Absorption Of Light Quanta Research Articles

Surface Properties of Precipitated Aerosol Microparticles of Indium(III) Oxide under Conditions of Ambient Air

Adsorption and photosorption properties of precipitated indium(III) oxide aerosol particles are studied under conditions close to those of ambient air. The composition of the adsorbed surface layer of precipitated aerosol particles is analyzed. The quantum yields and their spectral dependences of oxygen photosorption and carbon monoxide photocatalytic oxidation are determined. Photocatalytic activity in the reaction of carbon monoxide oxidation is detected during the absorption of light quanta from the region of indirect electron transitions (quantum energy of <2.9 eV).

Effect of Indirect Electronic Transitions in Indium Oxide on Photoadsorption of Oxygen and Photocatalytic Oxidation of Carbon Monoxide

The photocatalytic activity of indium(III) oxide in the oxidation of carbon monoxide upon absorption of light quanta from the region of indirect electronic transitions (quantum energies <2.9 eV) has been found. The quantum yields and spectral dependences of the quantum yields of photocatalytic oxidation of carbon monoxide and photoadsorption of oxygen have been determined. The adsorption, photoadsorption, and photocatalytic properties of indium oxide microparticles have been studied.

Synthesis, photophysical properties, and photochemical activity of the water-soluble dyad based on fullerene С60 and chlorin e6 derivatives

Chlorin e6 derivative and water-soluble dyad resulting from covalent bonding of polyanionic fullerene С60 derivative to chlorin e6 derivative were synthesized and studied for spectral properties and photochemical activity. A considerable change in the absorption spectra and pronounced fluorescence quenching for the chlorin moiety included in the dyad were identified. The singlet excited state of chlorin is quenched via electron transfer from the excited chlorin to the fullerene core. A comparison of the photochemical activities of the test compounds in aqueous solutions showed a tenfold increase in the photochemical activity of the chlorin–fullerene dyad compared with free chlorin per absorbed light quantum.

Synthesis of 5-Acyl-2-Amino-3-Cyanothiophenes: Chemistry and Fluorescent Properties.

Independent of the substrate structure and reaction conditions, 3-amino-2-cyanothioacrylamides, which contain two active electrophilic centers, were shown to interact with various active halo methylene compounds under mild conditions to afford 5-acyl-2-amino-3-cyanothiophenes as the only products. A series of new polyfunctional thiophene derivatives with a rare combination of functionalities were synthesized, and their photophysical properties were experimentally and computationally investigated. The calculated electronic characteristics of the ground and excited states were compared to the experimental results, which provided a good understanding of the relationship between the optoelectronic properties and the molecular structures. After absorption of light quanta, the systems populate an intramolecular charge-transfer (ICT) Franck-Condon state, and emission occurs from a twisted ICT minimum.

Particular features of photochemical transformations of molecules initiated by continuous external impact

Photochemical transformations induced by long-term continuous exposure to light have been studied. Using the developed theory and methods for calculating photochemical processes, computer experiments have been performed and their results have been analyzed for a large number of model systems and particular reactions. The excitation process has been considered in an explicit form, and the population of energy levels in excited states by absorption of light quanta has been taken into account. It has been shown that continuous optical excitation qualitatively changes the situation in comparison with pulsed excitation: the quantum yield is not zero even at very low quantum beat frequencies. For fast reactions, the kinetics of the process is exponential; in the case of slow reactions, concentration oscillations (on the order of 25% or more) arise, which are clearly manifested in the stationary state of the molecular system. The choice of the optimal photoirradiation time is an important factor in experimental design aimed to obtain a desired amount of the product. Molecular modeling makes it possible to a priori evaluate this quantity.

Anti-Fowler Temperature Regime in Photoemission from &amp;lt;i&amp;gt;n&amp;lt;/i&amp;gt;-Type Semiconductors with Surface Accumulation Layer

According to the Fowler theory and numerous experiments the quantum efficiency for photoemission from conductors increases with temperature. Here we show that an opposite temperature dependence is also possible, when the photoemission is from quasi-metallic surface accumulation layers of n-type semiconductors. This is due to the temperature dependence of the Fermi level energy in semiconductors. The Fermi level energy increases with decreasing temperature; this leads to a decrease of the semiconductor work function and consequently an increase of the quantum efficiency photoemission at constant value of absorbed light quanta of energy. We have calculated this effect for electron accumulation layer in n-GaN, induced by adsorption of positively charged cesium or barium ions. It is found that at low temperatures near liquid nitrogen, the quantum efficiency for photoemission increases to near 55%, which is comparable to the largest values, reported for any known photo-ca-thodes. This phenomenon may prove useful for efficient photo-cathodes operating at low temperatures.

