Low-temperature Spectra Research Articles

Relativistic effects in a phosphorescent Ir(III) complex

We compare high field magnetic circular dichroism, absorption, and photoluminescence spectra for Ir(ptz)(3) with time-dependent density functional theory. By gradually turning on the relativistic effects we identify several distinct relativistic effects in the spectra of this complex. We show that relativistic effects must be included to accurately predict the low-temperature spectra. This leads to new insights into the low-lying excitations responsible for the observed phosphorescence, and suggests new avenues to improve the performance of organic light emitting diodes.

Effect of different annealing temperature on Sb-doped ZnO thin films prepared by pulsed laser deposition on sapphire substrates

Influence of annealing temperature on the properties of Sb-doped ZnO thin films were studied. Hall measurement results indicated that the Sb-doped ZnO annealed at 950 °C was p-type conductivity. X-ray diffraction (XRD) results indicated that the Sb-doped ZnO thin films prepared at the experiments are high c-axis oriented. It was worth noting that p-type sample had the worst crystallinity. The measurements of low-temperature photoluminescence (PL) spectra indicate that the sample annealed at the temperatures of 950 °C showed strong acceptor-bound exciton (A 0X) emission, and confirmed that it is related to Sb-doping by comparing with the undoped ZnO low-temperature PL spectrum.

Optical Conductivity Spectral Anomalies in the Off-Center Rattling Systemβ−Ba8Ga16Sn30

We present optical conductivity studies of the type-I clathrate Ba8Ga16Sn30, using a terahertz time-domain spectrometer (0.3-3.0THz). The lowest-lying spectral peak at 0.72THz due to the Ba(2) ion's off-center vibration in the oversized cage shows a drastic and anomalous temperature dependence. Below about 100K, the single broad peak splits into two subpeaks, and with further lowering of the temperature, the spectral shape of this so-called rattling phonon shows non-Boltzmann broadening to the point that the linewidth becomes comparable to the peak frequency. Whereas the initial splitting can be understood by assuming a multiwell anharmonic potential, the strong linewidth broadening toward low temperature cannot, since the Boltzmann factor generally sharpens the low-temperature spectra. The observed behavior suggests strong interaction between the local anharmonic phonons and other excitations.

Effects of Seed Layers on Structural, Morphological, and Optical Properties of ZnO Nanorods

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.

Impurity spectroscopy at its ultimate limit: relation between bulk spectrum, inhomogeneous broadening, and local disorder by spectroscopy of (nearly) all individual dopant molecules in solids

We present a technique for the measurement of the low-temperature fluorescence excitation spectra and imaging of a substantial fraction of all single chromophore molecules (hundreds of thousands and even more) embedded in solid bulk samples as nanometre-sized probes. An important feature of our experimental studies is that the full information about the lateral coordinates and spectral parameters of all individual molecules is stored for detailed analysis. This method enables us to study a bulk sample in a broad spectral region with ultimate sensitivity, combining excellent statistical accuracy and the capability of detecting rare events. From the raw data we determined the distributions of several parameters of the chromophore spectra and their variations across the inhomogeneous absorption band, including the frequencies of the electronic zero-phonon lines, their spectral linewidths, and fluorescence count rates. Relationships between these distributions and the disorder of the matrix were established for the examples of two polycrystalline solids with very different properties, n-hexadecane and o-dichlorobenzene, and the amorphous polymer polyisobutylene. We also found spatially inhomogeneous distributions of some parameters.

Unusual electric field gradient variation in the NpPd2Ge2 compound

NpPd2Ge2 has been studied by 237Np Mössbauer spectroscopy between 4.2 K and 80 K. The value of the isomer shift suggests a Np3+ electronic configuration. The low temperature spectra provide a value of 2.10 μB for the magnetic ordered moment on the Np site. These values suggest a rather localized character of the 5f electrons in this system. The magnetic transition around 60 K is accompanied by a large increase of the quadrupolar interaction that is evidence of a redistribution of the electronic charges around the neptunium nucleus.

FLUORESCENCE EMISSION SPECTRA OF MARINE AND BRACKISH-WATER ECOTYPES OF FUCUS VESICULOSUS AND FUCUS RADICANS (PHAEOPHYCEAE) REVEAL DIFFERENCES IN LIGHT-HARVESTING APPARATUS(1).

