Showing papers on "Absorption band published in 2012"

Persistent Spectral Hole-Burning: Science and Applications

Abstract: 1. Introduction.- 1.1 Fundamental Requirements for Persistent Spectral Hole-Burning.- 1.2 Significance for Science and Applications.- 1.3 Historical Overview and Survey of Mechanisms.- 1.4 Synopsis of the Book.- References.- 2. Basic Principles and Methods of Persistent Spectral Hole-Burning.- 2.1 Background.- 2.2 Homogeneous Spectrum of an Electron-Vibrational Transition.- 2.2.1 Integrated Intensities of Purely Electronic Lines and Phonon Sidebands, Electron-Phonon Interactions and Temperature Dependence.- 2.2.2 Relative Width (Q-factor) of PEL. Peak Intensities.- 2.2.3 Role of Local Modes.- 2.3 Inhomogeneous Broadening of the Vibronic Spectrum.- 2.3.1 Inhomogeneous Broadening of Purely Electronic Lines Inhomogeneous Distribution Function.- 2.3.2 Selectivity of the Spectral Response of an Inhomogeneous Absorption Band.- 2.3.3 Inhomogeneous Distribution Function Under Monochromatic Laser Excitation. Site-Selection Spectroscopy.- 2.4 Persistent Spectral Hole-Burning.- 2.4.1 Burning of Spectral Holes in the Inhomogeneous Distribution Function.- 2.4.2 Early Observations of Persistent Spectral Hole-Burning.- 2.5 Kinetics of Persistent Spectral Hole-Burning.- 2.6 Spectroscopic Applications.- 2.6.1 Homogeneous Zero-Phonon Line Broadening and Dephasing in Crystals.- 2.6.2 Photochemical Hole-Burning in Glassy Matrices.- 2.6.3 Homogeneous Linewidths of Vibronic Transitions and Relaxation.- 2.6.4 Off-Resonance Hole-Burning and Non-Correlation Effects.- 2.6.5 Hole-Burning in the Spectra of Chlorophyll-like Molecules.- 2.7 Special Methods of Hole-Burning and Detection.- 2.7.1 Detection of Holes by Doppler Scanning.- 2.7.2 Holographic Detection of Spectral Holes.- 2.7.3 Creation of Sharp Antiholes.- 2.8 Hole-Burning Time-and-Space-Domain Holography.- 2.8.1 Hole-Burning by Picosecond Pulses.- 2.8.2 Theory of Time-and-Space-Domain Holographic Recording and Playback.- 2.8.3 Experimental Results and Discussion.- 2.9 Concluding Remarks.- References.- 3. Photochemical Hole-Burning in Electronic Transitions.- 3.1 Photochemical, Photophysical, and Spin Hole-Burning.- 3.1.1 Historic Survey.- 3.1.2 Radiation-Induced Saturation Versus Chemical Depletion.- a) Transient Saturation.- b) Chemical Depletion.- 3.1.3 Photochemical Systems and Mechanisms.- 3.2 Spectroscopic Analysis of Hole-Burning Experiments.- 3.2.1 General Remarks.- 3.2.2 Fast Relaxation Processes and Excited State Dephasing.- a) Lineshape Analysis.- b) Temperature Dependence of the "Homogeneous" Linewidth.- 3.2.3 Spectral Diffusion in Glasses.- a) TLS Parameters and Tunnelling Rates.- b) Spectroscopic Parameters.- 3.3 Field Effects in Hole-Burning Spectroscopy.- 3.3.1 Introduction: The Site Memory Function.- 3.3.2 Electric-Field Effects.- a) Stark Effect for Molecules with Inversion Symmetry.- b) Stark Effect for Molecules Without Inversion Symmetry.- 3.3.3 Strain-Field Effects.- References.- 4. Persistent Spectral Hole-Burning in Inorganic Materials.- 4.1 Introduction.- 4.2 Hole-Burning Mechanisms.- 4.3 Color Centers.- 4.4 Rare Earth Compounds.- 4.4.1 Trivalent Rare Earth Ions in Glasses.- 4.4.2 Divalent Rare Earth Ions in Crystals.- a) CaF2:Sm2+.- b) SrF2:Sm2+.- c) BaClF:Sm2+.- 4.5 Transition Metal Ions.- 4.5.1 LiGa5 O8:Co2+.- 4.5.2 Y3Al5O12:Ti3+.- 4.6 Conclusion.- References.