A Key Role of Xanthophylls That Are Not Embedded in Proteins in Regulation of the Photosynthetic Antenna Function in Plants, Revealed by Monomolecular Layer Studies.

The main physiological function of LHCII (light-harvesting pigment-protein complex of photosystem II), the largest photosynthetic antenna complex of plants, is absorption of light quanta and transfer of excitation energy toward the reaction centers, to drive photosynthesis. However, under strong illumination, the photosynthetic apparatus faces the danger of photodegradation and therefore excitations in LHCII have to be down-regulated, e.g., via thermal energy dissipation. One of the elements of the regulatory system, operating in the photosynthetic apparatus under light stress conditions, is a conversion of violaxanthin, the xanthophyll present under low light, to zeaxanthin, accumulated under strong light. In the present study, an effect of violaxanthin and zeaxanthin on the molecular organization and the photophysical properties of LHCII was studied in a monomolecular layer system with application of molecular imaging (atomic force microscopy, fluorescence lifetime imaging microscopy) and spectroscopy (UV-Vis absorption, FTIR, fluorescence spectroscopy) techniques. The results of the experiments show that violaxanthin promotes the formation of supramolecular LHCII structures preventing dissipative excitation quenching while zeaxanthin is involved in the formation of excitonic energy states able to quench chlorophyll excitations in both the higher (B states) and lower (Q states) energy levels. The results point to a strategic role of xanthophylls that are not embedded in a protein environment, in regulation of the photosynthetic light harvesting activity in plants.

Coherent processes in formation of primary products of rhodopsin photolysis

194 The absorption of the light quantum by rhodopsin (R 498 ), a visual pigment, results in the cis → trans isomerization of its chromophore, 11cis -retinal, over approximately 200 fs [1, 2] with a quantum yield of 0.65 [3]. The physical meaning of this fast and efficient photoreaction is in the maximal utilization of the absorbed light quantum energy for isomerization of 11cis -retinal rather than its dissipation into heat or loss as fluorescence. The isomerization of 11cis -retinal leads to conformational rearrangement in the protein moiety of rhodopsin molecule; as a result, rhodopsin as a G-protein binding receptor acquires the ability to interact with G-protein (transducin), thereby initiating phototransduction in the photoreceptor cell. The dynamics of the 11cis -retinal photoisomerization in rhodopsin has been studied by laser kinetic spectroscopy with a femtosecond resolution [1, 2, 4–6]. It has been demonstrated that the first reaction product, photorhodopsin (photo 570 ), is formed over 200 fs to transform into the next product, bathorhodopsin (batho 535 ), in 2–3 ps [1, 4, 5]. The oscillations of the absorption signals of photo 570 , batho 535 , and initial state R 498 , observed during several picoseconds after excitation, have been also discovered. A high rate and quantum yield of 11cis -retinal photoisomerization in rhodopsin as well as the oscillations of absorption signals of the reaction products suggested that this is a coherent reaction. This means that the photoreaction proceeds via passing of the system through nonstationary oscillation states. In terms of the model of two states [1, 7], when two potential energy surfaces (PESs) are involved in photochemical reaction of rhodopsin (Fig. 1), the corresponding processes can be represented as follows. The action of a femtosecond excitation pulse results in a coherent population of certain oscillatory states in the electron-excited molecule, the set of which is named a coherent oscillatory wave packet (hereinafter, wave packet). The coherent photoreaction can be represented as the movement of wave packet over PES S 1 of the excited state along the reaction coordinate. The reaction products are formed Coherent Processes in Formation of Primary Products of Rhodopsin Photolysis

The Effect of High Temperatures on the Photosynthetic Activity of Intact Barley Leaves at Low and High Irradiance

