Wavelength Dependence Of Quantum Yield Research Articles

Determining wavelength-dependent quantum yields of photodegradation: importance of experimental setup and reference values for actinometers.

Accurate quantum yields are crucial for modeling photochemical reactions in natural and engineered treatment systems. Quantum yields are usually determined using a single representative light source such as xenon lamps to mimic sunlight or UVC light for water treatment. However, photodegradation modeling can be improved by understanding the wavelength dependence of quantum yields and the potential errors introduced by the experimental setup. In this study, we investigated the effects of experimental setup on measured quantum yields using four photoreactor systems and up to 11 different light sources. When using a calibrated spectroradiometer to measure incident irradiance on an open solution surface, apparent quantum yields were up to two times higher if light reflection and light screening were not accounted for in the experimental setup. When the experimental setup was optimized to allow for accurate irradiance measurements, quantum yields were reproducible across photoreactors. The optimized experimental setup was then used to determine quantum yields of uridine, atrazine, p-nitroanisole (PNA), sulfamethoxazole, and diclofenac across the UV spectrum. No significant wavelength dependence of quantum yields was observed for sulfamethoxazole and diclofenac, in contrast to wavelength-dependent quantum yields for uridine, atrazine, and PNA. These reference values can be used for determining wavelength-dependent quantum yields of other compounds of interest. Additionally, more accurate results can be obtained when using (1) an actinometer with similar light absorption and photoreactivity compared to that of the target chemical, (2) optically transparent actinometer solutions that can account for light reflection within reaction vessels, and (3) a quantum yield that corresponds to the spectrum of the selected light source.

Assessing the quantum yield spectrum of photochemically produced reactive intermediates from black carbon of various sources and properties

Black carbon (BC) is ubiquitous in sunlit waters and atomosphere. Recent studies revealed that under sunlight irradiation BC is photoactive on producing photochemically produced reactive intermediates (PPRIs), a group of key species in accelerating earth's surface biogeochemical processes and pollutant dynamics. Nevertheless, reported PPRIs productions from BC exhibit large inconsistency and the intrinsic capacities of BC in producing PPRIs remain poorly characterized. This work provided a wavelength-dependent quantum yields (QYs) assessment of four environmentally-relevant PPRIs (excited triplet state BC (3BC*), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (·OH)) from various BC. The QYs of all investigated PPRIs exhibit high dependence on incident light wavelength. For instance, the QYs of 1O2 dramatically decreased from 4.4% to 0.4% with light wavelength increasing from 375 to 490 nm and decreased to 0 above 490 nm. Suprisingly, PPRIs QYs only varied by 2.0–2.5-fold among BC prepared from different biomasses (i.e., pine needle, shell, straw, and wood), while the pyrolysis temperature and size of BC demonstrate higher impacts on the PPRIs QYs by up to 30.3- and 7.1-fold variations, respectively. Analyses on the physicochemical properties of BC demonstrate that QYs of 3BC* and 1O2 were linked to the optical properties of BC, while the QYs of H2O2 and ·OH were determined by multiple factors including the surface redox characteristics. Further, PPRIs productions from BC follow similar paths and efficiencies compared to those from natural organic matter. The revealed QYs of BC-derived PPRIs establish a key basis for evaluating PPRIs-mediated element cycles and pollutant transformation in natural waters, which are becoming increasingly important in the context of higher BC input from more frequent wildfires and artificial sources.

Photochemistry of hexachloroosmate(IV) in ethanol.

The photochemistry of the OsIVCl62- complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump-probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d-d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(iv) to Os(iii), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(iii) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(iv) and Os(iii) intermediates determines the synthetic potential of OsIVCl62- photochemistry.

On the Theoretical Determination of Photolysis Properties for Atmospheric Volatile Organic Compounds.

