Low-temperature Spectra Research Articles

AlGaAs/GaAs/AlGaAs quantum wells as a sensitive tool for the MOVPE reactor environment

We present in this work a simple quantum well (QW) structure consisting of GaAs wells with AlGaAs barriers as a probe for measuring the performance of arsine purifiers within a metalorganic vapour phase epitaxy system. Comparisons between two different commercially available purifiers are based on the analysis of low-temperature photoluminescence emission spectra from thick QWs, grown on GaAs substrates misoriented slightly from (1 0 0). Neutral excitons emitted from these structures show extremely narrow linewidths, comparable with those that can be obtained by molecular beam epitaxy in an ultra-high vacuum environment, suggesting that purifications well below the 1 ppb level are needed to achieve high quality quantum well growth.

Redox Behavior of Rhodium 9,10-Phenanthrenediimine Complexes

New square-planar rhodium complexes of the redox-active ligand 9,10-phenenthrenediimine (phdi) have been prepared and studied. The complexes [dpp-nacnac(CH3)]Rh(phdi) (2a; [dpp-nacnac(CH3)](-) = CH[C(Me)(N-(i)Pr(2)C(6)H(3))](2)(-)) and [dpp-nacnac(CF3)]Rh(phdi) (2b; [dpp-nacnac(CF3)](-) = CH[C(CF(3))(N-(i)Pr(2)C(6)H(3))](2)(-)) have been prepared from the corresponding [nacnac]Rh(CO)(2) synthons by treatment with Me(3)NO in the presence of the phdi ligand. Complexes 2a and 2b are diamagnetic, and their absorption spectra are dominated by intense charge-transfer transitions throughout the visible region. Electrochemical studies indicate that both the phdi ligand and the rhodium metal center are redox-active, with the [nacnac](-) ligands serving to modulate the one-electron-oxidation and -reduction redox potentials. In the case of 2a, chemical oxidation and reduction reactions provided access to the one-electron-oxidized cation, [2a](+), and one-electron-reduced anion, [2a](-), the latter of which has been characterized in the solid state by single-crystal X-ray diffraction. Solution electron paramagnetic resonance spectra of [2a](+) and [2a](-) are consistent with S = (1)/(2) spin systems, but surprisingly the low-temperature spectrum of [2a](-) shows a high degree of rhombicity, suggestive of rhodium(II) character in the reduced anion.

Structure and sensitized near-infrared luminescence of Yb(III) complexes with sulfonylamidophosphate type ligand

A series of new lanthanide complexes with dimethyl(phenylsulfonyl)amidophosphate (H SP) was synthesized ({Na[Ln(SP) 4]} n ( Ln1), [Ln(SP) 3phen] ( Ln2) and [Ln(SP) 3bpy]·H 2O ( Ln3) (where Ln = Gd III, Er III, Yb III and Lu III; phen = 1,10-phenantroline; and bpy = 2,2′-bipyridine) and the crystal structures of [Er(SP) 3phen] and [Yb(SP) 3bpy]·H 2O were resolved. Absorption (at 295, 4 K), emission (at 295, 77, 4 K) and IR (at 295 K) spectra as well as luminescence decay time measurements were used to characterize the photophysical properties of single crystals. Effective energy transfer from a [SP] − ligand to the Yb III ion was demonstrated despite a large energy gap Δ E = E trip − E 2 F 5 / 2 . Replacing one molecule of the [SP] − ligand with the phen or bpy molecule caused reduction of the emission decay time from 35 μs for Yb1 to 20 and 15.5 μs for Yb2 and Yb3, respectively at the temperature of 295 K. Using the absorption spectra τ rad (1.1 ms— Yb1, 943 μs— Yb2 and 914 μs— Yb3) and Q Ln Ln (3.14%— Yb1, 2.19%— Yb2, 1.64%— Yb3) were estimated. On the basis of low temperature, high-resolution absorption and luminescence spectra, the ligand-field splittings of the excited and ground states of Yb III complexes were determined.