Light curves of CO2 fixation by barley seedling leaves preliminarily heated at 30–43°C for 5 min were measured. The slope of the linear part of the light curve decreased after leaf heating at temperatures above 35°C; whereas, at a high light level, the photosynthesis rate decreased only at temperatures of 40°C and higher. The linear relationships between the photosynthetic CO2-fixation rate and a photon flux density up to 1400 μmol/(m2 s) were found in leaves preheated at 42°C; this indicates the strong nonphotochemical dissipation of absorbed light quanta. The lowering of the oxygen concentration from 21 to 1% led to a CO2 fixation maximum quantum yield and a photosynthesis-rate increase at the highest light intensity in leaves preheated at temperatures above 40°C as compared to the control leaves. Nevertheless, the linear relationship between the photosynthetic CO2 fixation and the light intensity was found in leaves heated at 42°C at O2 concentrations of both 21 and 1%. The latter fact suggests that the proton gradient of the thylakoid membrane, which causes an increase in the nonphotochemical dissipation of the quanta absorbed, could also be formed due to the cyclic electron transport over photosystem I.

Bio-optical modelling of oxygen evolution using in vivo fluorescence: Comparison of measured and calculated photosynthesis/irradiance (P-I) curves in four representative phytoplankton species

Summary The measurement of variable fluorescence yields relative electron transport rates which can not directly be converted into absolute values of photosynthetic rates. We have developed a measuring device which provides identical optical geometry for simultaneous fluorescence and oxygen measurements in single cell or chloroplast suspensions and also allows the determination of the absorbed photosynthetic radiation (Q phar ). In this set-up we have simultaneously measured photosynthesis-irradiance curves (P-I curves) of algal cell suspensions with both PAM fluorometry and a Clark-type electrode. From the electron flow through photosystem II per absorbed light quanta (Q phar ) we have modeled oxygen based P-I curves. In order to evaluate the validity of the model we have compared modeled with directly measured oxygen based P-I curves. This comparison was extended to four different algal taxa which exhibit significantly different absorption spectra leading to changes in Q phar . The results show that in the initial slope of the P-I curves (α), calculated and measured oxygen production is quite similar on an absolute scale, whereas at the transition part (l k = P max /α) and at maximal photosynthetic rates (P max ) both methods can deviate significantly from each other depending on the species used. Therefore, the conversion of fluorescence based electron transfer rates through photosystem II into oxygen production is a reliable procedure for light intensities below the l k . However, at light intensities close to light saturation, physiological regulation processes can cause problems in the interconversion of fluorescence- and oxygen-based photosynthetic rates.

Possible Impact of Heterogeneous Photocatalysis on the Global Chemistry of the Earth's Atmosphere

Abstract Photochemistry is recognized to be important for various physicochemical processes in the atmosphere, such as formation of the ozone layer and smogs, degradation of waste substances, etc. [1]. However, up to the present the emphasis in atmospheric photochemistry has been mainly on the study of photochemical reactions that occur with molecules directly excited by absorption of light quanta. However, the major components and impurities of the earth's atmosphere (such as nitrogen, oxygen, water, carbon dioxide, methane, methane halides, etc.) are totally transparent to most solar radiation. Electronically excited states of these molecules are formed only upon absorption of vacuum ultraviolet light quanta with energy hv ≥ 5 eV (i.e., with wavelength λ ≤ 200 nm). Only a small portion of the energy of solar light is found in this spectral region. In other words, most of the energy of the solar flux cannot participate in such direct photochemical reactions.

Effect of the Long-Term Elevation of CO2 Concentration in the Field on the Quantum Yield of Photosynthesis of the C3 Sedge, Scirpus olneyi

CO(2) concentration was elevated throughout 3 years around stands of the C(3) sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO(2) atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO(2) uptake (ø(abs) and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO(2) concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. An Ulbricht sphere cuvette incorporated into an open gas exchange system was used to determine ø(abs) and a portable chlorophyll fluorimeter was used to estimate the photochemical efficiency of photosystem II. When measured in an atmosphere with 10 millimoles per mole O(2) to suppress photorespiration, shoots showed a ø(abs) of 0.093 +/- 0.003, with no statistically significant difference between shoots grown in elevated or control CO(2) concentrations. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO(2) atmosphere. Shoots grown and measured in 680 micromoles per mole of CO(2) in air showed a ø(abs) of 0.078 +/- 0.004 compared with 0.065 +/- 0.003 for leaves grown and measured in 351 micromoles per mole CO(2) in air; a highly significant increase. In accordance with the change in ø(abs), the light compensation point of photosynthesis decreased from 51 +/- 3 to 31 +/- 3 micro-moles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO(2) in air, respectively. The results suggest that even after 3 years of growth in elevated CO(2), there is no evidence of acclimation in capacity for photosynthesis under light-limited conditions which would counteract the stimulation of photosynthetic CO(2) uptake otherwise expected through decreased photorespiration.