The Bothnian Sea in the northerly part of the Baltic Sea is a geologically recent brackish-water environment, and rapid speciation is occurring in the algal community of the Bothnian Sea. We measured low-temperature fluorescence emission spectra from the Bothnian Sea and the Norwegian Sea ecotypes of Fucus vesiculosus L., a marine macroalga widespread in the Bothnian Sea. Powdered, frozen thallus was used to obtain undistorted emission spectra. The spectra were compared with spectra measured from the newly identified species Fucus radicans Bergström et L. Kautsky, which is a close relative of F. vesiculosus and endemic to the Bothnian Sea. The spectrum of variable fluorescence was used to identify fluorescence peaks originating in PSI and PSII in this chl c-containing alga. The spectra revealed much higher PSII emission, compared to PSI emission, in the Bothnian Sea ecotype of F. vesiculosus than in F. radicans or in the Norwegian Sea ecotype of F. vesiculosus. The results suggest that more light-harvesting chl a/c proteins serve PSII in the Bothnian Sea ecotype of F. vesiculosus than in the two other algal strains. Treatment of the Bothnian Sea ecotype of F. vesiculosus in high salinity (10, 20, and 35 practical salinity units) for 1 week did not lead to spectral changes, indicating that the measured features of the Bothnian Sea F. vesiculosus are stable and not simply a direct result of exposure to low salinity.

Clock Shift and Interstate Coherence of Multi-Level Atoms

In contrast to two-level atoms, the interstate coherence of both three-level bosons and fermions contributes to the collisional frequency shift of internal transitions in a cold homogeneous gas. In particular, the measured contact shift of the b−c (a−d) hyperfine transition in 3D atomic hydrogen in the presence of incoherently populated state a (b) yields the difference at−as=30(5) pm between the triplet and singlet s-wave scattering lengths, which is a factor of 2 smaller than the previous result. The dynamic contact shift of the b−c resonance due to the Rabi oscillations between the coherently populated states a and b under the simultaneous continuous excitation of the a−b transition can explain the low-temperature electron-nuclear double resonance spectra of 2D hydrogen. The role of field inhomogeneity, spin diffusion, relaxation and recombination is discussed.

Changes in antenna of photosystem II induced by short-term heating.

Changes in antenna of photosystem II, induced by short-term heating, were studied using characteristics of a short-wavelength band in low-temperature fluorescence spectra (77K) of pea chloroplasts. Heating for 5min was carried out at 25 and 45°C in the darkness or in the presence of white light with intensity of 260 or 1,400μmol/m(2)s. Most modes of thermal treating induced a decrease in integral intensity of the band and an increase of its half-width. The changes were more prominent at high-temperature heating. The second derivative of the contour of a short-wavelength band showed its three components around 680, 685, and 693nm, the first of which belongs to emission of the outer antenna of Photosystem II, and the other two to its inner antenna. As the fourth derivative shows, high-temperature heating in the presence of light evokes an appearance of some additional components in a short-wavelength region (654, 658, 661, 666, 672, and 675nm) as well as of two additional components, 682 and 689nm, in the region of 685-nm peak. Two subcomponents, 692 and 694nm, can be detected in the 693-nm component. The results are discussed on the basis of the data concerning energy levels and pathways of energy transfer in pigment-protein complexes of the outer and the inner antennas of photosystem II. It is assumed that a protective role of low light relates to inducing of an essential disarrangement in the outer and the inner antennas and of a subsequent decrease in energy funneling to reaction centers, which, in turn, lowers the extent of photoinhibition.

Optical conductivity in the vicinity of a quantum critical point

We demonstrate an interrelation between the magnetic properties and optical conductivity for 3d electron systems with active orbital degree of freedom at a transition-metal ion: t2g in d 1 and eg in d 7 configuration. Both systems are described within the two-band Hubbard model which we analyze using exactdiagonalization technique for a two-site molecule at quarter filling. We highlight the main features of the low-temperature optical conductivity spectra for eg and t2g electrons, in the presence of the crystal-field splitting and show that these spectra provide a way to determine both Hund’s exchange JH and intraorbital Coulomb interaction U. The orbital polarization and the entanglement between spin and orbital degrees of freedom are also discussed, together with possible violations of the Goodenough-Kanamori rules.

