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Showing papers on "Absorption spectroscopy published in 1996"


Journal ArticleDOI
07 Feb 1996-Langmuir
TL;DR: In this paper, the use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model, and the absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying.
Abstract: The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads to anion desorption. Underpotential deposition (upd) of electropositive metal layers results in dramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in t...

3,454 citations


Journal ArticleDOI
TL;DR: By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free-carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders as mentioned in this paper.
Abstract: By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free‐carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free‐carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low‐voltage field emission displays is discussed.

1,888 citations


Journal ArticleDOI
TL;DR: In this article, the absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media.
Abstract: The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

535 citations


Journal ArticleDOI
TL;DR: In this paper, quantum-confined InP nanocrystals from 20 to 50 A in diameter have been synthesized via the reaction of InCl3 and P(Si(CH3)3 )3 in trioctylphosphine oxide (TOPO) at elevated temperatures.
Abstract: Quantum-confined InP nanocrystals from 20 to 50 A in diameter have been synthesized via the reaction of InCl3 and P(Si(CH3)3)3 in trioctylphosphine oxide (TOPO) at elevated temperatures. The nanocrystals are highly crystalline, monodisperse, and soluble in various organic solvents. Improved size distributions have been obtained by size-selectively reprecipitating the nanocrystals. The UV/vis absorption spectra of the particles show the characteristic blue shift of the band gap of up to 1 eV due to quantum confinement, a moderately well-resolved first excitonic excited state, and, in some cases, the resolution of a higher excited state. Structurally, the nanocrystals are characterized with powder X-ray diffraction and transmission electron microscopy. Raman spectroscopy reveals TO and LO modes near the characteristic bulk InP positions as well a surface mode resulting from finite size. The Raman line widths, line positions, and relative intensities are all size-dependent . X-ray photoelectron spectroscopy ...

485 citations


Journal ArticleDOI
TL;DR: In this article, a thin-film quantum dot composites incorporating bare and overcoated CdSe nanocrystals in a ZnSe matrix were synthesized by electrospray organometallic chemical vapor deposition (ES-OMCVD).
Abstract: Electronic and chemical passivation of CdSe nanocrystals (quantum dots) has been achieved with a thin ZnSe overlayer grown in solution from trioctylphosphine selenide and diethylzinc. Layered particles with a [ZnSe/CdSe] ratio ranging from 0 to ∼5.0 were prepared and characterized by optical absorption spectroscopy, photoluminescence, high-resolution transmission electron microscopy, Auger electron spectroscopy, and X-ray scattering. The overgrown particles were crystalline and displayed band-edge absorption and emission characteristic of the initial CdSe nuclei. Thin-film quantum dot composites incorporating bare and overcoated CdSe nanocrystals in a ZnSe matrix were synthesized by electrospray organometallic chemical vapor deposition (ES-OMCVD). The photoluminescence spectra of the composites with bare CdSe dots were dominated by broad deep-level emission and the photoluminescence yield deteriorated with increasing deposition temperature. In contrast, the composites incorporating the overcoated dots sho...

448 citations


Journal ArticleDOI
01 Nov 1996
TL;DR: In this article, a series of oxidize-stable CdTe nanoclusters with narrow size distributions and extremely small particle sizes ranging from 1.3 to 2.4 nm has been prepared in aqueous solution using 2-mercaptoethanol and 1-thioglycerol as stabilizers.
Abstract: In order to expand the range of high-quality nanosized semiconductor materials that can be obtained as quantum dots through a wet chemical route a series of oxidize-stable CdTe nanoclusters with narrow size distributions and extremely small particle sizes ranging from 1.3 to 2.4 nm has been prepared in aqueous solution using 2-mercaptoethanol and 1-thioglycerol as stabilizers. It has been investigated by means of UV-vis absorption and photoluminescence spectroscopy, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and energy dispersive x-ray analysis (EDX).