Optical Spectroscopy of Sm3+ and Dy3+ Doped ZnO Nanocrystals

ABSTRACT Sm3+ and Dy3+ ions doped hexagonal wurtzite ZnO nanocrystals were fabricated by a sol-gel method. The obtained ZnO nanocrystals were characterized using the X-ray diffraction, transmission electron microscopy, ultraviolet-visible reflectance spectra, and low-temperature luminescence spectroscopy. Intense and well resolved emission lines for Sm3+ and Dy3+ ions can be achieved via an energy transfer process from ZnO host to the dopants. It was found that the host sensitized emissions were more efficient than that of direct excitation for Sm3+ and Dy3+ ions. Moreover, multiple sites of Sm3+ and Dy3+ ions in ZnO nanocrystals were identified based on the low-temperature photoluminescence spectra.

Highly Ionized sodium X-ray line emission from the solar corona and the abundance of sodium

{The \ion{Na}{x} X-ray lines between 10.9 and 11.2~\AA\ have attracted little attention but are of interest since they enable an estimate of the coronal abundance of Na to be made. This is of great interest in the continuing debate on the nature of the FIP (first ionization potential) effect. } {Observations of the \ion{Na}{x} lines with the Solar Maximum Mission Flat Crystal Spectrometer and a rocket-borne X-ray spectrometer are used to measure the Na/Ne abundance ratio, i.e. the ratio of an element with very low FIP to one with high FIP.} {New atomic data are used to generate synthetic spectra which are compared with the observations, with temperature and the Na/Ne abundance ratio as free parameters.} {Temperature estimates from the observations indicate that the line emission is principally from non-flaring active regions, and that the Na/Ne abundance ratio is $0.07 \pm 50$\%.} {The Na/Ne abundance ratio is close to a coronal value for which the abundances of low-FIP elements (FIP $< 10$~eV) are enhanced by a factor of 3 to 4 over those found in the photosphere. For low-temperature ($T_e \leqslant 1.5$~MK) spectra, the presence of \ion{Fe}{xvii} lines requires that either a higher-temperature component is present or a revision of ionization or recombination rates is needed. }

Structural, optical and electrical properties of Al–N codoped ZnO films by RF-assisted MOCVD method

N-doped ZnO films were produced using N 2 as N source by metal-organic chemical vapor deposition (MOCVD) system which has been improved with radio-frequency (RF)-assisted equipments. The data of secondary ion mass spectroscopy (SIMS) indicate that the concentration of N in N-doped ZnO films is around 5 × 10 20 cm −3, implying that sufficient incorporation of N into ZnO can be obtained by RF-assisted equipment. On this basis, the structural, optical and electrical properties of Al–N codoped ZnO films were studied. Then, the effect of RF power on crystal quality, surface morphologies, optical properties was analyzed using X-ray diffraction, atomic force microscopy and photo-luminescence methods. The results illustrate that the RF plasma is the key factor for the improvement of crystal quality. Then the observation of A 0X recombination associated with N O acceptor in low-temperature PL spectrum proved that some N atoms have occupied the positions of O atoms in ZnO films. Hall measurements shown that p-type ZnO film deposited on quartz glasses was obtained when RF power was 150 W for the Al–N codoped ZnO films, while the resistivity of N-doped ZnO films was rather high. Compared with the Al-doped ZnO film, the obviously increased resistivity of codoped films indicates that the formation of N O acceptors compensate some donors in ZnO films effectively.

Computational study of the equilibrium geometry and anharmonic vibrational spectra of PbX2 ··· NO and PbX2 ··· ON (X = F, Cl, Br, I) complexes