Early and late plastid development in response to chill stress and heat stress in wheat seedlings

Five-day-old etiolated wheat (Triticum aestivum L.) seedlings were transferred to 7°C (chill stress), 25°C (control), and 42°C (heat stress) and were kept in the dark or light for different time periods. Plastids were isolated from the control and stressed seedlings, and their low-temperature (77K) fluorescence emission spectra were monitored. Most of the Protochlorophyllide (Pchlide) present in heat-stressed etiolated seedlings were in nonphototransformable form. The phototransformable Pchlide (F657) rapidly decreased when 5-day-old etiolated seedlings were transferred to 42°C in the dark for 24h. A flash illumination of 0.2s given to etiolated heat-stressed seedlings resulted in substantial arrest of Shibata shift, while in chill-stress conditions, it was only partially affected. In high temperature, due to disaggregation of polymeric Pchlide-Pchlide oxidoreductase (POR)-nicotinamide adenine dinucleotide phosphate (NADPH) molecules, the conversion of nonphototransformable Pchlide to its phototransformable form is substantially delayed resulting in impaired Shibata shift and belated development of the core antenna CP47 Photosystem II (PSII). Chill stress, however, did not disaggregate the polymeric Pchlide-POR-NADPH molecule-suppressed Pchlide and Chl synthesis and impaired of the assembly of PSII core antenna CP47 that emits F695 and PSI that emits F735. The decreased gene/protein expression and reduced posttranslational import of plastidic proteins, importantly POR in temperature-stressed plants, may be responsible for the delay in conversion of nonphototransformable to phototransformable form of Pchlide and plastid biogenesis.

Vibronic Coupling in J-Aggregates and Beyond: A Direct Means of Determining the Exciton Coherence Length from the Photoluminescence Spectrum

Exciton coherence in a J-aggregate with exciton−phonon coupling involving a single intramolecular vibration is studied. For linear aggregates with no disorder and periodic boundary conditions, the 0−0 to 0−1 line strength ratio, S(R), corresponding to the low-temperature photoluminescence spectrum is rigorously equal to N/λ2, where N is the number of chromophores comprising the aggregate and λ2 is the Huang−Rhys factor of the coupled vibrational mode. The result is independent of exciton bandwidth and therefore remains exact from the weak to strong exciton−phonon coupling regimes. The simple relation between S(R) and N also holds for more complex morphologies, as long as the transition from the lowest exciton state to the vibrationless ground state is symmetry-allowed. For example, in herringbone aggregates with monoclinic unit cells, the line strength ratio, defined as SR ≡ I(b)(0−0)/I(b)(0−1) (where I(b)(0−0) and I(b)(0−1) correspond to the b-polarized 0−0 and 0−1 line strengths, respectively) is rigorously equal to N/λ2. In the presence of disorder and for T > 0 K, λ2S(R) is closely approximated by the exciton coherence number N(coh), thereby providing a simple and direct way of extracting N(coh) from the photoluminescence spectrum. Increasing temperature in linear J-aggregates (and herringbone aggregates) generally leads to a demise in S(R) and therefore also the exciton coherence size. When no disorder is present, and under the fast scattering and thermodynamic limits, S(R) is equal to N(T)/λ2, where the thermal coherence size is given by N(T) = 1 + [4πω(c)/k(b)T](d/2) for an aggregate of dimension d, where ω(c) is the exciton band curvature at k = 0.

Synthesis of Metal−Hydrazone Complexes and Vapochromic Behavior of Their Hydrogen-Bonded Proton-Transfer Assemblies

