Low-temperature Luminescence Spectra Research Articles

Growth and optical properties of BiI3 and PbI2 microcrystals

A study has been made of low-temperature absorption and luminescence spectra of bismuth iodate microcrystals in various hosts (layered cadmium iodate, microporous glasses and polymers), as well as of the spectra of mesoscopic domains formed in bulk bismuth iodate by mechanical strains. The structure of the spectra and the quantum shifts of the exciton levels yielded information on the dimensions and size dispersion of the microcrystals and domains, as well as on the effect of microcrystal size on the Stokes losses. The quantum exciton-line shifts in microcrystalline lead-iodate films grown in pores of the glass host have been used to calculate the film thicknesses. A photoinduced change in the mechanism of radiative recombination was observed in PbI2 microcrystals. The spectrum of resonant Raman scattering in lead iodate microcrystals consists of broad bands corresponding to the branches of optical phonons undergoing strong angular dispersion.

Appearance of resonant Γ-X mixing in gradient GaAs/AlAs short-period superlattices

We have experimentally investigated the low-temperature (10 K) luminescence and reflection spectra of a gradient GaAs/AlAs superlattice. We have examined the behavior of phonon satellites in the vicinity of the X-Γ resonance. Smooth passage through the resonance was achieved by scanning an exciting light beam along the surface of a gradient sample. Based on our experimental results, we have determined the functional dependence of the Γ-X mixing potential on the resonance detuning.

Synthesis and Optical Spectroscopy of MnO42--Doped Crystals of Cs2CrO4, SrCrO4, CsBr, and CsI

The first low-temperature absorption and luminescence spectra of MnO42--doped crystals of Cs2CrO4, SrCrO4, CsBr, and CsI are reported. Single crystals of Cs2CrO4:MnO42-, which crystallize in the orthorhombic β-K2SO4 structure, were grown from basic aqueous solutions. An alkali−chloride flux composition was used to obtain single crystals of SrCrO4:MnO42- with the monoclinic monazite structure. Crystals of CsBr:MnO42- and CsI:MnO42- were prepared by slowly cooling the melt. The site symmetry in SrCrO4 is C1, but the deviations from D2 symmetry are small, and the spectra are analyzed in the approximate D2 symmetry. The 2E ↔ 2T2 ligand-field transitions are strongly c polarized, and in absorption only the transition to the lowest-energy component of the split 2T2 state is observed with its origin at 10 303 cm-1. Luminescence is exclusively observed to the lower-energy component of the 2E state. By a combination of absorption and luminescence data, it is shown that, in the cubic CsBr and CsI lattices, MnO42- i...

An oscillating visible light optical center in some natural green to yellow diamonds

Visible spectra taken at 72 K of a group of ten green-to-yellow-colored natural gem-quality diamonds show more than 30 sharp absorption bands between 13140∗ and 18420 cm−1 (1.63 and 2.28 eV), suggesting one or two defects that oscillate to visible light, a phenomenon not previously reported for diamond. Preliminary analysis of these spectra shows two oscillations, one dominating the lower energy end of this range with an energy spacing of 0.021 eV, and another appearing at the higher energy end of the range with a spacing of 0.025 eV. All samples show weak to moderate strength infrared absorptions typical of hydrogen, and of single substitutional nitrogen. The green color in nine of these diamonds is caused by the broad overlap at the base of these many absorption bands, not by a GR1 center, and thus forms a new cause of naturally occurring green color in diamond. Similar “many-banded” features have been observed by others in the low temperature cathodoluminescence spectra of diamond and in the low temperature luminescence spectra of minerals containing a molecular ion; perhaps the hydrogen or nitrogen (or both) play a role in causing these oscillations.

Excitonic luminescence spectra of hexagonal CuBr nanocrystals

The structure of low-temperature excitonic luminescence spectra of CuBr nanocrystals of hexagonal modification is studied. It is shown that the observed spectral structure can be explained assuming the formation of exciton-phonon complexes with their subsequent annihilation and radiative recombination of the exciton.