426 citations


Journal ArticleDOI
TL;DR: In this paper, photoinduced charge separation and subsequent charge recombination were observed in a series of zincporphyrin-C60 dyads by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy.
Abstract: Four different kinds of C60-linked zincporphyrins have been prepared by changing systematically the linking position at meso-phenyl ring from ortho to para and their photophysical properties have been investigated. Regardless of the linkage between the two chromophores, photoinduced charge separation (CS) and subsequent charge recombination (CR) were observed in a series of zincporphyrin-C60 dyads by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy. In THF the CS occurs from both the excited singlet state of the porphyrin and the C60 moieties, implying that the increase of the absorption cross section by both the chromophores results in the efficient formation of the ion pair (IP) state. On the other hand, in benzene the IP state generated by the photoinduced CS from the excited singlet state of the porphyrin to the C60 produces or energetically equilibrates with the locally excited singlet state of the C60. Both the CS and CR rates for the meta isomer are ...

358 citations


BookDOI
01 Jan 1996
TL;DR: In this article, the photoacoustic method in photosynthesis - Monitoring and analysis of Phenomena which Lead to Pressure Changes Following Light Excitation S.J. Hoff and P.M. Small.
Abstract: Preface. Part One: Optical Methods. 1. Developments in Classical Optical Spectroscopy J. Amesz. 2. Linear and Circular Dichroism G. Garab. 3. Fluorescence K. Sauer, M. Debreczeny. 4. Ultrafast Spectroscopy of Photosynthetic Systems R. Jimenez, G.R. Fleming. 5. Data Analysis of Time-Resolved Measurements A.R. Holzwarth. 6. Photosynthetic Thermoluminescence as a Simple Probe of Photosystem II Electron Transport Y. Inoue. 7. Accumulated Photon Echo Measurements of Excited State Dynamics in Pigment-Protein Complexes T.J. Aartsma, R.J.W. Louwe, P. Schellenberg. 8. Spectral Hole Burning: Methods and Applications to Photosynthesis N. Raja, S. Reddy, G.J. Small. 9. Infrared and Fourier-Transform Infrared Spectroscopy W. Mantele. 10. Resonance Raman Studies in Photosynthesis - Chlorophyll and Carotenoid Molecules B. Robert. 11. Stark Spectroscopy of Photosynthetic Systems S.G. Boxer. 12. The Photoacoustic Method in Photosynthesis - Monitoring and Analysis of Phenomena which Lead to Pressure Changes Following Light Excitation S. Malkin. Part Two: Magnetic Resonance. 13. Magnetic Resonance: an Introduction A.J. Hoff. 14. Time-Resolved Electron Paramagnetic Resonance Spectroscopy - Principles and Applications H. Levanon. 15. Electron Spin Echo Methods in Photosynthesis Research R.D. Britt. 16. ENDOR Spectroscopy W. Lubitz, F. Lendzian. 17. Optically Detected Magnetic Resonance (ODMR) of Triplet States in Photosynthesis A.J. Hoff. 18. MagicAngle Spinning Nuclear Magnetic Resonance of Photosynthetic Components H.J.M. de Groot. Part Three: Structure and Oxygen. 19. Structure Determination of Proteins by X-Ray Diffraction M. Schiffer. 20. Electron Microscopy E.J. Boekema, M. Roegner. 21. X-Ray Absorption Spectroscopy:Determination of Transition Metal Site Structures in Photosynthesis V.K. Yachandra, M.P. Klein. 22. Moessbauer Spectroscopy P.G. Debrunner. 23. Characterization of Photosynthetic Supramolecular Assemblies Using Small Angle Neutron Scattering D.M. Tiede, P. Thiyagarajan. 24. Measurements of Photosynthetic Oxygen Evolution H.J. van Gorkom, P. Gast.

329 citations


Journal ArticleDOI
TL;DR: A new analysis procedure was developed, modeling atmospheric spectra with the absorption structures of the individual trace gases, to determine their concentrations and corrects differences in the wavelength-pixel mapping of these spectra.
Abstract: Differential optical absorption spectroscopy (DOAS) has become a widely used method to measure trace gases in the atmosphere. Their concentration is retrieved by a numerical analysis of the atmospheric absorption spectra, which often are a combination of overlapping absorption structures of several trace gases. A new analysis procedure was developed, modeling atmospheric spectra with the absorption structures of the individual trace gases, to determine their concentrations. The procedure also corrects differences in the wavelength-pixel mapping of these spectra. A new method to estimate the error of the concentrations considers the uncertainty of this correction and the influence of random residual structures in the absorption spectra.

