Effect Of Photoexcitation Research Articles

Photoinduced alteration of the inner electric field in a Zn-phthalocyanine/C 60 heterojunction solar cell

Electroabsorption (EA) and photocurrent measurements are carried out for a Au/Zn-phthalocyanine(ZnPc)/C 60/In/Al heterojunction cell in order to elucidate the effect of photoexcitation of ZnPc or C 60 on the inner electric field E 0. By a pumping excitation of either the ZnPc or C 60 layer, the E 0 of the pumped layer is decreased and that of the other layer is increased. The photocurrent due to a probing-light illumination besides the pumping-light illumination has a good correlation with E 0 under the pumping excitation of ZnPc but has no correlation under the pumping excitation of C 60.

Photo-bleaching response in chlorophyll solutions

Photo-excitation processes were investigated in chlorophyll solutions by irradiation with UV light sources. Photo-excited bulk effects during the bleaching experiments were diagnosed. The optical diagnostic was expressed by a Beer–Lambert matrix formulation and applied to the experimental results. The chromophores evaluated optical cross-section was 10 −15 cm 2 and the optical absorption coefficient dynamic variation was fitted to a simple reaction rate model. The bleaching rates were expressed by this model and fitted to the experimental data.

Photoemission measurements of transition-metal oxides under laser illumination

The effects of photoexcitation on Sr 14−xCa xCu 24O 41 and La 2−2xSr 1+2xMn 2O 7 single crystals were measured via X-ray photoemission spectroscopy. Changes observed in the Sr 14−xCa xCu 24O 41 Cu 2p core-level line shape indicate that the cleaved surface is surprisingly unstable under vacuum and that changes in the surface stoichiometry are easily obtained with laser ablation. In La 2−2xSr 1+2xMn 2O 7, the O 1s core-level peak binding energy irreversibly increases upon optical excitation. The magnitude of the increase depends on the sample hole doping and is observed to be largest in a x = 0.5 sample, where phase competition between electronic ordered states is strongest. It appears that the photoinduced electronic structural change is permanent due to stabilization by strong electron-lattice coupling.

The Effect of Photoexcitation during the Epitaxial Thin Film Growth of Organic Molecular Materials(<Special Issue>Organic Crystals)

The Effect of Photoexcitation during the Epitaxial Thin Film Growth of Organic Molecular Materials(<Special Issue>Organic Crystals)

Electron spin resonance study of GaAs:Er,O grown by organometallic vapor phase epitaxy

Electron spin resonance (ESR) measurements of GaAs:Er,O grown by organometallic vapor phase epitaxy have been performed at 9.49GHz from 4.5to13K. Several anisotropic ESR signals (A, B, C, and D) are observed, which is consistent with the previous reports. We reexamined the angular dependence and temperature dependence of the signals in detail. Our ESR results indicate that the direction of alignment of two oxygen atoms or two arsenic atoms for an Er-2O center deviates from the ⟨110⟩ direction in various ways, resulting in the various Er-2O centers. Our ESR results also indicate that the paramagnetic states of the Er centers in GaAs change to a singletlike nonmagnetic state below 8K. The ESR measurements under illumination have been performed to investigate the effect of photoexcitation of the host. This result indicates that the energy transfer efficiency from the host to the B center is especially large. We found that the luminescent Er-2O center B and another Er-2O center C considerably differ in the coupling with the lattice.

Photoinduced chain-oxygen ordering in detwinnedYBa2Cu3O6.7single crystals studied by reflectance-anisotropy spectroscopy

We present a reflectance-anisotropy spectroscopy (RAS) investigation of photoexcitation and annealing effects in oxygen deficient detwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.7}$ single crystals. Well-resolved RAS spectral features are either bleached or enhanced on a time scale of hours upon laser illumination with polarization parallel to the $\mathrm{Cu}(1)\text{\ensuremath{-}}\mathrm{O}(1)$ chains. These photoinduced effects recover with room temperature annealing in the dark. Based on previous ellipsometric studies and on cluster models for the oxygen $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{O}(1)$ chain-fragments we are able to assign the RAS peaks that depend on illumination to optical transitions involving copper atoms located either on short chain-fragments or in isolated $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{O}{(4)}_{2}$ sites. This provides strong evidence that photoinduced chain-oxygen ordering is indeed at the origin of persistent photoconductivity and Raman vibrational mode bleaching in these materials.

