Relaxation Of Selection Rules Research Articles

YInMn blue - 200 Years in the making: New intense inorganic pigments based on chromophores in trigonal bipyramidal coordination

Materials with various colors have always fascinated humans from prehistory to the present. They bring joy to our daily life by making the world a more beautiful place to live in and influence our feelings. The important factor that decides the color of a material is its interaction with light in the visible range (400–700 nm) sensitive to human eye. Transition metal cations engender color because they have electronic transition energies that resonate with frequencies in the visible range. In general, transition metal chromophores with a coordination environment of less symmetry and more mixing between the p and d orbitals will have a greater probability of producing intense colors due to the relaxation of selection rules for the d-d transitions. This review chronicles the historical importance of inorganic pigments, reviews the origin of color in solids and highlights recent advancements in designing color pigments. Although fundamentals of the color science about gemstones and minerals are qualitatively well-understood, it is difficult to predict the color of inorganic solids before they are prepared experimentally. In spite of recent advancements in quantum mechanical theories and computational methods, predicting a crystal lattice that will produce an intense inorganic pigment of a desired color is still elusive. Serendipity often plays a role in the discovery of commercially important pigments and dyes such as Prussian Blue, Perkin's aniline dyes and recently YInMn Blue. We conclude that designing durable, nontoxic and multifunctional inorganic pigments with brilliant colors is quite challenging, and a deeper understanding of the structure-optical property relationship in this respect can be helpful.

Characterization of oxygen vacancies in SrTiO3 by means of anelastic and Raman spectroscopy

Oxygen vacancies in reduced SrTiO3 are investigated by Raman spectroscopy and dynamic elastic modulus measurements. The anelastic spectrum is used to evaluate the amount and state of the O vacancies, which result to be almost completely paired and otherwise aggregated at a concentration of 0.5 mol. %, with only 1/6 of them isolated at room temperature. Raman spectra recorded as a function of temperature in as-grown and reduced samples are examined regarding the different processes which can contribute to them. Chemometric analysis is used to highlight the specific influence of oxygen vacancies on the Raman spectra. It is shown that oxygen vacancies induce a relaxation of selection rules leading to the activation of first-order lines even in the cubic phase. Furthermore, regions around oxygen vacancies are demonstrated to be responsible for the asymmetric broadening of Raman lines.

Imaging of 180∘ ferroelectric domain walls in uniaxial ferroelectrics by confocal Raman spectroscopy: Unraveling the contrast mechanism

In recent years, Raman spectroscopy has been used to visualize and analyze ferroelectric domain structures. The technique makes use of the fact that the intensity or frequency of certain phonons is strongly influenced by the presence of domain walls. Although the method is used frequently, the underlying mechanism responsible for the changes in the spectra is not fully understood. This inhibits deeper analysis of domain structures based on this method. Two different models have been proposed. However, neither model completely explains all observations. In this work, we have systematically investigated domain walls in different scattering geometries with Raman spectroscopy in the common ferroelectric materials used in integrated optics, i.e., ${\mathrm{KTiOPO}}_{4}$, ${\mathrm{LiNbO}}_{3}$, and ${\mathrm{LiTaO}}_{3}$. Based on the two models, we can demonstrate that the observed contrast for domain walls is in fact based on two different effects. We can identify on the one hand microscopic changes at the domain wall, e.g., strain and electric fields, and on the other hand a macroscopic change of selection rules at the domain wall. While the macroscopic relaxation of selection rules can be explained by the directional dispersion of the phonons in agreement with previous propositions, the microscopic changes can be explained qualitatively in terms of a simplified atomistic model.