- 5. Two-Level-System Relaxation in Amorphous Solids as Probed by Nonphotochemical Hole-Burning in Electronic Transitions.- 5.1 Background.- 5.2 Survey of NPHB Systems.- 5.2.1 Hydrogen-Bonded Crystals.- 5.2.2 Molecules in Amorphous Polyacene Films.- 5.2.3 Molecules in Organic Glasses.- 5.2.4 Molecules in Polymers.- 5.2.5 Rare-Earth Ions in Glasses and Polymers.- 5.3 Optical Linewidths and Dephasing in Amorphous Solids.- 5.3.1 Single-Impurity Single-TLS System Hamiltonian.- 5.3.2 Optical Dephasing due to Off-Diagonal Modulation.- 5.3.3 Recent Experiments.- 5.3.4 New Theories.- 5.3.5 Comparison of Theories and Experimental Data.- 5.3.6 Hole Widths and TLS Relaxation Processes in Organic Systems.- 5.4 Density of States Functions for TLS.- 5.5 Laser-Induced Hole Filling.- 5.5.1 Rhodamine 640 in Poly(vinylalcohol).- 5.5.2 Nd3+ and Pr3+ in Poly(vinylalcohol).- 5.5.3 A Tentative Model for LIHF.- 5.6 Recent Developments.- 5.7 Concluding Remarks.- References.- 6. Persistent Infrared Spectral Hole-Burning for Impurity Vibrational Modes in Solids.- 6.1 Introduction.- 6.1.1 Matrix-Isolated Molecules in Van der Waals and Ionic Solids.- 6.1.2 Persistent IR Hole-Burning in Vibrational Modes.- 6.2 Molecules in Van der Waals Matrices.- 6.2.1 1,2-Difluorethane (DFE).- a) Diode Laser Measurements.- b) CO2 Laser Measurements.- 6.2.2 Interpretation of Persistence.- 6.2.3 Molecular Aggregates of Methyl Nitrite or Methanol.- 6.3 ReO4? in Alkali Halide Crystals.- 6.3.1 Background and Spectroscopic Information.- 6.3.2 Measurements of Relaxation Times T1 and T2.- 6.3.3 Persistent Spectral Holes for ReO4? in Alkali Halides.- a) Summary of Characteristics.- b) Model for the PIRSH Process.- 6.3.4 Persistent Spectral Pegs.- 6.3.5 Ultrasonic Studies of Multiple Ground State Configurations.- 6.3.6 Conclusions on the ReO4? System.- 6.4 Persistent Spectral Hole-Burning for CN? Molecules in Alkali Halide Crystals.- 6.4.1 Background Information on Matrix-Isolated CN?.- 6.4.2 High-Resolution FTIR Spectroscopy in the CN? Stretch Region.- 6.4.3 Hole-Burning in the CN? Stretch Mode Region.- 6.4.4 A Study of the CN?:Na+ Center Dynamics.- a) Fluorescence.- b) Hole-Burning and ?l Center Geometry.- 6.4.5 Other CN? Complexes.- 6.5 Conclusion.- 6.5.1 Comparison of the Three Types of Vibrational Hole-Burning Systems.- 6.5.2 Systems Which do not Exhibit PIRSH Formation.- a) Derivatives of the CN? Molecule.- b) Spherical-Top Molecules Which Contain Hydrogen.- 6.5.3 Future Prospects.- a) NO2? in Alkali Halides.- b) Disordered Solids.- References.- 7. Frequency Domain Optical Storage and Other Applications of Persistent Spectral Hole-Burning.- 7.1 Introduction.- 7.2 Systems Issues for Frequency Domain Optical Storage.- 7.2.1 General Remarks.- 7.2.2 Engineering Studies.- 7.3 Materials Research for Frequency Domain Optical Storage.- 7.3.1 General Materials Requirements.- 7.3.2 Limitations of Single-Photon Recording Mechanisms.- 7.3.3 Photon-Gated Mechanisms.- 7.3.4 Limitations on Storage Density.- 7.4 Alternative Data-Storage Configurations.- 7.4.1 Time Domain Storage.- 7.4.2 Electric-Field Readout.- 7.4.3 Holographic Readout.- 7.5 Other Applications of Persistent Spectral Hole-Burning.- 7.5.1 General Remarks.- 7.5.2 Laser Pulse Shaping Based on Fourier Synthesis.- 7.5.3 Laser Pulse Shaping Based on Voltage Modulation.- 7.5.4 Frequency Multiplexed Optical Spatial Filters.- 7.6 Summary and Future Prospects.- References.