Atomic Data and Spectral Line Intensities for S x

Electron impact collision strengths, energy levels, oscillator strengths and spontaneous radiative decay rates are calculated for S XI. The configurations included are 2s(sup 2)2psup 2), 2s2p(sup 3), 2p(sup 4), 2s(sup 2)2p3l and 2s(sup 2)2p4l (I = s , p , d) giving rise to 72 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 32, 60, 90, 120, and 150 Ry. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved: The effects of resonances, proton rates, photoexcitation and cascades on level populations have been investigate. The predicted S XI line intensities are compared with EUV and UV observations of the quiet and active Sun.

Effect of photoexcitation on the electrical performance of ZnS: Mn thin-film electroluminescent structures

It is shown that the kinetics of the charge and current passing through a thin-film electroluminescent emitter, as well as the I-V characteristics of the emitter, greatly diverge under blue, red, and IR pulsed illumination with photon energies of ≈2.6, ≈1.9, and ≈1.3 eV, respectively, and a photon flux density of 4×1014–3×1015 mm−2 s−1. Results obtained indicate that, during the operation of the emitter, deep centers associated presumably with V Zn 2− zinc vacancies and V S + and V S 2+ sulfur vacancies exchange charge. These centers lie above the valence band by ≈1.1, ≤1.9, and ≤1.3 eV, respectively. Their concentrations are estimated as (3–4)×1016 cm−3 for V Zn 2− and V S + and ≈1.5×1016 cm−3 for V S 2+ . It is demonstrated that positive and negative space charges forming in the near-anode and near-cathode regions of the phosphor layer specify the electric performance of the emitters.

The effect of photoexcitation of the electrical properties of ZnS:Mn thin-film emitters

Significant distinctions have been observed in the current kinetics and current-voltage characteristics of ZnS: Mn electroluminescent thin-film emitters measured with pulsed photoexcitation in different (blue, red, and infrared) spectral intervals. The results show evidence that a recharge of deep centers related to the zinc and sulfur vacancies takes place in the course of the emitter operation, leading to the formation of space charges in the near-anode and near-cathode regions of the phosphor layer.

Optical hysteresis in a spin-crossover complex

Photoexcitation effects on the spin states were investigated for a prototypical spin-crossover complex $[\mathrm{Fe}(\mathrm{ptz}{)}_{6}]({\mathrm{BF}}_{4}{)}_{2}$ (ptz=1-propyltetrazole) by the pump and probe technique at 77 K. We have found (i) superlinear increase of density ${n}_{\mathrm{HS}}$ of the photoexcited high-spin molecules when the excitation power density P exceeded a critical value $(=120{\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2})$ and (ii) optical hysteresis between the P-increasing and the P-decreasing runs. These characteristic features are well reproduced by the Monte Carlo simulation based on the Ising model, in which the spin state of the respective ${\mathrm{Fe}}^{2+}$ ion is represented by the pseudospins.

CHIANTI—An Atomic Database for Emission Lines. VI. Proton Rates and Other Improvements

The CHIANTI atomic database contains atomic energy levels, wavelengths, radiative transition probabilities, and electron excitation data for a large number of ions of astrophysical interest. Version 4 has been released, and proton excitation data are now included, principally for ground configuration levels that are close in energy. The fitting procedure for excitation data, both electrons and protons, has been extended to allow nine-point spline fits in addition to the previous five-point spline fits. This allows higher quality fits to data from close-coupling calculations where resonances can lead to significant structure in the Maxwellian-averaged collision strengths. The effects of photoexcitation and stimulated emission by a blackbody radiation field in a spherical geometry on the level balance equations of the CHIANTI ions can now be studied following modifications to the CHIANTI software. With the addition of H I, He I, and N I, the first neutral species have been added to CHIANTI. Many updates to existing ion data sets are described, while several new ions have been added to the database, including Ar IV, Fe VI, and Ni XXI. The two-photon continuum is now included in the spectral synthesis routines, and a new code for calculating the relativistic free-free continuum has been added. The treatment of the free-bound continuum has also been updated.

Photoinduced dynamic effects in the sliding conduction of K0.3MoO3

The effects of photoexcitation on the dynamics of the CDW in K0.3MoO3 are demonstrated. A light illumination causes a relaxation of the CDW phase strain, which can be utilized to instantly prepare the stress-relieved phase configuration as a well-defined initial condition for each measurement. This was clearly observed in the optical modification of the time evolution of sliding threshold voltage and in the amount of conduction delay time after the step voltage application.