Drastic enhancement of blue-to-orange color conversion efficiency in heavily-doped ZnS:Mn2+ phosphor and its application in white LEDs

Heavily-doped ZnS:Mn2+ phosphors showed an extremely enhanced photoluminescence excitation intensity at 464nm peak wavelength originated from 6A1(6S)→4A1(4G) forbidden transition of Mn2+ ions. The best ZnS: Mn2+ phosphors showed the intensive orange color emission of 584nm peak wavelength from 4T1(4G) → 6A1(6S) forbidden transitions with an excellent lumen maintenance of 97% at 200°C, and their quantum efficiency of 75% under the 464nm excitation wavelength. The photoluminescence enhancement was explained in terms of the relaxation of selection rule on the forbidden intra-transitions of Mn2+ ion. It was caused by the spin-spin and the electron-phonon interactions supported by electron spin resonance signal and Raman spectroscopy, respectively. The excellent lumen maintenance was explained in terms of resonance-assistant direct intra-transition caused by the coupling between an excited electron (e*) on the 4A1(4G) state and multi-phonon energy. To evaluate its feasibility for white light-emitting diodes, the optimized ZnS:Mn2+ phosphor and its mixture with a green Lu3Al5O12:Ce3+ phosphor were applied to a 460nm blue-light-emitting diode. It showed high luminous efficiency of 68lm/W, suggesting a possibility on the application of Mn-doped ZnS phosphor for pc-WLED.

FT-IR and Raman vibrational spectroscopic studies of R2FeSbO7 (R3+ = Y, Dy, Gd, Bi) pyrochlores

Fourier-transform infrared (FT-IR) and Raman vibrational spectroscopic studies of four iron-antimonate pyrochlores R2FeSbO7 (R3+=Y, Dy, Gd, Bi) are reported for the first time. FT-IR spectra are recorded in the transmission mode in the wavenumbers range of 400–1000cm−1 and Raman spectra are taken in the range of 50–1000cm−1at the room temperature. The vibrational modes are assigned by comparison to the spectra of other pyrochlore structured materials, including the experimental and theoretical (ab initio and lattice-dynamical) calculations found in the literature. This paper provides the assignment of the observed modes which are either fundamental or originated from relaxation of selection rules due to local displacement disorder of R and O′ atoms in R4O′ tetrahedron, and some modes are attributed to the splitting due to different force constants within M(Fe/Sb)−O6 octahedron in all the compounds. Though seven F1u IR modes are usually observed for ideal cubic pyrochlores in the range 100–1000cm−1, we ascribe only four F1u modes in the IR spectra for these samples in the studied wavenumbers range. Six Raman active modes, ΓRaman=A1g+Eg+4F2g, are generally observed in the ideal pyrochlores R2M2O7 in the range 250–800cm−1, but our spectra show twelve Raman bands for each of the samples including only three strong intensity bands, assigned as A1g, Eg, F2g(1), decorated with various shoulders and weak bands. The A1g phonon mode lying in the wavenumbers 511–515cm−1 of the Raman spectra shifts toward the lower wavenumber, as the 48f oxygen x-parameter decreases due to substitution at R-site by large cation from Y→Dy→Gd→Bi. To identify the IR active F1u modes reappearing in the Raman spectra, FT-IR measurement below 400cm−1 is most welcome for these compounds.

Strong blue absorption of green Zn2SiO4:Mn2+ phosphor by doping heavy Mn2+ concentrations

Heavily-doped Zn2SiO4:Mn2+ phosphors showed a strong 422nm photoluminescence excitation from 6A1→4A1 forbidden transitions of Mn2+ ions and an intensive 530nm peak emission from 4T1→6A1 forbidden transitions with excellent thermal stability (90% @ 200°C). The photoluminescence enhancement under 422nm excitation was explained in terms of the relaxation of selection rule on the forbidden intra-transitions of Mn2+ ions. It is attributed to the increased Mn2+–O2− intermixing supported by electron spin resonance signal. Finally, the heavily-doped Zn2SiO4:Mn2+ phosphor was applied to white-light-emitting diode as a blue-to-green color conversion phosphor.