Equilibrium geometry parameters of the open shell PbX2 ··· NO and PbX2 ··· ON (X = F, Cl, Br, I) complexes have been computed by second-order Z-averaged perturbation theory (ZAPT2) with Stevens–Basch–Krauss–Jasien–Cundari (SBKJC) scalar-relativistic effective core potentials (RECP) and basis sets on all atoms. Equilibrium geometries of both PbX2 ··· NO and PbX2 ··· ON bonding isomers conform to Cs symmetry structure with end-on ligand coordination, and are characterized by Pb-N bond length within 266.6–271.7 pm range, Pb–O distance of 267.8–275.8 pm, Pb–N–O angle within 109.2–120.7° range, and Pb–O–N angle of 117.1–127.8°. Anharmonic vibrational spectra of the PbX2 ··· NO and PbX2 ··· ON complexes have been calculated by direct correlation-corrected vibrational self-consistent field (CC-VSCF) method enhanced with second-order perturbative correction using potential energy surfaces (PESs) determined at ZAPT2/SBKJC+(d) level in curvilinear (internal) coordinates. Fundamental ν(Pb–N) stretching mode has been computed at 232.8 to 209.0 cm−1 within PbX2 ··· NO series. whereas ν(Pb–O) stretching mode fundamental evaluation in PbX2 ··· ON series afforded wavenumbers within 183.2–150.7 cm−1 range. Blue shift of the ν(N=O) stretching mode wavenumber upon PbX2 ··· NO complex formation, computed in anharmonic approximation, 15.8–14.6 cm−1, correctly reproduces the effect observed in the low-temperature Ar matrix spectra of PbX2 ··· NO compounds. Influence of complex formation on the νs(Pb–X) and νas(Pb–X) fundamentals of PbX2 halides has also been discussed. Two-dimensional mapping of the (Qi , Qj ) mode-mode coupling potential has been used to rationalize the origin of mode coupling related anharmonic corrections.

XeF2Coordination to a Halogen Center; Raman Spectra (n= 1, 2) and X-ray Crystal Structures (n= 2) of [BrOF2][AsF6]·nXeF2and [XOF2][AsF6] (X = Cl, Br)

The syntheses and structural characterizations of the [XOF(2)][AsF(6)] (X = Cl, Br) salts and the XeF(2) adduct-salts, [BrOF(2)][AsF(6)].nXeF(2) (n = 1, 2), are described. Although the [XOF(2)][AsF(6)] salts have been known for some time, their crystal structures had not been reported until the present study. The crystal structure of [BrOF(2)][AsF(6)] shows a positional disorder among the oxygen atom and the fluorine atoms. Both ClOF(2)(+) and BrOF(2)(+) have pseudo-octahedral coordination with a primary tripodal coordination sphere consisting of an oxygen atom and two fluorine atoms and a secondary coordination sphere consisting of three long contacts to fluorine atoms of different AsF(6)(-) anions. The low-temperature Raman spectra of [XOF(2)][AsF(6)] have been assigned on the basis of the crystal structures and with the aid of quantum-chemical calculations using [XOF(2)][AsF(6)](3)(2-) as a model for the crystallographic environment of XOF(2)(+). Several examples of XeF(2) coordinated through fluorine to transition metal centers are known, but no crystallographically characterized examples of XeF(2) coordinated to a nonmetal center other than xenon are known. The complex cation salts, [BrOF(2)][AsF(6)].nXeF(2) (n = 1, 2), were synthesized, and their Raman spectra have been assigned with the aid of quantum-chemical calculations. Although the structure of [BrOF(2)][AsF(6)].2XeF(2) is similar to that of the recently reported krypton analogue, notable differences occur. The contact distances between bromine and the fluorine atoms of NgF(2) (Ng = Kr, Xe) are shorter in [BrOF(2)][AsF(6)].2XeF(2) than in the KrF(2) analogue, which is attributed to the more polar natures of the Xe-F bonds. Unlike [BrOF(2)][AsF(6)].2KrF(2), which has been shown in the prior study to be stable in HF solution at room temperature, [BrOF(2)][AsF(6)].2XeF(2) enters into a dissociative equilibrium in which fluoride ion abstraction by BrOF(2)(+) occurs to give Xe(2)F(3)(+) and BrOF(3). The ELF and QTAIM analyses of [BrOF(2)][AsF(6)](3)(2-) and [BrOF(2)][AsF(6)].2XeF(2) were carried out and are compared with those of [BrOF(2)][AsF(6)].2KrF(2) and for free BrOF(2)(+) to better understand the effect of Br(V) coordination number on the localization domain of the Br(V) valence electron lone pair.