Ionic and electronic processes in quartz:Mechanisms of thermoluminescence and optically stimulated luminescence

We suggest a model of the atomic and electronic processes responsible for the so-called 110 and 325 °C thermoluminescence (TL) peaks, including predose behavior, and for the room temperature optically stimulated luminescence (OSL) of quartz. Our model is based on defects and defect processes typical of those known from many previous studies of quartz. It explains the experimental observations that the two TL peaks and OSL are correlated with respect to the effects of thermal annealing and photoexcitation after irradiation. The model indicates that the energy for the two TL peaks and OSL is all stored by the same defect pairs. These defect pairs comprise [AlO4]− and [X/M+]+ generated by a radiolytic reaction [AlO4/M+]0→[AlO4]−+M+. Here [AlO4]− is an Al impurity center substituting a Si atom, M+ is an alkali ion, [X/M+] is M+ stabilized by a defect denoted by X and [AlO4/M+]0 is an [AlO4]− center charge compensated by M+. Even though the 110 and 325 °C TL peaks and OSL arise from the same defect pairs, they should emit different luminescence because they arise through different mechanisms: the 325 °C TL peak through the migration of M+, and the OSL through the migration of holes. The 110 °C TL peak is ascribed to the electron–hole recombination at the [AlO4]− centers. According to the model, the TL at 110 and 325 °C and OSL luminesce at different wavelengths.

Wavelength- and Time-Dependent Two-Photon Photoemission Spectroscopy of Dye-Coated Silicon Surface

Two dye molecules, rhodamine B (RB) and sulforhodamine (SR), were adsorbed on doped silicon. The effect of photoexcitation of the adsorbed species on the electronic properties of the surface was investigated applying wavelength- and time-dependent two-photon photoemission spectroscopy (WD-TPPE). Despite the very similar electronic properties of the two molecules, their adsorption affects very differently the electronic properties of the substrate. It was found that while the adsorbed RB interacts weakly with the substrate and hence preserves its spectral properties, SR interacts strongly with the substrate, causes a decrease in the work function, and looses its “molecular” spectral features. It is shown that photoexcitation of a small fraction (0.01%) of the weakly bound RB species causes a significant change in the electronic properties of the substrate.

Pressure- and photo-induced phase transition in gold complex

Effects of pressure and photo-excitation on the mixed-valence (MV) to single-valence (SV) phase transition in Cs 2Au 2Br 6 have been investigated by means of high-pressure Raman spectroscopy. Under pressures, we have found both the permanent (at 6.5 GPa) and transient (at 5.5 GPa) photo-induced phase transitions (PIPTs). The two types of PIPTs have been discussed in terms of the pressure-controlled free-energy potential.

Dynamics of excited electrons in copper and ferromagnetic transition metals: Theory and experiment

Both theoretical and experimental results for the dynamics of photoexcited electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and Ni are presented. A model for the dynamics of excited electrons is developed, which is based on the Boltzmann equation and includes effects of photoexcitation, electron-electron scattering, secondary electrons (cascade and Auger electrons), and transport of excited carriers out of the detection region. From this we determine the time-resolved two-photon photoemission (TR-2PPE). Thus a direct comparison of calculated relaxation times with experimental results by means of TR-2PPE becomes possible. The comparison indicates that the magnitudes of the spin-averaged relaxation time \tau and of the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation times for the different ferromagnetic transition metals result not only from density-of-states effects, but also from different Coulomb matrix elements M. Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.

Photo-excitation from dia- to ferri-magnetism in a Rb-Co-hexacyanoferrate Prussian blue analogue