Synthesis, Characterization, and Spectroscopic Properties of ZnO Nanoparticles

Abstract: ZnO nanoparticles have been synthesized by precipitation method from Zinc nitrate. The powder was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction, UV-vis optical absorption, and photoluminescence spectroscopy analyses. XRD patterns showed that ZnO nanoparticles have hexagonal unit cell structure. SEM and TEM pictures reveal the morphology and particle size of prepared ZnO nanoparticles. The UV-vis absorption spectrum shows an absorption band at 355 nm due to ZnO nanoparticles. The photoluminescence spectrum exhibits two emission peaks one at 392 nm corresponding to band gap excitonic emission and another located at 520 nm due to the presence of singly ionized oxygen vacancies. The synthesis method has potential for application in manufacturing units due to ease processing and more economical reagents.

Behaviour of electric and magnetic dipole transitions of Eu3+, 5D0 → 7F0 and Eu–O charge transfer band in Li+ co-doped YPO4:Eu3+

Abstract: The effect of Li+ co-doping on the photoluminescence properties of YPO4:Eu is discussed. Interesting behaviours, such as the presence of intermediate bands, shifting of the Eu–O charge transfer band (Eu–O CTB) to a lower wavelength, variation in intensities of magnetic (5D0 → 7F1) and electric dipole (5D0 → 7F2) transitions of Eu3+ and shift of 5D0 → 7F0 to higher energy with increasing excitation wavelengths are observed. The Eu3+ ion does not have an absorption band in the range 340–350 nm, but after excitation at these wavelengths, a broad emission band (370–570 nm), as well as sharp peaks of Eu3+, could be observed. This is due to strong energy transfer from the intermediate band of the host to the Eu3+ ion. X-ray photoelectron spectroscopy (XPS) study also confirms that intermediate band emission is not due to Eu2+ ion emission. The blue shifting of Eu–O CTB is because of the increase in the optical electronegativity of the Eu3+ ion on Li+ co-doping. The variation in intensities of the 5D0 → 7F2 and 5D0 → 7F1 dipole transitions is related to (i) overlapping interaction parameters within the ground and excited states, (ii) exchange interaction among atoms/ions and (iii) density of the incoming photons. Shift of 5D0 → 7F0 to a higher energy with increasing excitation wavelengths is because of change in the second order crystal field parameter B20 with excitation wavelength. The significant enhancement of luminescence intensity is found with Li+ co-doping due to the increase in crystallinity.