Hole-burning spectroscopy of photosynthetically active pigments of green sulphur photosynthetic bacteria

Spectral hole burning at low temperatures was used to study bacteriochlorophylls (BChl) e and d in natural chlorosomal structures as well as in artificially prepared monomeric and aggregated forms. Low-temperature (4.2 K) absorption and luminescence spectra were analysed to estimate spectral positions of different BChls' aggregated states. Excited state lifetimes for monomeric (~ 30 ns) and chlorosomal (~ 1–2 ps) BChl's were obtained from zero-phonon hole analysis. Huang-Rhys factor reflecting electron-phonon coupling was estimated for monomeric ( S ~ 1), lower-state aggregated ( S vary from S ~ 0.7 to S ~ 3 depending on spectral position) and oligomeric (chlorosomal) state ( S ~ 5) BChl e.

Investigation of uranyl nitrate extraction from ketone solutions by dimethyl sulfoxide

Based on an analysis of low-temperature luminescence spectra (T=77 K) of UO2(NO3)2·6H2O solutions in acetone, the mechanisms of formation of a wide group of uranyl complexes in uranyl extraction from solutions by dimethyl sulfoxide are studied. It is shown that to increase the coefficient of uranyl distribution between the solution and solid phase (in the form of UO2(NO3)2·2DMSO) it is necessary to add sulfoxide in small amounts, of about 0.35–0.5 mole per mole of uranyl. One-time introduction of DMSO in amounts of 1–3 mole per mole of uranyl leads to the formation of a number of uranyl complexes that are well soluble in acetone, and to a corresponding decrease in the distribution coefficient. The role of entropy and enthalpy in improvement of the stability of chelate complexes of uranyl nitrate is evaluated.

Absorption and Luminescence Spectroscopy of MnO(4)(2)(-)-Doped Crystals of BaSO(4).

The first polarized low-temperature absorption and luminescence spectra of manganese-doped crystals of BaSO(4) containing essentially MnO(4)(2)(-) are reported. By using a flux composed of NaCl, KCl, and CsCl we were able to grow BaSO(4):Mn(6+) crystals below 620 degrees C. This prevents the simultaneous presence of MnO(4)(3)(-) besides MnO(4)(2)(-), which was mainly responsible for the erroneous assignments of the absorption spectrum in the literature. In the BaSO(4) host the MnO(4)(2)(-) ion occupies a site of C(s)() symmetry, and the orbital degeneracies of the E and T states are thus lifted. Above 16 000 cm(-)(1) the absorption spectra consist of a series of intense ligand-to-metal charge transfer (LMCT) excitations. Their marked polarization dependence allows an unambiguous band assignment in the parent T(d)() symmetry. The three origins of the (2)E --> (2)T(2) ligand-field (LF) transition peak at 11 074, 11 570, and 11 790 cm(-)(1). The lowest-energy component of (2)T(2) serves as the initial state for broadband luminescence in the near-infrared (near-IR) region with a maximum at 9300 cm(-)(1). Below 100 K the quantum yield is unity and the radiative lifetime is 2.75 &mgr;s, and at 300 K the quantum yield is still 20%. In both the (2)E <--> (2)T(2) (d --> d) absorption and luminescence spectra the vibrational structure is dominated by progressions in O-Mn-O bending modes whereas coupling to the totally symmetric Mn-O stretching mode is less pronounced. The luminescence band shapes for the transitions to the two orbital components of (2)E are strikingly different; the Huang-Rhys parameters for the bending-mode progressions obtained from fits of simulated band shapes to the experimental spectra are 1.3 and 3.7, respectively. This is due to weak E multiply sign in circlee and stronger T(2) multiply sign in circlee Jahn-Teller (JT) effects in the ground and excited LF states, respectively. The linear vibronic coupling constants are f(E)() approximately 180 cm(-)(1) and f(T)() approximately -730 cm(-)(1) and the corresponding JT stabilization energies E(JT)((2)E) approximately 50 cm(-)(1) and E(JT)((2)T(2)) approximately 780 cm(-)(1), respectively.