We studied the photo-excitation process, the relaxation of the photo-excited state towards the stable state, and the photo-induced magnetic properties of the Prussian blue analogue Rb0.52Co [Fe(CN)6]0.84, 2.3 H2O. Magnetic, Mossbauer and reflectivity measurements have been performed during and after illumination. The efficiency of the photo-excitation device is maximum at \(\lambda \cong 700 - 750\) nm. The process, however, is severely hindered by bulk absorption of the light; it is rapidly completed at the surface of the sample and then proceeds slowly in the bulk. Under the effect of photo-excitation the system turns from a dia- to a ferri- magnet, with a Tc value ≥21 K, indicative of the transformation of the material due to the following optical electron transfer: \(F{e^{II}}(LS) + C{o^{III}}(LS) \to F{e^{III}}(LS) + C{o^{II}}(HS)\) Thermal relaxation towards the stable electronic state is observed in the 95-110 K temperature interval, obeying a self-accelerated kinetics. At low temperature, a weak, non-exponential, relaxation is detected. These features are discussed in terms of co-operative effects in the frontal process of photo- excitation. The peculiarities of a photo-excited state created below the magnetic ordering temperature are discussed. A metastable magnetic state has been observed in low fields, denoted “Raw Photo-Induced State" (RPI), with a magnetization curve in-between the field-cooled and zero-field-cooled curves.

Continuum Fluorescence Excitation of [Ni] and [Oi] Lines in Gaseous Nebulae

We investigate the effects of photoexcitation of [N I] and [O I] lines by stellar continuum radiation under nebular conditions. It is found that the [N I] optical lines at 5198 and 5200 Å are affected by fluorescence in many objects, including the Orion Nebula. [O I] lines are much less sensitive to fluorescence, except under conditions of low temperatures (≤5000 K) and high-radiation fields. Furthermore, it is shown that recent diagnostics by Esteban et al. of the physical conditions of partially ionized zones in Orion using combinations of [N I] and [O I] are unreliable because the lines from these species are likely to come from different regions. The hypothesis about different conditions in [N I]- and [O I]/[Fe II]-emitting regions is supported by the lack of correlation between these lines in samples of planetary nebulae, supernova remnants, and Herbig-Haro objects.

Effects of photo-excitation and organic vapor treatment on vapor sorption behavior of sputtered fluoropolymer films

Investigation of sorption capacities for volatile organic compounds of radio-frequency-sputtered fluoropolymer films has revealed that they are affected by the applied voltage and by supplemental photo-excitation using ultraviolet light irradiation during sputtering. The photo-assisted effects of producing unsaturated carbons on the sputtered tetrafluoroethylene film are revealed by infrared, electron spin resonance, and X-ray photoelectron spectroscopies. Measurements of sorption capacities and computational chemical calculation for six-membered cyclic hydrocarbons revealed that the π-character of the solute molecule is a decisive molecular descriptor for sorption behavior of the sputtered fluoropolymers. The π-character of the sputtered fluoropolymer films can be enhanced by a photo-assisted effect, which can also increase the affinities for π-characterized solutes by π– π interaction. This is further clarified by the fact that the photo-assisted fluoropolymer film has a larger sorption capacity for highly π-conjugated double aromatic ring systems (C 10H 8): naphthalene than for slightly π-conjugated, but polar, isomer: azulene. It was also found that saturated organic vapor ( p-xylene and 2-butanone) sorption can enhance the sorption capacities of the sputtered fluoropolymer by restructuring induced by solvation and radical reaction.

Detection of petroleum hydrocarbons at low ppb levels using quartz resonator sensors and instrumentation of a smart environmental monitoring system.

Petroleum hydrocarbon vapors at low ppb levels can be detected using a thickness shear mode resonator (TSMR) coated with a chemical-sensing overlayer, prepared by radiofrequency sputtering of porous sintered-polyethylene (PS-PE). The sensing capabilities of PS-PE sensors were profoundly affected by the sputtering methods; they were enhanced by the photo-excitation effect, and were reduced by carbonization and water treatment. The photo-assisted PS-PE sensor was extremely sensitive and could detect linear hydrocarbon (> C12) vapors below the ppb level. The time constant of the sorption curve, however, was large, indicating a slow sensing speed. Toward creating instrumentation for a smart environmental monitoring system, the TSMR sensors were arrayed on a circuit board equipped with a serial interface and signal processing chips of the oscillation drive and frequency counter. Co-sorption with water vapor at a relative humidity of about 10% has almost no effect on the sensing ability of PS-PE sensors for 1,2,4-trimethylbenzene. Conversely, it enhances the sensitivity of the TSMR sensor coated with a D-phenylalanine film. Upward shifts in the baseline are evident with elapsed time. However, a rigorous ten-cycle iteration test for 100 ppm toluene vapor demonstrated good reproducibility of the sensor's signals.