Multiple-peak structure in porous Si photoluminescence

Photoluminescence (PL) measurements have been carried out on anodic porous silicon (PSi) at temperatures between T=20 and 300 K. He–Ne laser (1.96 eV), green laser diode (2.33 eV), and He–Cd laser (3.81 eV) are used as excitation light sources. Low-temperature (T≤250 K) PL spectra show a multiple-peak structure even when laser excitation is far from resonance with the red PL band. The configurational-coordinate model explains such unique multiple-peak structure very well. Fourier transform (FT) technique is used for the analysis of the measured multiple-peak structure. The FT frequency is determined to be ∼64 meV, which is in exact agreement with the bulk long-wavelength optical phonon energy in crystalline Si. Although the origin of the multiple-peak structure is bulk-related, its strength is very sensitive to the presence of surface oxide.

Growth and high resolution spectroscopic investigation of YAB:Tm crystal

Tm-doped YAB single crystals were grown by the top-seeded flux technique. High resolution (0.04 cm −1) Fourier transform spectroscopy was employed to determine the term diagram of Tm 3+ ions in YAB. Six transitions were identified from the 3H 6 ground state of the Tm 3+ ions to the 3F 4, 3H 5, 3H 4, 3F 3, 3F 2, and 1G 4 manifolds. All of their Stark components were assigned by using the temperature dependence (9–100 K) of the absorption lines. The low temperature spectra consists of sharp lines, broadened ones and those split to components. These features may be attributed to Tm–Tm interaction and optical hyperfine structure.

Ultrafast Dynamics of Frenkel Excitons in Tetracene and Rubrene Single Crystals

Characteristics of thermally driven environmental motion (i.e., spectral densities) similarly control energy and charge-transport processes in self-assembled molecular aggregates, crystalline molecular solids, and photosynthetic antennae. A true microscopic understanding of these transport processes necessarily involves decomposing the spectral density of thermal noise into contributions from specific nuclear motions (i.e., modes). To this end, molecular solids may serve as excellent model systems due to their known crystal structures and well-defined intermolecular modes. Here, the electronic relaxation dynamics of tetracene (Tc) and rubrene (Rb) single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Parameterization of the model is achieved by comparing simulated optical signals with those measured in experiments. In addition, electronic population transfer rates are computed with this same set of parameters using a modified Redfield theory. An important aspect of the model is its use of femtosecond stimulated Raman spectroscopies to obtain nuclear mode-specific spectral densities (i.e., polarizability spectral densities). Attainment of the spectral densities facilitates the interpretation of electronic spectroscopies, which are sensitive to both exciton delocalization and population transfer kinetics. One important prediction of the model, which is based on the comparison of low-temperature linear absorption spectra and model calculations, is that the exciton sizes for both Tc and Rb are approximately 18 molecules at 200 and 78 K, respectively. In addition, transient grating experiments detect sub-100 fs intraband population transfer processes in both crystals. Photon echo experiments and model calculations further support the assignment of these dynamics to electronic population transfer. The role of spatial correlations in the spectral densities at different molecular sites is also investigated. Calculations predict a remarkable behavior in which variation in the amount of spatial correlation for just a single mode in the spectral density, among many, causes the electronic relaxation rates to vary over an order of magnitude. This finding holds for both crystals and will likely generalize to molecular aggregates and photosynthetic antennae, thereby contributing to a microscopic understanding of phenomena that originate in spatially correlated fluctuations (e.g., coherent energy transfer).

Conformational Space and Photochemistry of α-Terpinene

Alpha-terpinene is a natural product that is isolated from a variety of plant sources and is used in the pharmaceutical and perfume industries. In the atmosphere, under the influence of sunlight, alpha-terpinene undergoes a series of photochemical transformations and contributes to the formation of the secondary organic aerosols. In the present work, alpha-terpinene has been isolated in low-temperature xenon and argon matrices, and its structure and photochemistry were characterized with the aid of FTIR spectroscopy and DFT calculations. The theory predicts three conformers resulting from the rotation of the exocyclic CH(CH(3))(2) framework, that is, Trans (T) and Gauche (G+ and G-) forms. The two Gauche conformers were estimated to be higher in energy, by ca. 1.75 kJ mol(-1), than the most stable Trans form. The signatures of all three conformers were found to be present in the experimental low-temperature matrix spectra with the T form dominating in diluted matrices. The conformational ratio was found to shift in favor of the G+/G- forms upon annealing of the matrices as well as in the neat alpha-terpinene liquid. UV-C (lambda > 235 nm) irradiation of matrix-isolated alpha-terpinene led to its isomerization into an open-ring species, which is produced in the Z configuration and in the conformations that require the smallest structural rearrangements of both the reagent and matrix.