Time-resolved luminescent spectra of J-aggregates with exciton traps

The possibility of coexistence of free and self-trapped excitons in one-dimensional molecular chains (J-aggregates) has been proved on the basis of experimental study of time-resolved low-temperature luminescence spectra.

Near-infrared and visible luminescence and absorption spectroscopy of low-coordinate organometallic rhenium compounds

Low-temperature absorption and luminescence spectra of four organorhenium compounds were measured in the visible and near-infrared spectral regions. Effects of the temperature, formal oxidation state, and the oxo-, bromo-, and organic ligands on the metal center are reported. The emission maxima of the four compounds show a large variation between 9000 and 14 200 cm−1. Stokes shifts show a smaller variation from 3700 to 4200 cm−1. Luminescence intensities are used to qualitatively characterize nonradiative relaxation processes. Key words: luminescence spectra, absorption spectra, organorhenium compounds.

Low-temperature doublet luminescence and absorption spectra of multidentate hexaam(m)ine Cr(III) complexes. Effects of ligand induced distortion from Oh symmetry

Luminescence and absorption spectra of the complexes [Cr([18]aneN6)]Br 3, [Cr(sen)]Br 3, [Cr(taptacn)]Br 3, trans-[Cr(tn) 2(NH 3) 2](ClO 4) 3, cis-[Cr(tn) 2)NH 3) 2](ClO 4) 3 and [Cr([20]aneN6)]Br 3 have been obtained at low temperatures. ([18]aneN6 = 1,4,7,10,13,16-hexaazacyclooctadecane, [20]aneN6 = 1,4,7,11,14,17-hexaazacycloeicosane, sen = 4,4′,4″-ethylidynetris-(3-azabutan-1-amine), taptacn = 1,4,7-tris(aminopropyl)-1,4,7-triazacyclononane, tn = 1,3-diaminopropane). The observed temperature dependence of luminescence band intensities, together with the comparison of the luminescence and absorption spectra, has allowed the assignment of the 2E g electronic origins, generally found to be split into two orbital components. The energies of the lowest energy doublet component {the origin splitting} were found to be 14 646{29}, 14 729{16}, 14 818{5?}, 15 108{44}, 15 14 680{72} cm −1 for the above sequence of complexes, respectively. In addition to the differences in 2E g splitting, considerable variance was found in the proportion in which the luminescence intensity appeared in the electronic origins as opposed to the vibronic origins. None of the spectra gave evidence for progressions. The data reveal the effect of symmetry lowering at the metal centre on both band intensities and doublet splittings. The observed splittings are compared with those calculated from the angular overlap model based on X-ray diffraction determinations of the coordination geometries around the metal centre. Trans-[Cr(tn) 2(NH 3) 2][(ClO 4) 3 is interesting because it shows the usual doublet splitting plus an additional solid state splitting of 5 cm −1 observed in the electronic origin and repeated with varying magnitude in many of the lines in the spectrum. This is attributed to the presence of two crystallographically non-equivalent complex sites in the crystal, which we were unfortunately not able to verify by X-ray diffraction owing to disorder in the perchlorate anions.

Luminescence of quantum-dimensional centers with spatially ordered molecules of organic compounds adsorbed on their surfaces

A dimensional effect in the luminescence of a narrow-band silver sulfide semiconductor (with the width of the forbidden band E{sub g} = 0.9 eV) was discovered in 1977, when a luminescent method was used to study the nature and functions of the centers appearing as a result of sulfur sensitization of halogen-silver photographic emulsions. As is known, during the indicated sensitization, certain sulfur-containing compounds (usually, sodium thiosulfate) are introduced into a melted emulsion containing microcrystals of silver halide, thus promoting an increase in the photosensitivity of the emulsion layers. The reason for the observed increase in photosensitivity is the fact that during the interaction between sulfur-containing compounds and silver halide silver-sulfide centers arise on the surface of the emulsion microcrystals, which serve as deep traps for electrons and therefore determine the possibility for silver centers of latent image to be formed near them by a two-stage electron-ion mechanism. Measurement of the low-temperature (T = 77 K) luminescence spectra of different photographic emulsions containing AgBr, AgBr (I), or AgCl microcrystals made it possible to establish that the introduction of sulfur sensitizers (sodium thiosulfate or thiourea) into emulsion to increase its photosensitivity is accompanied by the appearance of new luminescence bands inmore » the near IR spectral region.« less

