orientation characteristics of polymers have been obtained. These formulas permit calculating the intensity of polarized absorption bands at different degrees of orientation of the polymer.

Synthesis, X-ray structure, polarized optical spectra and DFT theoretical calculations of two new organic-inorganic hybrid fluoromanganates(III): (bpaH2)[MnF4(H2O)2]2 and (bpeH2)[MnF4(H2O)2]2.

Two new fluoromanganates(III) of 1,2-bis(4-pyridyl)ethane (bpa) and trans-1,2-bis(4-pyridyl)ethylene (bpe), LH(2)[MnF(4)(H(2)O)(2)](2) (L = bpa or bpe), have been prepared and their structure have been solved by single-crystal X-ray diffraction. The [MnF(4)(H(2)O)(2)](-) anion displays an octahedral geometry with a strong Jahn-Teller tetragonal distortion along the H(2)O-Mn-OH(2) axis. The equatorial metal-ligand distances (Mn-F 1.827(1)-1.859(2) A) are shorter than the axial ones (Mn-O 2.203(2)-2.234(2) A). Three polarized absorption bands at 22,500, 18,300 and 14,500 cm(-1) are observed in the optical spectra of (bpaH(2))[MnF(4)(H(2)O)(2)](2). Finally, we present theoretical calculations on the equilibrium bond distances as well as the crystal-field electron structure using density functional methods. The calculated Mn-F bond distances (1.85 A) are in agreement with the experimental data but the obtained Mn-O distances (2.53-2.56 A) are higher than the experimental one as usually found in similar Jahn-Teller distorted systems. The calculated d-d transition energies are compared with experimental energies derived from the optical spectra. The variation of the HOMO energy and transition energies against the Mn-O distance is also shown.

The role of Fe and cation order in the crystal chemistry of surinamite, (Mg,Fe2+)3(Al,Fe3+)3O[AlBeSi3O15]: A crystal structure, Mössbauer spectroscopic, and optical spectroscopic study

The crystal structure of surinamite from Christmas Point, Enderby Land, Antarctica, has been newly refined by single-crystal X-ray diffraction: V I (Mg 2 . 2 6 Fe 2 + 0 . 7 4 Fe 3 + 0 . 3 9 Al 2 . 6 1 )O I V (Al 1 . 0 0 Be 1 . 0 0 Si 3 . 0 0 )O 1 5 (simplified formula), space group P2/n, a = 9.915(2), b = 11.368(2), c = 9.617(2), b = 109.30(2)°, Z = 4, wR(F 2 ) = 0.074 for 4876 independent reflections. The refined site occupancies agree well with the chemical composition determined by electron microprobe analysis and with the Fe 3 + /Fe 2 + ratio estimated from stoichiometry. The surinamite structure is characterized by an ordered Al/Be/Si distribution on the tetrahedral sites and by charge ordering with extensive Mg 2 + -Fe 2 + and Al 3 + -Fe 3 + exchange on the octahedral sites. This cation ordering is distinct from that observed in related phases of the sapphirine series and aenigmatite groups, and the difference is linked to the unique structural topology of the tetrahedral chains in surinamite. Optical and Mossbauer spectra of surinamite have been fully interpreted in terms of an octahedral distribution of Fe 2 + and Fe 3 + cations that agrees very well with the X-ray site populations. Both the X-ray and Mossbauer data establish the absence of significant tetrahedral Fe and the non-uniform distribution of octahedral Fe. Intense and strongly polarized absorption bands caused by IVCT-processes in clusters of Fe 3 + and Fe 2 + iron in edgesharing octahedral sites produce the unusual color and pleochroism observed in surinamite.

Position of Polarized Impurity Absorption Bands and Orientation Statistics of Impurity Molecules in Uniaxial Polymer Films

Position of Polarized Impurity Absorption Bands and Orientation Statistics of Impurity Molecules in Uniaxial Polymer Films

Position of polarized impurity absorption bands and orientation statistics of impurity molecules in uniaxial polymer films

The features of a static shift and splitting of polarized electronic absorption bands of uniaxial impurity molecules are studied in uniaxial polymer films. The specific properties of the orientation statistics of impurity molecules are found that are typical for stretched polymer films and result in the qualitative difference of the dependence of positions and splitting of polarized absorption bands on the order parameter of the impurity molecules compared to impurity nematic liquid crystals.