Light activation of the sodium pump in blowfly photoreceptors

1. The oxygen consumption of the compound eye in the blowfly was determined during light exposure and in darkness by a manometric measuring technique. Within the first 10 s of exposure to bright white light the oxygen uptake increased up to 20 × the resting value in darkness, which is 2.4 to 3×10−5ml oxygen x min−1 × eye−1. 2. The time course of oxygen consumption during 3-min light stimulation proceeded in two phases: an initial dynamic phase, followed by a decline to a maintained phase of almost constant amplitude. The time course of the dynamic phase varied with the intensity and wavelength of the stimulating light. In contrast, within the intensity range used, oxygen consumption during the maintained phase was about double the resting value in darkness, irrespective of intensity and wavelength of the light stimulus. The time course of the decline in oxygen consumption during light exposure paralleled the electrophysiologically recorded time course of decrease in amplitude of the photoreceptor response. 3. Light-induced oxygen consumption decreased continuously after replacement of the extracellular fluid in the eye by solutions of increasing potassium concentration. Oxygen uptake in darkness was influenced less. It is concluded that the additional oxygen consumption during light exposure is mainly caused by activation of the sodium-potassium pump. 4. Replacing extracellular sodium by choline did not markedly reduce the oxygen uptake. This result indicates that, after perfusion of the eye, sodium is pumped out of the cell, causing a sodium gradient to be re-established. Alternatively, the ion pump can accept choline and thus is not highly specific for sodium. 5. Complete exchange of extracellular sodium by calcium or magnesium 150 mmol/l caused only a small reduction in light-induced oxygen consumption. This observation can be explained by an ion exchange coupled to the ion transport mechanism. The intracellular store of sodium after the replacement is sufficient to form a small sodium gradient, which drives a sodium-calcium exchanger. Sodium itself is actively pumped out. This coupled transport mechanism seems to be sensitive to the extracellular anion composition, since chloride markedly reduced the transport rate. 6. The oxygen consumption above steady state, following excitation by blue light, was about six times higher in rhodopsin-rich eyes than in rhodopsin-poor eyes. Oxygen uptake following light exposure thus depends on the absolute rhodopsin content in the rhabdomeres. Sequential stimulations by blue-red or red-blue light showed that the higher oxygen uptake following light exposure in rhodopsin-rich eyes is caused by the excess in excited metarhodopsin molecules, which is strongly correlated with the PDA signal. 7. ATP consumption per absorbed light quantum decreases supralinearly with increasing stimulus intensity. Light intensities eliciting photoreceptor responses of a maximal amplitude of 0.64 induce an initial ATP consumption of about 54000 molecules per absorbed quantum. Exposure to bright white light, hitting all rhodopsin molecules many times within 10 s, initially induces a consumption of only 90 ATP molecules per quantum. After 3-min exposure, consumption is reduced to only 7 ATP molecules per quantum. This dramatic reduction in ATP consumption per absorbed quantum occurs within the upper range of physiological light intensities. The results demonstrate that, at low light intensities, most of the energy is used for pump activity. At high physiological intensities, the fraction of energy consumption used for biochemical amplification and adaptation processes becomes more important.

Efficiency of sensitization of titanium dioxide by adsorbed acridine yellow in photocatalytic hydrogen evolution

Quantum efficiency of the photocatalytic hydrogen evolution upon illumination in the self-and dyesensitized absorption bands of suspended titanium dioxide has been determined. The photocatalytic activity observed upon illumination in the sensitization band is shown to be due to absorption of light quanta by adsorbed molecules of the dye.

Excitation relaxation processes in amorphous and vitreous materials

In media with spatial dispersion of properties the energy of an absorbed light quantum is shown to localize on small (〈v〉 = 10 −22 + 10 −21 cm −3 ) space scales. Energy transfer from electron subsystem to ion lattice is performed in two steps: on the first step high frequency local phonons are generated in a microregion, limited by the structural correlation radius, while on the second step they are transmitted to long wave propagating oscillations. Basing on this idea some physical effects in glasses (photodarkening, photoinduced anisotropy, photoluminescence) are considered.