Microcharacterization of composition modulations in epitaxial ZnSe1-xTex

Molecular beam epitaxial growth of the ZnSe1-xTex (x=0.44-0.47) alloy on vicinal (001) GaAs substrates tilted four, six, and nine degree-[111]A or B results in partial phase separation of the alloy with a vertical modulation between different compositions. Transmission electron microscopy images of samples grown on four degree-tilted substrates showed superlattice-like structures, with periods in the range 13.4-28.9Â. Lattice images reveal diffuse interfaces between light and dark bands. Period variations were detected in isolated regions of some samples. We present evidence that the modulation develops at the growth surface, and remains stable in the bulk at temperatures up to 450°C. Satellite spot pairs with approximate indices (h k 1 + δ) were present near the zinc-blende spots in electron diffraction patterns and x-ray diffraction data, as expected from material with a sinusoidal composition profile. The orientation of the spots reveals that the modulation vector is parallel to the growth direction, rather than to [001]. The [111]A- and B-tilted samples showed significant modulation, while the five degree-[110] and on-axis material showed no detectable modulation. The modulation wavelength did not strongly depend on growth temperature in the range examined (285–335°C). Samples showing composition modulation did not exhibit significantly altered low-temperature luminescence spectra from material with no modulation.

Zero-phonon line as the foundation stone of high-resolution matrix spectroscopy, persistent spectral hole burning, single impurity molecule spectroscopy

The purely electronic zero-phonon line (ZPL) - the optical analog of the Mössbauer-γ-resonance line - is a remarkable feature of the low temperature impurity absorption and luminescence spectra of solids. The narrowness and high peak absorption cross section of ZPLs has opened new horizons for high resolution spectroscopy of impurity molecules (centers). The newest and most spectacular achievement is single impurity molecule spectroscopy. Persistent spectral hole burning (PSHB), serving as a tool to control by means of illumination the absorption coefficient, and bound to it, via the Kramers-Kronig relations, the refraction index, turns inhomogeneous broadening of ZPLs into a useful feature and opens a variety of applications including PSHB optical data storage and processing. Space-frequency and space-time domain holographies are developed. The latter comprises the causality relations between events subject to holography and opens new principal possibilities in optics.

Zur Elektronenstruktur metallorganischer verbindungen der f-Elemente : XXXIV. Ist der N, N′-bis(trimethylsilyl)benzamidinato-Ligand im Falle von Lanthanoid-Zentralionen ein elektronisches Äquivalent zum η 5-Cyclopentadienyl-Liganden?

Magnetic data and the results of low temperature absorption, luminescence and magnetic circular dichroism spectra of 4-methoxy-substituted lanthanide ( III)tris[ N, N′-bis(trimethylsily)benzamidinates [(MeOBA) 3Ln; Ln  Eu ( 1), Pr ( 2)], and of adducts derived from tris(η 5-cyclopentadienyl)-europium( III) [CP 3EuX; X  CNC 6H 11 ( 3), THF ( 4)] are reported. In case of 1, 3 and 4 the 151Eu Mössbauer spectra also were recorded. On the basis of the optical measurements the truncated crystal field (CF) splitting patterns of 1 and 2 were derived. The parameters of an empirical Hamiltonian were fitted to the experimentally derived splitting patterns. The parameters obtained suggest that the BA ligand may be considered to a certain extent as an electronic equivalent to the Cp ligand in case of Ln central ions with small electron affinity. In case of central ions with higher oxidation power such as Eu 3+, the BA ligand behaves in a conventional manner. The Cp ligand, however, causes unusual physical properties which cannot be explained on the basis of an Eu 3+, central ion which is surrounded by three Cp ligands.