Raman analysis of coupling of light to vibrational states of amorphous carbon

First-order and second-order Raman scattering can reveal the phase transformation from graphite to amorphous carbon. Analysis of C ij (ω) and coupling of light to vibrational states of amorphous carbon permits a rough identification of possible Raman active overtones of graphite. Low frequency Raman scattering is observed here for disordered state, where acoustic phonons can be seen in Raman scattering due to relaxation of selection rule.

Use of various growth catalysts for laser sputtering of ZnO micro- and nanorods

Pulsed laser deposition and various catalysts are used to fabricate ZnO micro- and nanorods at temperatures close to the optimum temperature for each catalyst. A comparative analysis of the optical and structural properties of the rods shows that, as the temperature of growth on Al2O3(11–20) substrates decreases, the internal stresses in the rods decrease, which improves their structure and optical properties. This effect is not observed for GaN/Si substrates because of the high stresses induced by the difference in the lattice parameters. An increase in the synthesis temperature leads to an increase in the lattice strains and the concentration of point defects, the relaxation of selection rules, and the appearance of numerous phonon A1(LO) overtones. The lattice strains calculated from unit cell parameter a and the shift in the phonon A1(LO) frequency agree qualitatively. The study of the photoluminescence of the rods shows that a decrease in the synthesis temperature decreases the imperfection of the ZnO rods and improves their optical properties for UV applications.

Analysis of surface effect on luminescent properties of Eu3+ in YVO4 nanocrystals

In this Letter, 7F2 crystal field (CF) levels of surface Eu3+ in YVO4 nanocrystals are calculated employing a refined electrostatic point charge model, where surface states are simulated by point charges. Based on the theoretical 7F2 CF levels, emission spectra of YVO4: Eu3+ nanocrystals are assigned to Eu3+ under different local environments. and relaxation of selection rules by surface effect is discussed.

Intraband absorption and Stark effect in silicon nanocrystals

Received 3 May 2005; published 27 October 2005A theoretical investigation of the combined effect of confinement geometry, valley degeneracy, crystallo-graphic orientations, and anisotropy of the effective mass on the intraband absorption of the conduction bandof Si nanocrystals is presented. We show that the coupling between the nanocrystal shape and arbitraryorientations of the crystalline Si core plays a dramatic role in the intraband optical properties through inter-esting changes in the electronic structure and intra- and inter-valley degeneracy. Different orientations of thecrystalline Si also affect the shape and orientation of the orbital wave functions, thereby modifying transitionrules and fine structure of the intraband absorption. The anisotropy of the effective mass is also responsible forthe emergence of absorption peaks that are not present in isotropic nanocrystals. Additional intraband transi-tions are also induced by changes in the nanocrystal shape with respect to spherical dots. Moreover, thequantum confined Stark effect blueshifts the absorption peaks and induces the appearance of peak structures asa consequence of the relaxation of selection rules caused by the displacement of the wave functions in responseto external electric fields. This effect was found to be only appreciable in large nanocrystals.DOI: 10.1103/PhysRevB.72.155438 PACS number s : 73.21.La, 73.22.Dj, 78.67.Bf, 78.20.Bh

Fluorescence of C60 and Its Interaction with Pyridine

Well-resolved strong fluorescence peaks for C60 dissolved in pyridine in the range from 399 to 750 nm at room temperature are reported. These indicate that C60 has important optical properties and great potential as a new photosensitive and luminescent material. In a C60−pyridine solution, interaction between pyridine and C60, which is further proved by the electroreduction of C60 in pyridine as well as the surface-enhanced Raman scattering (SERS) spectrum of C60, distorts the icosahedral symmetry of C60. The relaxation of selection rules, due to the reduced symmetry of C60 molecule, induces the observed strong fluorescence peaks.