328 citations


Journal ArticleDOI
O. Humbach, H. Fabian, U. Grzesik, U. Haken, W. Heitmann1 
TL;DR: In this article, the spectral position and relative intensities of bound hydroxyl (SiOH) absorption bands in state-of-the-art synthetic silica were investigated.
Abstract: The presence of bound hydroxyl (SiOH) in silica is well known to produce an optical fundamental absorption band at about 2.7 μm. For optical fiber applications the influence of the corresponding overtones and combination modes on the absorption spectrum are of significant importance. A literature review is presented which reveals uncertainties regarding the correct absorption band intensities as well as their spectral positions. We present precise data on spectral position and relative intensities of OH absorption bands in state of the art synthetic silica. Our investigations cover the influence of different manufacturing techniques, OH content, and a comparison of bulk and fiber data. With the knowledge of the conversion factors between the intensities of different OH absorption bands it is possible to predict the entire OH related transmission performance of an optical component by measurement of a single absorption band, e.g., the fundamental mode at 2.7 μm or the 1.38 μm band in the low loss range of optical fibers.

324 citations


Journal ArticleDOI
TL;DR: In this paper, transparent and crack free thin films of ZnO have been deposited on fused silica, soda glass, silicon wafers and KBr single crystals using the sol-gel technique.

Journal ArticleDOI
TL;DR: In this article, the optical and redox properties of coordination compounds of the general type Ru(dmb)2(LL)(PF6)2 were reported for the attachment to semiconductor metal oxide surfaces.
Abstract: Coordination compounds of the general type Ru(dmb)2(LL)(PF6)2, where dmb is 4,4‘-(CH3)2-2,2‘-bipyridine and LL is 4-(CH3)-4‘-(COOH)-2,2‘-bipyridine, or 4-(CH3)-4‘-((CH2)3COOH)-2,2‘-bipyridine, or 4-(CH3)-4‘-((CH2)3COCH2COOC2H5)-2,2‘-bipyridine were prepared for the attachment to semiconductor metal oxide surfaces. The optical and redox properties of these compounds in dichloromethane solution are reported. Binding to porous nanostructured TiO2 films was analyzed with the Langmuir adsorption isotherm model. Photoelectrochemical measurements of the modified TiO2 electrodes in regenerative solar cells are reported. The results indicate that intimate electronic coupling between the surface link and the chromophoric ligand is not a strict requirement in the design of sensitizers for photovoltaic applications. Interfacial kinetics for recombination of the electron in the solid with the oxidized form of the sensitizer were quantitated by excited state absorption spectroscopy.

Journal ArticleDOI
TL;DR: In this article, the authors analyzed indocyanine green in methanol, water and aqueous albumin solution and found that the strong affinity of the dye to albumin shifts the onset of dimerization to higher concentrations.
Abstract: Absorption spectra, fluorescence quantum distributions, fluorescence quantum yields and degrees of fluorescence polarization vs. dye concentration were determined for indocyanine green in methanol, water and aqueous albumin solution. The monomer fluorescence quantum yield is limited to a few per cent by internal conversion. In water, dimerization starts at low concentrations (approximately 3 × 10 −8 m ol dm −3 ) and lowers the fluorescence quantum yield. The strong affinity of the dye to albumin shifts the onset of dimerization to higher concentrations. In methanol, dimerization is weak and closely spaced pair formation dominates at high concentrations. Dye adsorption to albumin and dye aggregation in the solvents were analysed experimentally and theoretically.