Nonequilibrium phonons and photostructure transformations in chalcogenide glasses

The characteristic properties of the energy transfer from an absorbed light quantum to a lattice in amorphous solids are considered. These properties are connected with the presence of medium-range order and non-thermal vibration spectrum of phonons arising under nonradiative photoelectron recombination. It is shown that effective microregion temperature can be much higher than that obtained on a basis of elementary evaluation with a heat capacity C = 3 NK.

Orientational optical nonlinearity of liquid crystals

The orientational optical nonlinearity of the mesophases of liquid crystals (LCs) is attracting ever more attention, owing to the high values of the constants characterizing it, owing to the many interesting specific features that LCs introduce even into traditional nonlinear optical effects, and owing to the potentialities of studying the physical properties of LCs. This article presents the state of the theory and experiment of orientational interaction of light waves with LCs. Especial attention is paid to the physical mechanisms of interaction?the light-induced Freedericksz effect, recording of director gratings, and the action of surface light waves on the orientation of the director. Varied manifestations of the effects of orientational self-action and interaction of light waves are discussed: self-focusing of light, nonlinear optical activity, mutual focusing, stimulated light scattering, and wave-front conjugation. Optical nonlinearities specific for the mesophase of a liquid crystal and associated with absorption of light quanta are also discussed. They include photoconformational nonlinearity, thermal-orientational nonlinearity, and liquid-crystal light valves. Especial attention is paid to the methodological problem of deriving the Euler?Lagrange?Rayleigh variational equations and to the correct choice of the free energy for the system LC + electromagnetic field. Section 8 traces the history of the studies of orientational optical nonlinearity of LCs and reviews the studies whose results are not directly reflected in the main text, and also reviews the studies on practical applications of light-induced orientational effects.

Photon-like signals following weak rhodopsin bleaches.

The rod visual system in its dark-adapted state behaves as a near-ideal light detector. Psychophysical studies on the reliability of light detection in man, analysis of the dark noise in rod bipolar cells and observation of photon-like events in rods in the dark suggest that the visual pigment, rhodopsin, is very stable against spontaneous isomerization. When a light which bleaches a small fraction of the rhodopsin is extinguished, the visual threshold may be increased by several orders of magnitude. The eye is then 'light-adapted' and the process (or processes) by which the sensitivity returns constitutes dark adaptation. We report here that bleaching of a small fraction of the rhodopsin produces a prolonged increase in the noise observed in the dark in rod bipolar cells of the dogfish retina. The associated noise events are similar to those produced by the absorption of light quanta and presumably have their origin in the rods which transmit their signals to the bipolar cells. This increased noise after bleaching would decrease the reliability of detection of light quanta and contribute to the elevation of visual threshold.

Picosecond fluorometry in primary events of photosynthesis

Many laboratories in different countries are involved in the study of the mechanism of conversion of light energy into chemical energy, namely photosynthesis. As is evident from the literature, the initial phases of photosynthesis, which determine the character of this process, proceed at time intervals of 10(-8) and 10(-13) s. They are associated with absorption of light quanta and energy transfer from the molecules of light-harvesting antenna (LHA) chlorophyll and accesory pigments to the reaction centers (RC), where the key reaction of photosynthesis occurs: photo-induced charge separation. Evidently it is of importance to study experimentally the process that occurs within the 10(-8) -10(-13)s time domain.

Optical nuclear polarization of protons in fluorene-d 8,h 2 single crystals

The optical nuclear polarization has been studied in single crystals of octadeutero-9,9′-proto-fluorene (fluorene-d 8,h 2). In this case, only two protons with spin I = 1 2 per molecule interact with the excited triplet states of spin S = 1. From the proton NMR data it is concluded that the crystalline proton positions agree with those predicted for the known crystal structure of fluorene-h 10. Proton ONP was observed at room temperature as a function of the magnetic field H p as well as of its orientation in all crystalline planes. The maximum ONP is p L ≈ 10 −3, corresponding to a spin temperature of 8 mK in the optimal polarization field H p = 80 G. The quantum efficiency, i.e., the number of polarized proton spins per absorbed light quantum, is about 10 −2. The basic experimental results can be understood with a general ONP mechanism which is reviewed briefly. Some striking properties of the field and orientation dependence are tentatively related to particular cross-relaxation processes, which occur if the transition frequencies between different pairs of the electron and nuclear spin sublevels are so close that they overlap within their respective linewidth.