Spectral features of polarized light absorption and anisotropy of a local field in the D ho discotic

The polarized IR absorption spectra of monodomain planar-oriented films of a discotic liquid crystal Dho are obtained for the first time. The spectral effects induced by the resonance dipole-dipole interaction of molecules, which have been predicted earlier, were observed. It was found that molecular excitations in isotropic and liquid-crystal phases are mixed due to local-filed effects and this mixing affects the relative intensity and dichroism of the absorption bands. New methods were suggested for measuring the local-field parameters in the Dho phase from intensities and positions of the polarized absorption bands taking into account the mixing of molecular excitations. The two-dimensional crystal lattice of molecular columns was shown to reduce the local-field anisotropy of this phase.

Er incorporation into congruent LiNbO 3 crystals

Er-doped LiNbO 3 crystals with a melt concentration ranging from 0.5 to 8 mol% have been grown by the Czochralski technique to study the Er incorporation into the LiNbO 3 lattice. The Er concentration modifies the crystal lattice parameters and the intensity of the optical absorption band. A limit for the Er incorporation into the crystal was found around 3 mol% through the saturation behavior of the dopant crystal concentration, the lattice parameters and the integrated area of the optical polarized absorption bands.

Thermochromic properties of the ferroelectric -doped : study of the temperature-induced dichroism

A new thermochromic material formed by doping into the ferroelectric crystal is presented. The change of colour from green (room temperature) to red (low temperature) under polarized light observation is accompanied by a strong temperature-induced dichroism at low temperature. The origin of these phenomena is investigated through the polarized charge transfer spectra associated with complexes formed in the title compound, and their dependence on temperature in the 10-300 K range. Attention is paid to correlating the optical spectra with the local structure around . The results are compared with those obtained in the tribromide and , and the tetrabromide and crystals, where forms complexes of nearly symmetry, and tetrahedral distorted complexes of symmetry, respectively. An interesting feature is the presence of a strongly polarized absorption band at 15 800 in , which is responsible for the observed dichroism and thermochromism. The existence of this band is associated with the formation of highly distorted complexes in the monoclinic host crystal. The enhancement of dichroism exhibited by this crystal with decreasing temperature is noteworthy. The analysis of the spectra reveals that this unusual thermal behaviour can be explained in terms of thermally activated reorientations rather than structural changes of the complex.

Radiation-induced defects in lithium yttrium fluoride

Aerospace applications may result in exposing the laser host material LiYF4 to ionizing radiation which may degrade its performance as a host material by introducing loss processes which affect the threshold energy. Ionizing radiation produces a series of partially polarized absorption bands in the UV—visible region of the optical spectrum. Comparison of the optical spectra obtained after irradiation at 300 and 195 K did not show a difference in the number and spectral location of the absorption bands at 77 K. The absorption bands observed at 1.96, 2.98, 3.7, and 4.94 eV were found to be polarized more strongly perpendicular to the c-axis. Other bands at 1.97, 2.3, 2.85, 3.7, and 4.43 eV were observed to be polarized parallel to the c-axis. Using optical relations and the previously identified F-center band at 3.7 eV, the 2.88 and 4.43 eV bands have been identified as also due to the F-center, the 1.97 eV band as due to the M-center, and the 2.3 eV band as due to the R-center.

Optical-absorption studies on noncubic s2 centers in single crystals of KH2PO4 and RbH2PO4.

Effects of a noncubic crystal field on the ${\mathit{s}}^{2}$ ion are investigated. Optical-absorption spectra of thallous ion centers produced in single crystals of ${\mathrm{KH}}_{2}$${\mathrm{PO}}_{4}$ (KDP) and ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ (RDP) are presented. They consist of polarized absorption bands. Since it is confirmed that the ${\mathrm{Tl}}^{+}$ centers in KDP and RDP exhibit quite similar absorption characteristics, a model is proposed to interpret the interaction mechanisms between the ${\mathit{s}}^{2}$ ions and the KDP-like crystal lattices. Among five typical absorption bands named ${\mathit{A}}_{\mathit{z}}$, ${\mathit{A}}_{\mathit{x}\mathit{y}}$, ${\mathit{B}}_{\mathit{x}\mathit{y}}$, ${\mathit{C}}_{\mathit{x}\mathit{y}}$, and ${\mathit{C}}_{\mathit{z}}$, the ${\mathit{A}}_{\mathit{x}\mathit{y}}$ band has a noticeable doublet structure and the ${\mathit{B}}_{\mathit{x}\mathit{y}}$ band is as intense as the ${\mathit{A}}_{\mathit{x}\mathit{y}}$ band even at liquid-nitrogen temperature and is strongly temperature dependent. The observed results are qualitatively explained by taking into account the spin-orbit, crystal-field, and electronic-vibrational interactions.