We synthesized and investigated a new series of metal-hydrazone complexes, including deprotonated [MX(mtbhp)] and protonated forms [MX(Hmtbhp)](ClO(4)) (M = Pd(2+), Pt(2+); X = Cl(-), Br(-); Hmtbhp = 2-(2-(2-(methylthio)benzylidene)hydrazinyl)pyridine) and hydrogen-bonded proton-transfer (HBPT) assemblies containing [PdBr(mtbhp)] and bromanilic acid (H(2)BA). The mtbhp hydrazone ligand acts as a tridentate SNN ligand and provides a high proton affinity. UV-vis spectroscopy revealed that these metal-hydrazone complexes follow a reversible protonation-deprotonation reaction ([MX(mtbhp)] + H(+) ⇋ [MX(Hmtbhp)](+)), resulting in a remarkable color change from red to yellow. Reactions between proton acceptor [PdBr(mtbhp)] (A) and proton donor H(2)BA (D) afforded four types of HBPT assemblies with different D/A ratios: for D/A = 1:1, {[PdBr(Hmtbhp)](HBA)·Acetone} and {[PdBr(Hmtbhp)](HBA)·2(1,4-dioxane)}; for D/A = 1:2, [PdBr(Hmtbhp)](2)(BA); and for D/A = 3:2, {[PdBr(Hmtbhp)](2)(HBA)(2)(H(2)BA)·2Acetonitrile}. The proton donor gave at least one proton to the acceptor to form the hydrogen bonded A···D pair of [PdBr(Hmtbhp)](+)···HBA(-). The strength of the hydrogen bond in the pair depends on the kind of molecule bound to the free monoanionic bromanilate OH group. Low-temperature IR spectra (T < 150 K) showed that the hydrogen bond distance between [PdBr(Hmtbhp)](+) and bromanilate was short enough (ca. 2.58 Å) to induce proton migration in the [PdBr(Hmtbhp)](2)(BA) assembly in the solid state. The hydrogen bonds formed not only between [PdBr(Hmtbhp)](+) and HBA(-) but also between HBA(-) and neutral H(2)BA molecules in the {[PdBr(Hmtbhp)](2)(HBA)(2)(H(2)BA)·2Acetonitrile} assembly. The H(2)BA-based flexible hydrogen bond network and strong acidic host structure result in an interesting vapor adsorption ability and vapochromic behavior in this assembly because the vapor-induced rearrangement of the hydrogen bond network, accompanied by changes in π-π stacking interactions, provides a recognition ability of proton donating and accepting properties of the vapor molecule.

Band-Edge Exciton Fine Structure of SingleCdSe/ZnSNanocrystals in External Magnetic Fields

We report a spectroscopic study of the two lowest-energy exciton levels of individual CdSe/ZnS nanocrystals under applied magnetic fields. Field-induced coupling between the bright and the dark excitonic states is directly observed in the low-temperature photoluminescence spectrum and decay and allows the determination of the angle between the nanocrystal c axis and the field. Orientation-dependent Zeeman splittings of the dark and bright exciton sublevels are measured and provide the corresponding exciton Landé factors, as well as spin-flip relaxation rates between Zeeman sublevels.

Axial Thiophene−Boron(subphthalocyanine) Dyads and Their Application in Organic Photovoltaics

We report the synthesis and characterization of boron(subphthalocyanine) derivatives with bithiophene and quaterthiophene as axial ligands, i.e., thiophene−subphthalocyanine dyads (nT-SubPcs), and their application in organic photovoltaic cells (OPVs). Thin films of nT-SubPcs prepared via solution processing can act as the electron donor in bilayer OPVs with evaporated C60 as the electron acceptor. The photophyscial and morphological properties of the nT-SubPcs are studied to rationalize OPV device parameters. The single-crystal X-ray structure is solved for two dyads to show the molecular structures in the solid state, and UV−vis spectroscopy and fluorescence spectroscopy are used to characterize the effect of conjugated thiophene ligands on the photophysical properties, i.e., absorption and photoluminescence quantum yield. Cyclic voltammetry, density functional theory (DFT) calculations, and low-temperature photoluminescence spectra show that photoluminescence yields depend on the overall flexibility of the SubPc derivatives and not on the oxidation potential or electronic relationship of the ligand and macrocycle molecular orbitals. We show with grazing-incidence X-ray scattering and atomic force microscopy (AFM) that careful choice of ligand structure can improve the crystallinity of thin films that leads to a relative increase in short-circuit current in OPV device. Our work clearly demonstrates that SubPcs can be used as light-harvesting chromophores in a matrix of a crystalline organic semiconductor for OPVs.

Quantum Hall regime in emission spectra of single self-organized InP/GaInP quantum dots

We used high spatial resolution near-field scanning optical microscopy (NSOM) to measure low-temperature (10K) emission spectra of single InP/GaInP QDs (lateral size ~100 nm) at magnetic fields up to 10 T. A multi-electronic structure having energy splitting 3–5 meV, resulting from charging of the dot up to 20 electrons, has been observed for these QDs. We measured a strong variation of the near-field emission intensity on a length scale ~30 nm related to Wigner localization. Magnetic-field-induced shifts of the multi-shell peaks for some dots follow remarkably well the energy levels of a Fock-Darwin Hamiltonian, which allow the observation of a Wigner-molecular-droplet transition.