On the origin of the 265 nm absorption band in AlN bulk crystals

Abstract: Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes An absorption band centered around 47 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN

Passively Q-switching induced by gold nanocrystals

Abstract: Passive Q-switching was experimentally demonstrated in an erbium-doped fiber laser (EDFL) by using gold nanocrystals (GNCs). The GNCs were mixed with sodium carboxymethylcellulose (NaCMC) to form GNCs-NaCMC films. The films exhibited a broad absorption band in the range of 400–1750 nm. By placing the GNCs-NaCMC film in an EDFL cavity pumped by a 980 nm laser diode, stable passive Q-switching was achieved for a threshold pump power of ∼30 mW, and 3.2 μs pulses at 1560 nm with a repetition rate of 24.2 kHz were obtained for a pump power of ∼125 mW.

Optical properties of oligothiophene substituted diketopyrrolopyrrole derivatives in the solid phase: joint J- and H-type aggregation.

Abstract: Photophysical properties of diketopyrrolopyrrole derivatives substituted with oligothiophenes are investigated. All compounds are found to be fluorescent both in solution and in the solid phase. At low temperature in the solid, fluorescence originates from excimer-like excited states. Comparison of absorption and fluorescence excitation spectra taken under matrix isolated conditions and on solid films show the presence of both J- and H-type absorption bands in the solid phase. Quantum-chemical calculations, including exciton–phonon coupling to account for deviations from the Born–Oppenheimer approximation, are performed to simulate the band shape of the lowest absorption band in the molecular solid. The joint presence of J- and H-bands is explained by the presence of two molecules in the unit cell. The Davydov splitting is substantial for molecules with linear alkyl substituents on the nitrogen atom (on the order of 0.2 eV) but can be reduced to almost zero by introducing branching at the β-carbon of the ...

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

Abstract: We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO2 in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO2. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO2, are observed by analyzing the results of optical properties, which locate in the region of 400–1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO2 shift to the long wavelength region compared with the pure TiO2. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO2. Our results show that the visible light response of TiO2 can be modulated by substitutional doping of Cu, Ag, and Au.

Spectrally resolved size-dependent third-order nonlinear optical properties of colloidal CdSe quantum dots

Abstract: Nonlinear absorption and nonlinear refraction of colloidal CdSe quantum dots (QDs) of two sizes were investigated in a wide spectral range with the Z-scan technique using a tunable femtosecond laser system. The nonlinear absorption was found to be the strongest close to twice the wavelength of the second exciton absorption band of the QDs. Based on nonlinear optical parameters the exciton binding energy has been determined. The current results are compared to the nonlinear properties of CdSe QDs presented in the literature. The features of this system relevant for multiphoton fluorescence microscopy applications are discussed.

Structural and optical characterization of Zn doped TiO2 nanoparticles prepared by sol–gel method

Abstract: Undoped and zinc-doped TiO2 nanoparticles (Ti1−xZnxO2 where x = 0.00–0.10) were synthesized by a sol–gel method. The synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–VIS spectrometer. XRD pattern confirmed the tetragonal structure of synthesized samples. Average grain size was determined from X-ray line broadening using the Debye–Scherrer relation. The crystallite size was varied from 10 to 40 nm as the calcination temperature was increased from 350 to 800 °C. The incorporation of 3–5 mol% Zn2+ in place of the Ti4+ provoked a slight decrease in the size of nanocrystals as compared to undoped TiO2. The SEM and TEM micrographs revealed the agglomerated spherical-like morphology with a diameter of about 10–30 nm and length of several nanometers, which is in agreement with XRD results. Optical absorption measurements indicated a blue shift in the absorption band edge upon 3–5 mol% zinc doping. Direct allowed band gap of undoped and Zn-doped TiO2 nanoparticles measured by UV–VIS spectrometer were 2.95 and 3.00 eV at 550 °C, respectively.

Solvent Effects on the UV/ Visible Absorption Spectra of Some Aminoazobenzene Dyes

Abstract: Absorption spectra of three aminoazobenzene dyes with various molecular size and alkyl tails have been recorded in solvents in the range between 200 to 600 nm. The photophysical behavior of a dissolved dye depends on the nature of its environment, i.e. the intensity, shape, and maximum absorption wavelength of the absorption band of dye in solution depends strongly on the solvent-solute interactions and solvent nature. The solvatochromic behaviors of aminoazobenzene dyes and solvent- solute interactions can be analyzed by means of linear solvation energy relationships concept proposed by Kamlet and Taft. Bathochromic shift of absorption maxima of these dyes, λmax , occurs in solvent with the highest proton acceptor ability and dipolarity/polarizability.