Structure, energy and charge transport in two-dimensional crystals of cyanine dyes

In order to develop systems with controlled energy or charge transfer across molecular dimensions it is highly desirable to build two-dimensional crystals of functional dye molecules: they can be addressed from the third dimension and the lateral intermolecular interactions can be well characterized due to a fixed and defined geometry. In an effort to study these possibilities we prepared crystals of different, negatively charged cyanine dyes, formed after adsorption from a water subphase to a positively charged monolayer. These crystals are one monolayer thick, of uniform dimensions between 10 and 100 μm (depending on nucleation conditions) and of rectangular shape. Single crystals are studied by transmission electron diffraction and by polarized absorption and emission spectroscopy. We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicular polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centred rectangular one of the dyes. It is incommensurate and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice. The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands. Replacing the positively charged lipid molecule with saturated bonds by one with a diacetylene group in the aliphatic region, one can sensitize the photopolymerization of diacetylenes. In accordance with the established mechanism for photopolymerization this indicates efficient charge transfer from the dye crystal to the linear polymer.

Two-dimensional dye crystals with controllable optical properties

The structure, molecular ordering and optical properties of single crystals of cyanine dyes grown by adsorption from a water subphase to a positively charged lipid monolayer are discussed. These crystals are one monolayer thick, of uniform dimensions between 10 and 100μm (depending on nucleation conditions) and of rectangular shape. Single crystals were studied by transmission electron diffraction and by polarized absorption and emission spectroscopy. We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicularly polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centered rectangular one of the dyes. It is incommensurate, and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice. The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands.

Polarized Fourier transform infrared microscopy as a tool for structural analysis of adsorbates in molecular sieves

Using FTIR microscopy with polarized IR radiation on silicalite I single crystals fully loaded with p-xylene, the existence of an ordered adsorbate could be proven for the first time by IR spectroscopy. By analyzing the polarized absorption bands the orientation of the p-xylene molecules relative to the host structure could be determined. The results agree well with structural data obtained from X-ray diffraction experiments. These first results suggest that polarized IR microscopy could develop into a powerful tool for the analysis of adsorbate structures, assisting in complete structure resolution by diffraction techniques

Order parameters from U.V. spectra with incompletely polarized absorption bands

Abstract A new method is developed for the determination of Saupe's order parameters S* and D* from I.R. or U.V. spectra and the temperature dependence of their degree of anisotropy R. This method can be applied to molecules with one uniformly polarized band and another band of mixed polarization if all bands concerned are polarized in the same plane. For that a relation D* = f(S*) between the order parameters as a function of temperature is necessary, which can be obtained from the lines of constant entropy in the order triangle. These isentropics are calculated from the orientational distribution function of the ordered system. The resulting function D* = f(S*) is equal to that of the mean field theory given by Luckhurst et al., but can be derived here in a very easy way. The method is applied to diaminoanthraquinones with a point symmetry C2v.

Spectral and orientation parameters for a nematic liquid crystal containing a dopant having biaxial molecules

The spectra of doped liquid crystals (LC) are of interest for electrooptic devices [i] and for research on molecular interactions in partly ordered media. The current trend is to use complicated dye molecules having spatially separated chromophores [2], where research is conducted on compounds containing condensed benzene rings, which give numerous well-resolved bands differing in polarization [3], which gives importance to research on how various molecular interactions and orientation relationships affect the spectra for biaxial dopant molecules. We have examined the effects from static dopant-matrix interactions on the electronic polarized absorption band positions and splittings for such compounds in nematic LC.

The vibrational fine structure of the 1A 2g ← 1A 1g polarized absorption band of trans-[Co(NH 3) 4(CN) 2] +. The excited state geometry

The low temperature polarized absorption, far-infrared, and Raman spectra of trans-[Co(NH 3) 4(CN) 2]Cl (H) and of the deuterated (D) compound