Journal ArticleDOI
16 May 1996-Nature
TL;DR: A baryon density that is 5% of the critical density required to close the Universe is calculated, consistent with that in the interstellar medium (after allowing for Galactic chemical evolution).
Abstract: THE primordial ratio of deuterium to hydrogen nuclei (D/H), created as a result of the Big Bang, provides the most sensitive measure of the cosmological density of baryons1–5. Measurements of the D/H ratio in the interstellar medium of our Galaxy place a strict lower limit on the primordial ratio6, because processing of gas by stars reduces the abundance of deuterium relative to hydrogen. Absorption of radiation from distant quasars by intervening clouds of gas offers a means of probing D/H ratios at large redshifts, where the effects of stellar processing should be negligible. Measurements7,8 on one absorption system have indicated an extremely high primordial abundance ratio of 24 × 10–5. Here we report a measurement of the D/H ratio in another high-redshift absorption system, and obtain a value that is an order of magnitude lower than that reported previously7,8. The measured ratio of 2.3 × 10–5 is consistent with that in the interstellar medium (after allowing for Galactic chemical evolution9,10), and indicates that the absorption spectra on which the earlier estimates are based may have been subject to strong contamination. We calculate a baryon density that is 5% of the critical density required to close the Universe.

Journal ArticleDOI
15 Dec 1996
TL;DR: Chattoraij et al. as discussed by the authors showed that steady-state and picosecond time-resolved absorption and fluorescence spectroscopy on the green fluorescent protein (GFP) have been interpreted by a mechanism where the key process is an excited state deprotonation of the chromophore.
Abstract: Recently steady-state and picosecond time-resolved absorption and fluorescence spectroscopy on the Green Fluorescent Protein (GFP) have been interpreted by a mechanism where the key process is an excited state deprotonation of the chromophore (M. Chattoraij, B.A. King, G.U. Bublitz and S.G. Boxer, Proc. Natl. Acad. Sci. USA, 93 (1996) 8362–8367). Such a conclusion was borne out by the mirror image of the picosecond decay of the protonated species RH∗ in the blue and the concomitant picosecond rise of the green fluorescence of the deprotonated fluorophore R−∗ as well as the significant slowing of both kinetic features upon deuteration. We report similar experiments confirming this mechanism. The results of ultrafast spectroscopy on wild-type GFP together with two important mutants combined with the recent crystal structures are shown to shed more light on the interplay between absorption and emission phenomena in GFP. Beyond some differences with previous results pertaining, for instance, to the assignment of vibronic progressions in absorption spectra and the temperature dependence of excited state deprotonation, several new features have been identified. These concern the deprotonated ground state R− in equilibrium as well as the excited state RH∗. In particular, we have studied the distributed fluorescence kinetics in the time and frequency domain, excited state absorption features observed in femtosecond time-resolution, and the dependence of excited state proton transfer kinetics on the aggregational state of the protein.

Journal ArticleDOI
TL;DR: In this article, the ability of polarization modulation IR reflection absorption spectroscopy (PM-IRRAS) to study the air/water interface is presented and a brief description of the set-up and of the experimental procedure is given.
Abstract: The ability of polarization modulation IR reflection absorption spectroscopy (PM–IRRAS) to study the air/water interface is presented. A brief description of the set-up and of the experimental procedure is given. Theoretical simulations accounting for the uniaxial nature of the spread monolayer lead to optimum experimental conditions (71 ° for the angle of incidence) and to a specific surface selection rule. Application to the study of cadmium arachidate, dimyristolyl phosphatidylcholine (DMPC) and polypeptidic Langmuir films illustrates the potential uses of this method.

Journal ArticleDOI
TL;DR: Epitaxial growth in a CdS/HgS heterostructure of nanometer dimensions, prepared by methods of wet chemistry, is demonstrated and photophysical measurements provide evidence for charge-carrier localization within the HgS well.
Abstract: Epitaxial growth in a CdS/HgS heterostructure of nanometer dimensions, prepared by methods of wet chemistry, is demonstrated High-resolution transmission-electron microscopy is used to determine the shape and crystallinity of this system consisting of a quantum well in a quantum dot The homogeneous absorption and fluorescence spectra are investigated by transient hole burning and fluorescence line-narrowing spectroscopy The photophysical measurements provide evidence for charge-carrier localization within the HgS well {copyright} {ital 1996 The American Physical Society}