Luminescence of oxyfluoride glasses co-doped with Ag nanoclusters and Yb3+ ions

Abstract: Bulk oxyfluoride glasses co-doped with Ag nanoclusters and Yb3+ ions have been prepared by a melt quenching technique. When excited in the absorption band of the Ag nanoclusters between 300 to 500 nm, these glasses emit a broad band characteristic of the Ag nanoclusters between 400 to 750 nm as well as an emission band between 900 to 1100 nm, originating from Yb3+ ions. The intensity ratio of the Yb3+/Ag emission bands increases with the Ag doping level at a fixed concentration of Yb3+, indicating the presence of energy transfer mechanism from the Ag nanoclusters to the Yb3+ ions. Comparison of time-resolved decay kinetics of the luminescence in the respectively Ag nanocluster-Yb3+ co-doped and single Ag nanocluster doped glasses, hints towards an energy transfer from the red and infrared emitting Ag nanoclusters to the Yb3+ ions.

Fluorescence and two-photon absorption of push–pull aryl(bi)thiophenes: structure–property relationships

Abstract: Photophysical and TPA properties of series of push–pull aryl(bi)thiophene chromophores bearing electron-donating (D) and electron-withdrawing (A) end-groups of increasing strength are presented All compounds show an intense intramolecular charge transfer (ICT) absorption band in the visible region Increasing the D and/or A strength as well as the length of the conjugated path induces bathochromic and hyperchromic shifts of the absorption band as reported for analogous push–pull polyenes Yet, in contrast with corresponding push–pull polyenes, a significant increase in fluorescence is observed In particular, chromophores built from a phenyl–bithienyl conjugated path and bearing strong D and A end-groups were found to combine very large one and two-photon brightness as well as strong emission in the red/NIR region These molecules hold promise as biphotonic fluorescent probes for bioimaging

A wide-angle dual-band infrared perfect absorber based on metal-dielectric-metal split square-ring and square array

Abstract: The optical properties of a dual-band plasmonic absorber are delineated numerically by employing a finite element method. Based on a metal–dielectric–metal scheme, the key structure consists of a lossless dielectric layer that is sandwiched by a silver split square-ring enclosing a silver square array and ground silver plane. The simulation results clearly show that near-perfect absorption efficiency can be realized for two absorption bands in the near-infrared wavelength range. The near-perfect absorption bands exhibit polarization insensitivity and wide-angle incidence. Notably, the absorption band is independently governed by the size of each part of the patterned films where the silver split square-ring determines the shorter wavelength band and the silver square determines the longer wavelength band, respectively. The repositioning of two near-perfect absorption peaks possesses a linear relationship that is related to the dimensions of the patterned silver layer. This allows for a flexible reconfigurability over the entire near-infrared regime.

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films

Abstract: The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO2 and ZrO2 thin films, all in contact with air, using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from ≤70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149˙+ radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149˙+ in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149˙+–electron complex to isolated D149˙+ and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149˙+ by interactions with ZnO. TiO2 exhibits similar dynamics to ZnO. In the case of ZrO2, electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149˙+ in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D0 → D1 absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149˙+ spectrum employing the MPW1K functional.