Low-temperature photoluminescence spectra of CdO–In2O3 thin films prepared by sol–gel

(CdO)1−x–(InO3/2)x thin films were deposited on glass substrates by the sol–gel method. The precursor solutions for the mixed oxide films were obtained from the mixture of the precursor solutions for CdO and In2O3 prepared separately. The investigated In atomic concentrations in the solution, x, were 0.0, 0.16, 0.33, 0.50, 0.67, 0.84, and 1. X-ray diffraction measurements showed that the films were mainly constituted of CdO, In2O3, and CdIn2O4. CdO and In2O3 were obtained for x=0 and 1, respectively. For x=0.67, which is the stoichiometric composition of the CdIn2O4 compound, only this oxide was formed. CdO and CdIn2O4 crystals were obtained in the Cd-rich region, whereas In2O3 and CdIn2O4 crystals were formed in the In-rich region. The PL spectra at 15 K for CdO showed the presence of two main emission bands at energies close to 2.2 and 3.0 eV. A blue-shift of these bands took place for increasing In concentration, which is related to the increase in the band gap energy of the mixed system in going from CdO, with a band gap energy of 2.46 eV, to CdIn2O4, with 3.2 eV, to In2O3, with 3.6 eV.

ChemInform Abstract: Dynamic Binuclear Enediolato- and Catecholato-Zirconocene Complexes Relevant to the Structural Problem Associated with Sharpless Oxidation Catalysts.

Dimethylzirconocene complexes [ZrMe2(C5H4R)2](R = H or CH3) were treated with catechol to give the (catecholato)zirconocene complexes. Low-temperature NMR spectra indicated dimeric metallatricyclic structures for these complexes and the corresponding (ethene-1,2-diolato)- and (but-2-ene-2,3-diolato)-zirconocenes. Dynamic temperature-dependent NMR spectroscopy revealed that all these complexes underwent a thermally induced degenerate rearrangement process, similar to the reorganization reaction of some Sharpless catalyst precursors, with Gibbs activation energies close to 50 kJ mol–1. A labelling experiment showed the intramolecular nature of this automerization. (4-tert-Butylcatecholato)zirconocene formed two diastereoisomeric dimers which did not interconvert on the NMR time-scale (ΔG‡inter >90 kJ mol–1) although the intramolecular rearrangement was observed [ΔG‡intra(241 K)= 52 ± 2 kJ mol–1]. The intermolecular equilibration process probably proceeding via monomeric (catecholato)zirconocene was monitored in the case of the bulky (4-tert-butyl-catecholato)bis(tert-butylcyclopentadienyl)zirconium system by dynamic NMR spectroscopy at high temperature [ΔG‡inter(364 K)= 78 ± 5 kJ mol–1].

ChemInform Abstract: C-N Bond Atropisomerism of 1-(2,4-Dimethyl-3-pentyl)cyclohepta(b) pyrrol-2(1H)-one.

Two retainers of 1-(2,4-dimethyl-3-pentyl)cyclohepta[b]pyrrol-2(1H)-one could be isolated by means of the HPLC at 0 °C. Its rotational barrier, measured by the line shape analysis of the low-temperature 1H NMR spectra, was 86.8 kJ/mol at 25 °C in CDCl3.

Calculated photoelectron spectra of isotopomers of the propargyl radical (H 2C 3H): An explicitly correlated coupled cluster study

Explicitly correlated coupled cluster theory at the (U)CCSD(T∗)-F12a level has been employed to study the vibrational structure of the first two bands of the low-temperature photoelectron spectra of four different isotopomers of the propargyl radical with C 2v symmetry (H 2C 3H, H 2C 3D, D 2C 3H, and D 2C 3D). A five-dimensional anharmonic model is employed to calculate the peak positions and relative intensities. While the first band of the PE spectra of all four isotopomers is dominated by the adiabatic peak, the second band shows a progression in the pseudoantisymmetric CC stretching vibration v 3 with relative intensities of 100:68:23:5:1 for n = 0–4 in the case of the most abundant isotopomer.