Journal ArticleDOI
TL;DR: In this paper, the formation of a chalchocite (Cu2S) phase was inferred using absorption spectroscopy, and the role of thiourea in the tailoring of particles to the micellar periphery was confirmed by synthesizing the nanoparticles using other S2-agents like H2S, and Na2S.
Abstract: Nanoparticles of copper sulfide have been synthesized by reacting a copper ammonia complex with an equimolar thiourea solution in Triton-X 100/cyclohexane water-in-oil microemulsions. The presence of an exceptionally sharp and blue-shifted peak at 475 ± 2 nm in the UV−vis spectrum reveals the formation of quasi-monodispersed, size-quantized particles. Using absorption spectroscopy, the formation of a chalchocite (Cu2S) phase is inferred. The peak position in the absorption spectra was found to be independent of net micellar water content as well as aging effect. It is attributed to the formation of a Cu(I) thiourea complex on the surface of the particles, which are hydrogen bonded to the polyoxyethylene (POE) chain of Triton-X 100 (TX-100). The role of thiourea in the tailoring of particles to the micellar periphery was confirmed by synthesizing the nanoparticles using other S2- agents like H2S, and Na2S. The role of the POE chain in mediating the adsorption was brought out by carrying out the reaction in...

Journal ArticleDOI
TL;DR: In this paper, the authors used femtosecond time-resolved UV−visible absorption spectroscopy to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C.
Abstract: Femtosecond time-resolved UV−visible absorption spectroscopy has been used to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C. Photolysis of t-AB at 303 nm results in transient absorption at 370−450 nm, the decay of which can be fitted by a sum of two exponential components. The shorter-lived component has a lifetime of 0.9 ± 0.2 ps in hexane, cyclohexane, and hexadecane and 1.2 ± 0.2 ps in acetonitrile; this is attributed to the S2(ππ*) excited state of t-AB. The longer-lived component has a lifetime which is similar to the recovery time of the ground-state absorption of t-AB at 303 nm, found to be 13 ± 1 ps in hexane, cyclohexane, and hexadecane and 16 ± 1 ps in acetonitrile. This longer-time-scale process is attributed to the internal conversion of an intermediate excited state, S†, into ground state t-AB, and this intermediate is tentatively assigned as a twisted conformer of excited t-AB on the S2 or S1 potential energy surface. The vibrational relaxation of hot t-AB molec...

Journal ArticleDOI
TL;DR: In this paper, the general characteristics and spectrometric features of a high resolution four-crystal reflection x-ray monochromator with wavelength analysis installed at the HASYLAB beam line L at DESY are presented.
Abstract: The general characteristics and spectrometric features of a high resolution four‐crystal reflection x‐ray monochromator with wavelength analysis installed at the HASYLAB beam line L at DESY are presented. The monochromator is part of a spectrometer developed to calibrate x‐ray absorption edge spectra in the energy range of 6–36 keV with a relative uncertainty ΔE/E from 10−5 to 10−6. This requires an extremely effective suppression of harmonics and also a negligible instrumental influence in order to obtain almost intrinsic spectra. As the results show, the monochromator fulfills the requirements, including very high stability. An example of the calibration procedure for the copper K edge is given as well as a comparison of the remeasured absorption edge energies with the previously tabulated data.

Journal ArticleDOI
TL;DR: In this article, the absorption, photoexcitation and internal and external luminescence efficiencies for Alq3 films grown by vacuum deposition have been measured accurately for the first time, and the internal quantum efficiency was found to be (32 ± 2)% independent of film thickness from 100 A to 1.35 μm.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the third-order nonlinear optical properties of thin Au/SiO2 composite thin films by means of a degenerate four-wave mixing at room temperature.
Abstract: Third‐order nonlinear optical properties of Au/SiO2 composite thin films have been investigated by means of a degenerate four‐wave mixing at room temperature. In the optical‐absorption spectra the absorption peak due to the surface plasmon resonance of Au particles is observed at the wavelength around 530 nm. With increasing the mean diameter of Au particles, the absorption at the peak is increased and the full width at half‐maximum of the absorption band is decreased from 130 to 80 nm. The third‐order nonlinear susceptibility χ(3) exhibits a peak at the wavelength of the absorption peak and the maximum value of χ(3) is obtained to be 2.0×10−7 esu. The size‐dependent enhancement of the χ(3) in Au particles with the mean diameter of 3.0–33.7 nm has been also investigated. The value of χ(3)/α (α: absorption coefficient) for the films is increased upon an increase of mean diameter of Au particles. This is explained by the size dependences of the local‐field factor and the imaginary part of dielectric constant of the metal particles.

Journal ArticleDOI
TL;DR: Cavity ring down absorption spectroscopy with a continuous light source is used to measure the transition frequencies and absolute absorption coefficient of the weak b1Σg+(v′ = 2) ← X 3 Σg−(v″ = 0) transition of 18O2.

Journal ArticleDOI
TL;DR: In this article, low-temperature optical absorption, circular dichroism, magnetic circular dichroidism, and sulfur K-edge X-ray absorption spectra have been measured for the green "blue" copper center (type 1) in Achromobacter cycloclastes nitrite reductase.
Abstract: Low-temperature optical absorption, circular dichroism, magnetic circular dichroism, and sulfur K-edge X-ray absorption spectra have been measured for the green “blue” copper center (type 1) in Achromobacter cycloclastes nitrite reductase. Combined with density functional calculations, the results of these spectroscopies have been used to define the extremely “perturbed” electronic structure of this site relative to that of the prototypical “classic” site found in plastocyanin. Experimentally calibrated density functional calculations have been further used to determine the specific geometric distortions which generate the perturbed electronic structure. These studies indicate that the principal electronic structure changes in nitrite reductase, relative to plastocyanin, are a rotation of the Cu dx2-y2 half-filled, highest occupied molecular orbital (HOMO) and an increase in the ligand field strength at the Cu center. The HOMO rotation increases the pseudo-σ interaction and decreases the π interaction of ...

Journal ArticleDOI
TL;DR: In this article, a comprehensive electrochemical and static spectroscopic study is reported for a series of dimeric and trimeric porphyrin-based arrays, which consist of tetraarylporphyrins linked via ethyne groups at the ppositions of the aryl rings.
Abstract: A comprehensive electrochemical (cyclic and square-wave voltammetry, coulometry) and static spectroscopic (absorption, resonance Raman (RR), electron paramagnetic resonance (EPR)) study is reported for a series of dimeric and trimeric porphyrin-based arrays. All the arrays consist of tetraarylporphyrins linked via ethyne groups at the p-positions of the aryl rings. The complexes investigated include zinc-free base and bis-zinc dimers which contain varying degees of torsional constraint between the porphyrin rings and the aryl group of the linker, and linear and right-angle trimers in which two zinc porphyrins are bridged by either a zinc or free base porphyrin. The spectroscopic studies were performed on singly and multiply oxidized complexes as well as the natural species. The electrochemical and spectral properties of the arrays indicate that the electronic communication between the macrocycles is relatively weak in the ground and excited electronic states. This communication is through-bond, rather than through-space, and is mediated by the diarylethyne linker. The EPR spectra of the oxidized arrays exhibit complex temperature-dependent signatures that reflect hole/electron hopping and/or spin exchange interactions in the ground electronic state. Exchange interactions in the multiply oxidized arrays are significant (probably 1000 MHz pr greater) in both liquid and frozen solutions and, in certain cases,more » are enhanced upon solvent freezing. 45 refs., 12 figs., 3 tabs.« less

Journal ArticleDOI
TL;DR: In this paper, local vibrational modes (LVMs) were reported for Mg-doped GaN grown by metalorganic chemical vapor deposition, and the stretch modes of the MgH and MgD complexes in GaN were assigned to the stretch mode with the vibrational frequencies indicative of a strong N−H bond.
Abstract: Local vibrational modes (LVMs) are reported for Mg‐doped GaN grown by metalorganic chemical vapor deposition. Hetero‐epitaxial layers of GaN:Mg, either as‐grown, thermally activated, or deuterated, were investigated with low‐temperature, Fourier‐transform infrared absorption spectroscopy. The as‐grown material, which was semi‐insulating, exhibits a LVM at 3125 cm−1. Thermal annealing increases the p‐type conductivity, as established with Hall effect measurements, and proportionally reduces the intensity of this LVM. Deuteration of the activated material creates a LVM at 2321 cm−1. The isotopic shift establishes the presence of hydrogen in the vibrating complex. The new LVMs are assigned to the stretch modes of the Mg–H and Mg–D complexes in GaN, with the vibrational frequencies indicative of a strong N–H bond as recently proposed from total‐energy calculations.

Journal ArticleDOI
TL;DR: In this article, the emission cross-sections and the net gain have been determined from the measured absorption spectra for both the ions on the basis of principle of reciprocity for Tm3+ singly-and Tm 3+ Ho 3+ doubly-doped glasses pumped with 790 nm diode laser.
Abstract: Spectroscopic measurements and analysis of energy transfer processes for Tm3+ singly- and Tm 3+ Ho 3+ doubly-doped glasses pumped with 790 nm diode laser have been performed. The emission cross-sections and the net gain have been determined from the measured absorption spectra for both the ions on the basis of principle of reciprocity. Tm3+ singly-doped fluorozircoaluminate glass, which has a higher fluorescence efficiency at 1.82 μm and a longer upper state lifetime, is a material suitable for Tm laser compared with oxide glasses. The quantum efficiency of Ho3+ fluorescence of 5I75I8 at 2.05 μm for the Tm 3+ Ho 3+ doubly-doped glass is dominated by three energy transfer processes: Tm3+Tm3+ cross-relaxation, net Tm3+Ho3+ energy transfer, and energy transfer upconversion. The first process allows pumping efficiency to approach 2, and the second one contributes to population of the upper 5I7 laser level, while the final process gives rise to a sublinear increase in upper state population with pump power. Optimum doping levels for laser performance are easily predicted based on spectroscopic measurements and are found to be 2–8Tm3+ (1020/cm3) for Tm3+ singly-doped and 2–8Tm3+/0.3–1Ho3+ (1020/cm3) for Tm3+/Ho3+ doubly-doped systems, respectively.


Journal ArticleDOI
Taiha Joo1, Yiwei Jia1, Jae-Young Yu1, David M. Jonas1, Graham R. Fleming1 
TL;DR: In this paper, the authors studied the underlying dynamics of the B800 absorption band in isolated LH2 of Rb. sphaeroides at room temperature by transient absorption, transient grating, and photon echoes using 30 fs pulses.
Abstract: The underlying dynamics of the B800 absorption band in isolated LH2 of Rb. sphaeroides at room temperature is studied by transient absorption, transient grating, and photon echoes using 30 fs pulses. The energy transfer time from B800 to B850 is determined to be 800 fs, similar to the value reported previously. The three pulse stimulated photon echo identifies several important contributions to the B800 absorption line shape and thereby the dynamics of the system involved: several low frequency intramolecular vibrations, ultrafast bath (solvent and protein) responses, and static inhomogeneity longer than the time scale of B800 to B850 energy transfer make significant contributions. Transient absorption decay is nonexponential as found previously. It is argued that the fast component in the two-exponential analysis of the transient absorption signal originates from vibrational relaxation within the B800 absorption band. Calculations of the nonlinear signals based on the optical transition frequency correl...

Journal ArticleDOI
TL;DR: In this paper, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near-edge x-ray absorption spectroscopy with time resolutions of 1.5-3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A).
Abstract: Ultrafast x‐ray techniques can, in principle, allow us to more directly watch the time evolution of matter, with atomic spatial resolution and with time resolution on the scale of atomic motions such as the making and breaking of chemical bonds, in order to more directly observe the fundamental molecular dynamics underlying the concept of ‘‘mechanism’’ in inorganic, organic, and biochemical reactions. As a step toward this goal, we have observed a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near‐edge x‐ray absorption spectroscopy with time resolutions of 1.5–3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A).