Understanding plasmon resonances of metal-coated colloidal crystal monolayers

Abstract: Metal films deposited over two-dimensional colloidal crystals (MFoCC) constitute a low-cost periodic structure with interesting photonic and plasmonic properties. It has previously been shown that this structure exhibits a behaviour similar to the well-known Extraordinary Optical Transmission (EOT) of metallic hole arrays in planar films. Here, we explore the transmission characteristics of AgFoCC by systematic comparison with that of the bare CC. Furthermore with additional reflectivity measurements we evaluate the AgFoCC overall plasmonic response, which, notably, exhibits a strong plasmon absorption band at wavelengths larger than those of the transmitted maximum. By corroborating these results with finite-difference time-domain electromagnetic simulations, we identify a hybrid metal-dielectric propagative mode in the transmission mechanism. On the contrary a strongly localized mode is responsible for the maximum light absorption by this structure. These results shed new light on the current understanding of this highly promising plasmonic structure, being useful for the design of surface-enhanced Raman scattering and enhanced fluorescence substrates.

Photophysics of diphenylacetylene: light from the "dark state".

Abstract: A weak band at the tail of the known tolane (diphenylacetylene, DPA) fluorescence spectrum in several solvents is assigned to the forbidden 11Au → 11Ag transition on the basis of its lifetime (∼200 ps) and its fluorescence excitation spectra. The 11Au state, generally called the dark state, is not truly dark. We report the temperature (T) dependence of DPA fluorescence quantum yields (ϕf) in methylcyclohexane (MCH) solution and the fluorescence and phosphorescence quantum yields of DPA in glassy MCH at 77 K. Significant differences between fluorescence and phosphorescence excitation spectra reveal that, in addition to the 11B1u ← 11Ag transition, the first DPA absorption band includes a transition to another excited state, most probably the 11B2u state, from which intersystem crossing is more efficient. The T dependence of ϕf values in MCH solution is shown to be consistent with the previously reported T dependence of the lifetimes of transient DPA singlet excited state absorptions in the picosecond time ...

Discovery of crystallized water ice in a silhouette disk in the m43 region

Abstract: We present the 1.9-4.2 μm spectra of the five bright (L ≤ 11.2) young stars associated with silhouette disks with a moderate to high inclination angle of 39°-80° in the M42 and M43 regions. The water ice absorption is seen toward d121-1925 and d216-0939, while the spectra of d182-316, d183-405, and d218-354 show no water ice feature around 3.1 μm within the detection limits. By comparing the water ice features toward nearby stars, we find that the water ice absorption toward d121-1925 and d216-0939 most likely originates from the foreground material and the surrounding disk, respectively. The angle of the disk inclination is found to be mainly responsible for the difference of the optical depth of the water ice among the five young stars. Our results suggest that there is a critical inclination angle between 65° and 75° for the circumstellar disk where the water ice absorption becomes strong. The average density at the disk surface of d216-0939 was found to be 6.38 × 10–18 g cm–3. The water ice absorption band in the d216-0939 disk is remarkable in that the maximum optical depth of the water ice band is at a longer wavelength than detected before. It indicates that the primary carrier of the feature is purely crystallized water ice at the surface of the d216-0939 disk with characteristic size of ~0.8 μm, which suggests grain growth. This is the first direct detection of purely crystallized water ice in a silhouette disk.

Adsorption of Co-Phthalocyanine on the Rutile TiO2(110) Surface: A Scanning Tunneling Microscopy/Spectroscopy Study

Abstract: We investigated the adsorption properties of Co-phthalocyanines (CoPc's) on the rutile TiO2(110) surface using scanning tunneling microscopy and spectroscopy. Analytical results showed that the adsorption properties strongly depend on the deposition conditions. Under specific deposition conditions (elevated substrate temperatures and low deposition rates), the molecules are immobilized on the surface without aggregation up to the full monolayer coverage. The substrate temperature plays a key role in determining the bonding nature between the molecules and the substrate. The immobilized molecules introduce gap states at the interface and locally decrease the surface band gap to an energy value that is lower than that of the absorption band of the CoPc thin film. Coverage dependence studies show that the adsorption structures are dominated by molecule–substrate interactions for the first monolayer growth and by a complex interaction between molecules in the first and second monolayers for the second monolay...