Enhancement of Binding Energies and Polarized Emission of 0D Biexcitons

The energy states of quasi zero-dimensional biexcitons are studied in epitaxially grown CdS/ZnS quantum structures. Selecting a semiconductor structure with high bulk exciton energy, comparatively small ratio in effective masses and strong 3D quantum confinement, an engineering of biexciton binding energies was possible towards highest values even exceeding the bulk exciton Rydberg energies. At low temperatures, the biexciton–phonon interaction is dominated by coupling to acoustic phonons. A strong enhancement of exchange splitting effects is observed resulting in two linearly polarized exciton peaks. The unusual preference polarization of the biexciton emission is attributed to a relaxation of selection rules because of anisotropies in the confining potentials and mixing of valence band states.

Networks of Quantum Nanodots: The Role of Disorder in Modifying Electronic and Optical Properties

Disorder is shown to induce qualitative changes in the electronic spectrum and hence in the response of assemblies of quantum dots. Lattices of quantum dots have one unique source of disorder: the dots themselves can be prepared with a narrow distribution of properties but they are never quite identical. This is unlike a lattice of atoms or molecules. In addition, lattices of quantum dots have a configurational disorder and can also be prepared with compositional disorder. The relaxation of selection rules and the splittings of degeneracies due to symmetry breaking induced by these fluctuations can be probed by optical means. Special attention is given to the enhancement and to the variation of the second harmonic response as a function of the spacing between the dots.

Electronic spectra and electron structure of fullerene complex (?2-C60)Fe(CO)4

The electronic spectrum of the C60Fe(CO)4 complex was studied in a toluene solution. The more intense absorption of C60Fe(CO)4 in the visible region, relative to the free C60, can be attributed to the effect of lower symmetry of the C60 fullerene cage in C60Fe(CO)4 and, thus, relaxation of selection rules for forbidden internal electronic transitions of C60. No bands of the charge transfer from 3d(Fe) to C60 orbitals were observed in the visible region of the complex spectrum. Assignment of the bands was confirmed by semiempirical calculations of the electronic spectrum.

Theory of distortion induced infrared and Raman intensities of forbidden modes of molecular units in crystals: Relaxation of selection rules

The results of the theory of Distortion Induced Activity (DIA) of i.r. and Raman forbidden modes are not only applicable to molecular units in crystals but also to those in liquids and high pressure gases. This point has been demonstrated by discussing examples of molecules such as C 6H 6 (liquid), C 6D 6 (liquid), CCl 4 (liquid) and CF 4 (liquid and gas).

Far-infrared observation of cyclotron and spin-flip resonances in HgSe

Far-infrared (FIR) magnetotransmission experiments were carried out at 4.2 K on high-purity HgSe crystals, with the use of a C${\mathrm{O}}_{2}$-pumped FIR laser and magnetic fields up to 14 T. Measurements were performed in the Voigt geometry, at three FIR wavelengths: 96.5, 119, and 163 \ensuremath{\mu}m. In addition to the usual plasma-shifted cyclotron resonance occurring in the transverse Voigt geometry, we have observed electric-dipole-excited electron-spin-resonance (EDE-ESR) and combined-resonance transitions. These resonances are made possible by the relaxation of selection rules due to spin-orbit coupling, as a consequence of $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ interaction and/or inversion asymmetry. Our data suggest that the inversion-asymmetry mechanism is stronger in HgSe than in other materials studied by FIR magnetotransmission, e.g., InSb. We have also observed a small but unambiguous magnetoplasma shift in EDE-ESR occurring in the transverse Voigt geometry. The positions of the observed resonances at the three laser frequencies were used to obtain band-structure parameters for HgSe within the framework of the Pidgeon-Brown model.

Electronic Spectrum of Ethylene Single Crystal; Search for the Olefin Mystery Band

Ethylene single crystals have been grown as uv specimens. The near-uv spectrum (2000–2500 Å) consists of a number of bands quite analogous to the π—π* (N–V) transition in the gas-phase molecule. No drastic perturbations are observed. No additional electronic transition (e.g., CH*—π, n—π*) is observed in this region, in spite of the relaxation of selection rules in the crystalline site (Ci). Thus, if the ``mystery'' band (6 eV) exists at all, its excited state is most likely either a triplet state or a Rydberg-type state (or a combination of both).