Spectrum designation and effect of Al substitution on the luminescence of Cr 3+ doped ZnGa 2O 4 nano-sized phosphors

Low-temperature photoluminescent spectra of ZnGa 2O 4:Cr 3+ nano-sized phosphors calcined at different temperatures were reported. The fine structure of the emission spectra has been designated to Cr 3+ ions in different sites including ideal octahedral, Zn-interstitial, Ga ZN 4 ‐ Zn Ga 6 sites and Ga 2O 3 impurity. The vibronic sidebands for both Stokes’ and anti-Stokes’ sides are related to the host lattice vibrations, which were confirmed by IR and Raman spectra. Al 3+ is substituted in Ga 3+ sites to form Zn ( Ga 1 − y Al y ) 2 O 4 : Cr 0.01 3 + (0≤ y≤0.5). The blue shift and luminescent intensity variations of the charge transfer band and 3d–3d transitions in the spectra caused by Al substitution were related to larger band gap and stronger crystal field, respectively. The calculated crystal-field parameters indicated that Al incorporation enhanced the crystal field strength and induced more trigonal distortion due to different radii of Al 3+ and Ga 3+.

UV Emission from Patterned Growth of ZnO Nanowires

Patterned and high-density ZnO nanowires were fabricated on a large area P type silicon (100) through Vapor-Liquid-Solid approach. An aluminium mask was used for the patterned deposition of gold catalyst. ZnO nanowires grown with the aid of gold catalyst were well aligned and had dimensions of 30-80 nm diameter and 0.5-2 microm length. The perfect crystallinity and orientation [0002] of the nanowires were confirmed by high-resolution transmission electron microscopy (HR-TEM). Low-temperature photoluminescence spectrum exhibits an intensive UV emission at approximately 380 nm and a weak broad green band at approximately 530 nm which is detected under an optical excitation of 325 nm wavelength of He-Cd laser. These arrays of ZnO nanowires could have myriad applications in nanolasers, optoelectronic devices and optical interconnects.

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about $\ensuremath{-}0.6\text{ }\text{meV}$, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.

Main chain and segmental dynamics of semi interpenetrating polymer networks based on polyisoprene and poly(methyl methacrylate)

Main chain and segmental dynamics of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) chains in semi IPNs were systematically studied over a wide range of temperatures (above and below T g of both polymers) as a function of composition, crosslink density, and molecular weight. The immiscible polymers retained most of its characteristic molecular motion; however, the semi IPN synthesis resulted in dramatic changes in the motional behavior of both polymers due to the molecular level interpenetration between two polymer chains. ESR spin probe method was found to be sensitive to the concentration changes of PMMA in semi IPNs. Low temperature spectra showed the characteristics of rigid limit spectra, and in the range of 293–373 K, complex spectra were obtained with the slow component mostly arising out of the PMMA rich regions and fast component from the PI phase. We found that the rigid PMMA chains closely interpenetrated into the highly mobile PI network imparts motional restriction in nearby PI chains, and the highly mobile PI chains induce some degree of flexibility in highly rigid PMMA chains. Molecular level interchain mixing was found to be more efficient at a PMMA concentration of 35 wt.%. Moreover, the strong interphase formed in the above mentioned semi IPN contributed to the large slow component in the ESR spectra at higher temperature. The shape of the spectra along with the data obtained from the simulations of spectra was correlated to the morphology of the semi IPNs. The correlation time measurement detected the motional region associated with the glass transition of PI and PMMA, and these regions were found to follow the same pattern of shifts in α-relaxation of PI and PMMA observed in DMA analysis. Activation energies associated with the T g regions were also calculated. T 50G was found to correlate with the T g of PMMA, and the volume of polymer segments undergoing glass transitional motion was calculated to be 1.7 nm 3. C-13 T 1ρ measurements of PMMA carbons indicate that the molecular level interactions were strong in semi IPN irrespective of the immiscible nature of polymers. The motional characteristics of H atoms attached to carbon atoms in both polymers were analyzed using 2D WISE NMR. Main relaxations of both components shifted inward, and both SEM and TEM analysis showed the development of a nanometer – sized morphology in the case of highly crosslinked semi IPN.