Synthesis and characterizations of red/near-IR absorbing A–D–A–D–A-type oligothiophenes containing thienothiadiazole and thienopyrazine central units

Abstract: A series of π-conjugated A–D–A–D–A-type oligothiophenes (1–3) comprising heterocyclic thieno[3,4-c][1,2,5]thiadiazole (TTDA) or thieno[3,4-b]pyrazine (TP) as the core and dicyanovinyl (DCV) or trifluoroacetyl (TFA) as terminal acceptor groups have been developed as red/near-IR absorbers. The resulting oligomers were characterized by UV-Vis spectroscopy, cyclic voltammetry and thermal analysis. The low energy absorption band of these oligomers was located at 620–707 nm in solution and at 695–830 nm in thin films. Theoretical studies reveal that the insertion of the bicyclic nonclassical thiophenes to the conjugated backbone imposed partial quinoidal character to the resulting oligomers and thus reduced the band gap. HOMO–LUMO energies of 1–3 derived from electrochemical measurements were found to be suitable for the use as donor material in combination with fullerene-C60 as acceptor in planar heterojunction solar cells prepared by vacuum deposition. Such devices incorporating these low band gap oligomers exhibited high FF values up to 0.60, and power conversion efficiencies of up to 1.3% under air mass (AM) 1.5G illumination. External quantum efficiency (EQE) spectra clearly showed the contribution of the low energy absorption to the overall photocurrent.

Direct and indirect electron emission from the green fluorescent protein chromophore.

Abstract: Photoelectron spectra of the deprotonated green fluorescent protein chromophore have been measured in the gas phase at several wavelengths within and beyond the S(0)-S(1) photoabsorption band of the molecule. The vertical detachment energy (VDE) was determined to be 2.68 ± 0.1 eV. The data show that the first electronically excited state is bound in the Franck-Condon region, and that electron emission proceeds through an indirect (resonant) electron-emission channel within the corresponding absorption band.

Photoabsorption cross-section measurements of32S, 33S, 34S, and 36S sulfur dioxide for the B1B1-X1A1 absorption band

Abstract: [1] We report measurements of the ultraviolet absorption cross-sections of32SO2, 33SO2, 34SO2 and 36SO2 recorded from 250 to 320 nm at 293 K with a resolution of 8 cm−1. This is the first reported measurement of the 36SO2cross-section. This work improves earlier measurements of the32SO2, 33SO2 and 34SO2cross-sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO2 samples were produced in an identical process via combustion of isotopically enriched S0, eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32SO2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32SO2was used to estimate the cross-sections of33,34,36SO2; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 ‰ respectively. While the peak-to-valley amplitude of36SO2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32SO2. The cross-sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B1B1-X1A1 and the C1B1-X1A1 bands.

Photoluminescence of bis(8-hydroxyquinoline) zinc (Znq2) and magnesium (Mgq2)

Abstract: Absorption and photoluminescence (PL) spectra, PL quantum efficiency, and PL lifetime have been investigated on bis(8-hydroxyquinoline) zinc (Znq2) and magnesium (Mgq2) in solutions and powder. Znq2 and Mgq2 have the lowest-energy absorption band at 376 and 396 nm in acetonitrile solution, respectively, and emission band with peak at 555 and 480 nm. The PL quantum efficiency is 0.03 and 0.45 for Znq2 and Mgq2 in the solution, respectively, while 0.45 and 0.36 in powder. Unlike the case of powders, two PL lifetimes are obtained in solutions. The longer lifetime is attributed to molecule having interaction with its neighboring molecule, while the shorter one to the isolated single molecule.

Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms.

Abstract: The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied Upon one photon absorption, the molecule is excited to the first singlet excited S1 state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S0 state or undergoes intersystem crossing to the first excited triplet T1 state, from where the molecule can dissociate over two main channels: the radical (CH3 + HCO) and the molecular (CO + CH4) channels Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T1 barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ∼308 nm Towards the blue edge, at 230 nm, an upper limit of ∼50% has been estimated for the contribution of the roaming channel The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations