I. Electronic states of heavily-doped molecular crystals--naphthalene. I. Theoretical. II. Experimental. II. Lowest singlet and triplet excited states of pyrazine

PART I

The energy spectrum of heavily-doped molecular crystals was treated in the Green’s function formulation. The mixed crystal Green’s function was obtained by averaging over all possible impurity distributions. The resulting Green’s function, which takes the form of an infinite perturbation expansion, was further approximated by a closed form suitable for numerical calculations. The density-of-states functions and optical spectra for binary mixtures of normal naphthalene and deuterated naphthalene were calculated using the pure crystal density-of-state functions. The results showed that when the trap depth is large, two separate energy bands persist, but when the trap depth is small only a single band exists. Furthermore, in the former case it was found that the intensities of the outer Davydov bands are enhanced whereas the inner bands are weakened. Comparisons with previous theoretical calculations and experimental results are also made.

PART II

The energy states and optical spectra of heavily-doped mixed crystals are investigated. Studies are made for the following binary systems: (1) naphthalene-h₈ and d₈, (2) naphthalene--h₈ and αd₄, and (3) naphthalene--h₈ and βd₁, corresponding to strong, medium and weak perturbations. In addition to ordinary absorption spectra at 4˚K, band-to-band transitions at both 4˚K and 77˚K are also analyzed with emphasis on their relations to cooperative excitation and overall density-of-states functions for mixed crystals. It is found that the theoretical calculations presented in a previous paper agree generally with experiments except for cluster states observed in system (1) at lower guest concentrations. These features are discussed semi-quantitatively. As to the intermolecular interaction parameters, it is found that experimental results compare favorably with calculations based on experimental density-of-states functions but not with those based on octopole interactions or charge-transfer interactions. Previous experimental results of Sheka and the theoretical model of Broude and Rashba are also compared with present investigations.

PART III

The phosphorescence, fluorescence and absorption spectra of pyrazine-h₄ and d₄ have been obtained at 4˚K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine-h₄ in d₄ were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure.

The analysis of the weak fluorescence spectrum resolves the long-disputed question of whether one or two transitions are involved in the near-ultraviolet absorption of pyrazine. The “mirror-image relationship” between absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as ¹B_3u ← ¹A_1g. The forbidden component ¹B_2g, predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain.

The phosphorescence spectrum when compared with the excitation phosphorescence spectra, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and a “mirror-image relationship” between absorption and emission. In accordance with previous work, the triplet state is designated as ³B_3u.

The vibronic structure of the phosphorescence spectrum is very complicated. Previous work on the analysis of this spectrum all concluded that a long progression of v_6a exists. Under the high resolution attainable in our work, the supposed v_6a progression proves to have a composite triplet structure, starting from the second member of the progression. Not only is the v_9a hydrogen-bending mode present as shown by the appearance of the C-D bending mode in the d₄ spectrum, but a band of 1207 cm^-1 in the pyrazine in benzene system and 1231 cm^-1 in the mixed crystal system is also observed. This band is assigned as 2v_6b and of a_1g symmetry. Its anonymously strong intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with the 2v_6a and v_9a band.

To help resolve the present controversy over the crystal phosphorescence spectrum of pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.

Electronic structure in five-coordinate complexes of nickel(II) with heavy-donor ligands

I.

Various studies designed to elucidate the electronic structure of the arsenic donor ligand, o-phenylenebisdimethylarsine (diarsine), have been carried out. The electronic spectrum of diarsine has been measured at 300 and 77˚K. Electronic spectra of the molecular complexes of various substituted organoarsines and phosphines with tetracyanoethylene have been measured and used to estimate the relative ionization potentials of these molecules.

Uv photolysis of arsines in frozen solution (96˚K) has yielded thermally labile, paramagnetic products. These include the molecular cations of the photolyzed compounds. The species (diars)⁺ exhibits hyper-fine splitting due to two equivalent ⁷⁵As(I=3/2) nuclei. Resonances due to secondary products are reported and assignments discussed.

Evidence is presented for the involvement of d-orbitals in the bonding of arsines. In (diars)⁺ there is mixing of arsenic “lone-pair” orbitals with benzene ring π-orbitals.

II.

Detailed electronic spectral measurements at 300 and 77˚K have been carried out on five-coordinate complexes of low-spin nickel(II), including complexes of both trigonal bipyramidal (TBP) and square pyramidal (SPY) geometry. TBP complexes are of the form NiLX⁺ (X=halide or cyanide,

L = Qƭ(CH₂)₃As(CH₃)₂]₃ or

P [hexagon - Q'CH₃] , Q = P, As,

Q’=S, Se).

The electronic spectra of these compounds exhibit a novel feature at low temperature. The first ligand field band, which is asymmetric in the room temperature solution spectrum, is considerably more symmetrical at 77˚K. This effect is interpreted in terms of changes in the structure of the complex.

The SPY complexes are of the form Ni(diars)₂X^z (X=CL, Br, CNS, CN, thiourea, NO₂, As). On the basis of the spectral results, the d-level ordering is concluded to be xy ˂ xz, yz ˂ z² ˂˂ x² - y². Central to this interpretation is identification of the symmetry-allowed ¹A₁ → ¹E (xz, yz → x² - y²) transition. This assignment was facilitated by the low temperature measurements.

An assignment of the charge-transfer spectra of the five-coordinate complexes is reported, and electronic spectral criteria for distinguishing the two limiting geometries are discussed.

Photoelectrochemistry of GaAs and Si liquid junctions

A quantitative study has been performed on the stability of GaAs surfaces in a 0.10 M K₂Se-0.01 M K₂Se₂ aqueous solution. In this electrolyte, n-type GaAs electrodes displayed significant photocorrosion in competition with faradaic charge transfer to Se^2-. Chemisorption of group VIIIB metal ions onto the GaAs surfaces yielded improved current-voltage behavior of the GaAs photoanodes, and also resulted in a significant reduction in photocorrosion. This behavior implies that the chemisorbed metal ions act to increase the rate of hole transfer to the Se^2- species. Related experiments on n-GaAs, pGaAs, and Sn-doped In₂O₃ electrodes in Te^2-/- aqueous solutions have also been performed.

The majority carrier (electrons) transfer rate constant at a highly doped n⁺-Si/Co(Cp)₂Cl-methanol junction has been measured directly using the chronoamperometry electrochemical technique. The reduction reaction rate of Co(Cp)₂+ was 0.03 cm-s^-1 at the Si electrode, and was more than 100 times slower than at a hanging mercury electrode. The slower rate was attributed to the smaller optical and static dielectric constants, and the lower density of electrons of the semiconductor. The experimental results were compared to the Marcus theory of charge transfer.

The unique properties of high purity Si/liquid junctions have been investigated under illumination conditions in which the photogenerated carrier concentration exceeds the dopant concentration. Under these high injection conditions, negligible electric fields exist at the semiconductor/liquid interface, and carrier motion is driven by diffusion. Studies of the current-voltage properties of the Si in methanol solutions containing various redox couples suggested that high efficiency photoelectrochemical cells could be established through selective collection of carriers at the semiconductor/liquid junction. The quasi-Fermi levels of electrons and holes were measured directly against the solution potential. Steady-state and transient photovoltage measurements, and theoretical modeliug of the carrier transport, generation, and recombination dynamics indicated that the quasi-Fermi levels were flat across the semiconductor sample. The recombination velocities at the Si/liquid junctions have also been measured, and were shown to vary with the solution potential following the Shockley-Read-Hall theory on recombination.

Large three-photon absorption cross-section in a novel class of bis-(N-carbazolyl) fluorene derivatives

Two novel symmetrical charge transfer fluorene derivatives (abbreviated as BCZF and BVCZF) with carbazole end-group as the donor moieties have been synthesized. Three-photon absorption cross-sections of these two compounds have been determined by using a Q-switched Nd:YAG laser pumped with 38 ps pulses at 1064 nm in DMF. The measured 3PA cross-sections are 140 x 10(-78) and 400 x 10(-78) cm(6) s(2) for BCZF and BVCZF, respectively. The geometries, electronic structures and electronic spectra of these two compounds are systematically studied by AM1 and ZINDO/S methods. On the basis of correct UV-vis spectra, the influence of different molecular structure on three-photon absorption cross-sections is discussed micromechanically. (C) 2005 Elsevier B.V. All rights reserved.

Two- and three-photon absorption in a novel fluorene-based compound

A novel symmetrical charge transfer fluorene-based compound 2,7-bis (4-methoxystyryl)-9, 9-bis (2-ethylhexyl)-9H-fluorene (abbreviated as BMOSF) was synthesized and its nonlinear absorption was investigated using two different laser systems: a 140-fs, 800-nm Ti:sapphire laser operating at 1-kHz repetition rate and a 38-ps, 1064-nm Nd:YAG pulsed laser operating at 10-Hz repetition rate, respectively. Unique nonlinear absorption properties in this new compound were observed that rise from multiphoton absorption. The nonlinear absorption coefficients were measured to be 6.02

TCNQ脂类衍生物及其铜复合物的合成与红外光谱研究

首次合成了7，7，8，8-四氰基对苯醌二甲烷的脂类衍生物：TCNQ（C2H4COOR）2（R=CH3，C2H5，C3H7）及其铜电子转移复合物。通过元素分析确定这些化合物的组成，对这些化合成物在4000～400cm^-1范围内的主要红外光谱吸收峰进行了归属，并讨论了取代基对TCNQ类衍生物红外光谱的影响及其规律。

Cooperative downconversion in Yb3+-RE3+ (RE = Tm or Pr) codoped lanthanum borogermanate glasses

We report on cooperative downconversion in Yb3+-RE3+ (RE = Tm or Pr) codoped lanthanum borogermanate glasses (LBG), which are capable of splitting a visible photon absorbed by Tm3+ or Pr3+ ions into two near-infrared photons. The results indicate that Pr3+-Yb3+ is a more efficient ion couple than Tm3+-Yb3+ in terms of cooperative downconversion. We have obtained a highest quantum yield of 165% and 138% for Pr3+-Yb3+ and Tm3+-Yb3+ codoped LBG glasses under 468 nm excitation, respectively. However, ultraviolet light excitation to the charge transfer band of Yb3+ does not result in quantum splitting as rapid relaxation from the charge transfer band to 4f(13) levels of Yb3+ dominates. (C) 2008 Optical Society of America

Three-photon-excited upconversion luminescence of niobium ions doped silicate glass by a femtosecond laser irradiation

We report on the bluish green upconversion luminescence of niobium ions doped silicate glass by a femtosecond laser irradiation. The dependence of the fluorescence intensity on the pump power density of laser indicates that the conversion of infrared irradiation to visible emission is dominated by three-photon excitation process. We suggest that the charge transfer from O-2-to Nb5+ can efficiently contribute to the bluish green emission. The results indicate that transition metal ions without d electrons play an important role in fields of optics when embedded into silicate glass matrix. (C) 2008 Optical Society of America.

Direct synthesis and intense luminescence of Y2O3 : Eu nanophosphors by complexation combustion

In this paper, we report the synthesis of high-luminance Y2O3:Eu nanocrystal through a citrate-nitrate complexation combustion method at a low temperature of 200-280 degrees C. The as-combusted Y2O3:Eu phosphors are almost equiaxed crystallites with an average size of 30-40 run, and have an intense red luminescence. The present fuel-deficient method suggests that by control of the ratio of citric acid to nitrates, it is valuable for the fabrication of Y2O3 nanoparticles without heat treatment. This process should be applicable to a wide range of nanocrystal oxides.

Sol-gel combustion synthesis and luminescent properties of nanocrystalline YAG : Eu3+ phosphors

EU3+ -doped Y3Al5O12 (YAG:Eu3+) phosphors were synthesized by a facile sol-gel combustion method. In this process, citric acid traps the constituent cations and reduces the diffusion length of the precursors. YAG phase is obtained through sintering at 900 degrees C for 2h. There were no intermediate phases such as YAlO3 (YAP) and Y4Al2O9 (YAM) observed. The charge transfer band of nanocrystalline phosphors shows a shift toward the high-energy side, compared with that of amorphous ones due to lower covalency of Eu-O bond for nanocrystalline phosphors. The higher concentration quenching in YAG:EU3+ nanophosphors may be caused by the confinement effect on resonant energy transfer of nanocrystalline. It also indicates that the sol-gel combustion synthesis method provides a good distribution of Eu3+ activators in YAG host. (c) 2005 Elsevier B.V. All rights reserved.

Effects of gamma irradiation and air annealing on Yb-doped YAlO3 single crystals

The effects of gamma irradiation on as-grown 5 at% Yb:YAlO3 (YAP) and air annealing on gamma-irradiated 5 at% Yb: YAP have been studied by the difference in the absorption spectra before and after treatment. The gamma irradiation and air annealing led to opposite changes of the absorption properties of the Yb: YAP crystal. After air annealing, the gamma-irradiation effects were totally removed over the wavelength range 390-800 nm and the concentrations of Fe3+ and Yb3+ were slightly increased. For the first time, the gamma-irradiation-induced valence changes between Yb3+ and Yb2+ ions in Yb: YAP crystals have been observed. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sol-gel combustion synthesis and luminescence Of Y4Al2O9 : Eu3+ nanocrystal

Y4Al2O9:EU3+ phosphor was synthesized through a sol-gel combustion method. The Y4Al2O9 phase can form through sintering at 800 degrees C. This temperature is much lower than that required via the solid state reaction method. The average grain size of the phosphor was about 30 run. Compared with the amorphous phosphor, the charge transfer band of crystalline phosphor shows a blue shift. The emission Of Y4Al2O9:Eu3+ indicates the existence of two luminescent centers, in agreement with the crystal structure of Y4Al2O9. Higher doping concentration could be realized in Y4Al2O9 nanocrystal host lattice. This indicates that the sol-gel combustion synthesis method can increase emission intensity and quenching concentration due to a good distribution of EU3+ activators in Y4Al2O9 host. (c) 2005 Elsevier B.V. All rights reserved.

Mg，Ti共掺Al2O3透明多晶陶瓷光谱性能研究

Transparent polycrystalline MgO and TiO2 codoped Al2O3 ceramics were fabricated by conventional solid-state pressureless processing. The absorption, emission and excitation spectra of ( Mg, Ti : Al2O3 ceramics were measured. Owing to charge compensation of Mg2+, only UV absorption around 250nm was observed due to O2- -> Ti4+ charge transfer transitions (CT) when Ti content was low. As a result, the emission peaks of isolated Ti4+ ion located at 280-290nm and 410-420nm were observed. Besides absorption peak of V, ion, the characteristic absorption peak of V, ion centered at 490nm was observed in Mg, Ti) : Al2O3 ceramics when Ti content was high. The emission spectra of Ti3+, ion in polycrystalline Al2O3 ceramics coincide with that of Ti: Al2O3 single crystal.

Visible luminescence in Yb3+-doped gadolinium gallium garnets

In this paper, some results on visible luminescence performed on Yb3+-doped gadolinium gallium garnets under 165 and 940 nm excitation were presented. The upconversion luminescence was ascribed to Yb3+ cooperative luminescence and the presence of rare earth impurity ions. The gain cross-sections of Yb:GGG crystal as a function of excited-state population fraction P were studied. Emission spectra under 165 nm at 20 K showed there was no charge transfer luminescence in Yb:GGG. (c) 2006 Elsevier B.V. All rights reserved.

EVIDENCE FOR TWO TELLURIUM SITES IN DILUTE LIQUID Te-Tl ALLOYS.

Recently, experimental evidence was presented which suggests that as the stoichiometric composition CuTe, NiTe, Tl//2Te and MnTe are approached from pure Te in the liquid state, substantial charge transfer takes place and Te exists in the form Te**y**31 ions with y close to 2. The system studied (Te-Tl) is one in which charge transfer localizes electrons on the tellurium and leads to semiconducting behavior at the stoichiometric composition Tl//2Te.