Emission characteristics of ZnO nanorods on nanosilicon-on-insulator: competition between exciton–phonon coupling and surface resonance effect

We investigated the optical properties of ZnO nanorods on nanosilicon-on-insulator using variable temperature photoluminescence (PL) spectroscopy, and explored the contribution of exciton–phonon coupling and surface resonance effect on the emission characteristics of the nanorods. The low-temperature (<100 K) PL spectra revealed different strengths of exciton–phonon interaction for nanorods of different surface structures. The exciton–phonon coupling strength was stronger for nanorods of rougher surfaces with enhanced contribution of longitudinal optical phonon replicas of free exciton. Despite exhibiting different coupling strengths of exciton–phonon interactions, the room-temperature PL showed an unchanged energy position at 3.28 eV for nanorods of different surface structures. The unchanged energy position of band-edge emission was caused by the competitive effect of the surface defects induced exciton–phonon interaction and the surface resonance effect in faceted nanorods.

Molecular beam epitaxial growth effects on type-II antimonide lasers and photodiodes

Two important classes of electro-optical devices, midwave-infrared interband cascade lasers (ICLs) and long-wave infrared photodiodes, employ type-II antimonide active regions grown by molecular beam epitaxy (MBE). The authors have studied how growth temperature and As flux affect the integrated intensity and linewidth of low-temperature photoluminescence spectra emitted from ICL active regions. Possible ramifications for defect-assisted Auger processes are discussed, and experimental Auger coefficients are extracted from the measured ICL thresholds and slope efficiencies. They also demonstrate that the effective dynamic impedance of a type-II photodiode with graded-gap depletion region is quite sensitive to the doping level in the absorber. Majority acceptor concentration is a key design parameter that must be carefully controlled in the MBE growth.

Sll0396 regulates transcription of the phycocyanin genes in Synechocystis sp. PCC 6803

An olive-green mutant was generated in Synechocystis sp. strain PCC 6803 by inactivation of the sll0396 gene. Whole-cell absorption spectra of the mutant revealed the missing of phycocyanin peak. An investigation of the low-temperature fluorescence emission spectra revealed that the sll0396Ω mutant has a reduced amount of phycocyanin. Western blot analysis showed that the mutant contained less phycocyanin β- and α-subunits and lacked the 30- and 32-kDa linker polypeptides, and northern blot analysis revealed that the transcription of the 1.4-kb cpcBA gene encoding the phycocyanin β- and α-subunits was lower in the mutant. The Sll0396 protein has a DNA-binding motif and shares homology with known response regulators. Our results indicate that Sll0396 plays a regulatory role in the transcription of the phycocyanin genes during phycobilisome synthesis.

Spatial distribution of rare-earth ions andGaS4tetrahedra in chalcogenide glasses studied via laser spectroscopy andab initiomolecular dynamics simulation

The spatial distribution of ${\text{Nd}}^{3+}$ ions and ${\text{GaS}}_{4}$ tetrahedral units in Nd-doped Ge-As-Ga-S glasses has been studied by laser spectroscopy and ab initio molecular dynamics (MD) simulations. A sharp increase in ${\text{Nd}}^{3+}$ fluorescence intensities and lifetimes was observed with increasing Ga content, and attributed to the formation of tightly bound ${\text{Nd}}^{3+}$ clusters in Ga-free glasses and the subsequent dissolution of such clusters upon Ga doping. A large modification in ${\text{Nd}}^{3+}$ sites was also identified from low-temperature site-selective excitation spectra, suggesting preferential spatial correlations between ${\text{Nd}}^{3+}$ and ${\text{GaS}}_{4}$ tetrahedra even at low Ga-doping levels. MD simulations of these materials in the liquid state showed a tendency for Ga cluster formation as well as spatial correlations between Nd and Ga atoms consistent with the experimental results. On the basis of this result, a comprehensive structural model for Nd- and Ga-doped sulfide glasses is proposed.