Spectroscopic properties of Cr^4+:Lu_3Al_5O_12

We report on the near-infrared absorption, emission, and lifetime data of Cr(4+):Lu(3)Al(5)O(12) (Cr:LAG) and compare the results with the known laser material Cr(4+)-doped Y(3)Al(5)O(12) (Cr:YAG). Lu(3)Al(5)O(12) has a smaller unit cell than Y(3)Al(5)O(12), and this feature is reflected in its spectroscopic properties. The low-temperature luminescence spectrum is shifted by 85 cm(-1) to higher energy compared with Cr:YAG. The luminescence lifetime of Cr:LAG at 10 K is 28.7 micros (Cr:YAG, 30.6 micros) and at 300 K is 4.3 micros (Cr:YAG, 3.5 micros). From these lifetimes we postulate that the quantum efficiency in Cr:LAG is higher than in Cr:YAG. We discuss the potential of Cr:LAG as a tunable near-infrared laser.

CN- molecular ions in silver halides: optical properties of a new isoelectronic exciton trap

Low-temperature optical absorption and luminescence spectra of AgCl and AgBr doped with CN- molecular defects are investigated. While in AgBr an amalgamation type of exciton is formed, CN- in AgCl is found to act as an isoelectronic trap, producing a bound exciton state with a localization energy of about 350 meV. Evidence for strong lattice relaxation to stabilize this state is provided by the positions and lineshapes of the spectra. An exciton model is discussed to account for the strong temperature dependence of the decay time and intensities. The results are compared with those for atomic-type isoelectronic impurities in AgCl, like Br- and I-.

Aluminum acceptor four particle bound exciton complex in 4H, 6H, and 3C SiC

New lines in the low temperature luminescence spectra of lightly aluminum doped p-type films of 3C, 6H, and 4H SiC are identified and associated with the recombination of a neutral aluminum acceptor four particle bound exciton complex.

Two-dimensional hole gas and Fermi-edge singularity in Be delta -doped GaAs.

The subband structure of the quasi-two-dimensional hole gas (2DHG) formed at a single Be \ensuremath{\delta}-doped layer in GaAs has been studied by photoluminescence spectroscopy. To confine the photogenerated minority carriers, and thus to enhance the efficiency of radiative recombination from the 2DHG, the \ensuremath{\delta}-doping spike was placed in the center of an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double heterostructure. Recombination involving different hole subbands has been resolved which enabled us to analyze the subband occupation as a function of dopant concentration and sample temperature. In sample structures where the Fermi level is located close to unoccupied subbands, a pronounced Fermi-edge singularity (FES) is observed in the low-temperature (20 K) luminescence spectrum. The temporal evolution of the FES has been studied by time-resolved luminescence spectroscopy. The enhancement in emission intensity at the Fermi edge can be understood in terms of a transfer of excitonic oscillator strength from the unoccupied subbands to nearby occupied states at the Fermi energy. Self-consistent subband calculations have been performed to compute the hole confining potential and the subband energies for the present \ensuremath{\delta}-doped structures. The results of these calculations, which take into account the finite spread of the dopant atoms in accordance with secondary-ion-mass spectroscopic data, are in good agreement with the measured subband spacings. The assignment of light- and heavy-hole transitions is supported by luminescence measurements using circularly polarized light.

The effect of size restriction on exciton relaxation in AgBr microcrystals

The low temperature luminescence and excitation spectra of the iodine bound exciton and of donor-acceptor pairs are investigated in monodispersed AgBr microcrystals. The crystallites have uniform cubic shape with 450 and 50 nm edge lengths. Significantly different from large samples, the excitation spectra above the exciton absorption edge exhibit strongly pronounced phonon structure which indicates that excitons that were scattered by LO intravalley phonons into states with low kinetic energy are preferentially trapped. Model calculations are consistent with the supposition that in the microcrystals surface annihilation of excitons becomes an increasingly important relaxation mechanism. The measurements demonstrate exciton trapping at iodine. They also provide direct evidence that intermediate-case excitons are formed and contribute to the donor-acceptor luminescence band. In the 50 nm sample the near-edge exciton luminescence suggests starting quantum confinement of states.