Volatile organic compounds (VOCs) are ubiquitous atmospheric molecules that generate a complex network of chemical reactions in the troposphere, often triggered by the absorption of sunlight. Understanding the VOC composition of the atmosphere relies on our ability to characterize all of their possible reaction pathways. When considering reactions of (transient) VOCs with sunlight, the availability of photolysis rate constants, utilized in general atmospheric models, is often out of experimental reach due to the unstable nature of these molecules. Here, we show how recent advances in computational photochemistry allow us to calculate in silico the different ingredients of a photolysis rate constant, namely, the photoabsorption cross-section and wavelength-dependent quantum yields. The rich photochemistry of tert-butyl hydroperoxide, for which experimental data are available, is employed to test our protocol and highlight the strengths and weaknesses of different levels of electronic structure and nonadiabatic molecular dynamics to study the photochemistry of (transient) VOCs.

Probing the role of nickel dopant in aqueous colloidal ZnS nanocrystals for efficient solar-driven CO2 reduction

Photocatalytic CO2 reduction reaction (CO2RR) on a heterogeneous catalyst offers the possibility for CO2 abatement and emerges as a promising avenue for renewable carbonaceous fuels production at ambient temperature and pressure using solar light as the sole energy input. Here, we report a newly aqueous colloidal comprised of monodispersed Ni-doped ZnS (ZnS:Ni) nanocrystals as excellent visible-light-responsive photocatalysts for CO2RR into formate. The wavelength-dependent quantum yield shows a significant contribution of Ni doping for visible light activity. A high selectivity (>95%) of HCOOH production and remarkable quantum efficiency of 59.1% at 340 nm and 5.6% at 420 nm are obtained over ZnS:Ni (0.1%) colloidal nanocrystals modified by Cd2+. The proper balance between sulfur vacancies and extended visible light absorption of the constructed colloidal ZnS:Ni nanocrystals contributes to the prominent performance for CO2RR. However, excessive doping of Ni does not guarantee an increase of photocatalytic CO2RR due to a diminish of sulfur vacancies. The regulation of sulfur vacancies by Ni doping and their interplay on photocatalytic CO2RR activity are presented and discussed. This work provides an in-depth insight of the role of dopant on vacancy modulation in photocatalyst beyond light absorption and a guidance for design of the potential photocatalyst for CO2RR.

Carbon dots prepared from citric acid and urea as fluorescent probes for hypochlorite and peroxynitrite

Carbon dots (CDs) were prepared from citric acid and urea, and their fluorescence was found to be quenched by hypochlorite and peroxynitrite. Microwave based synthesis gives CDs with excitation/emission wavelength-dependent quantum yields (8 % at 400/520 nm; 10 % at 360/451 nm; 12 % at 350/420 nm). Quenching of fluorescence depends on pH values, and response is most selective and sensitive to hypochlorite at pH 4, and to peroxynitrite at pH 9. The lower detection limits are 0.5 and 1.5 μM, respectively. The method was successfully applied to quantify hypochlorite and peroxynitrite in standard solutions and in spiked dilute serum samples.

Leaky mode analysis of luminescent thin films: The case of ZnO on sapphire

Zinc oxide (ZnO) epitaxial thin films grown on c-sapphire substrates by pulsed laser deposition were investigated using angle and polarization-resolved photoluminescence spectroscopy. Side-emission spectra differed significantly from surface-emission spectra in exhibiting dominant, narrow, polarization-resolved peaks. These spectral features were attributed to leaky substrate modes in the layers. Observations were first verified using transmission calculations with non-adjustable parameters, which took into account the dispersion, the anisotropy of the ZnO refractive index, and the dependence on film thickness. Results were consistent with Fabry-Perot-like interference being the origin of the distinctive ZnO luminescence observed at grazing incidence angles. A second analysis, based on the source terms method, was used in order to retrieve the bulk emission properties, including the wavelength-dependent quantum yield and the emission anisotropy. While ZnO thin films were considered here, this analysis method can be extended to any luminescent thin film of similar geometry, demonstrating the potential of leaky mode analysis for probing passive and active material properties.

Dye-Sensitized Photocathodes: Efficient Light-Stimulated Hole Injection into p-GaP Under Depletion Conditions

The steady-state photoelectrochemical responses of p-GaP photoelectrodes immersed in aqueous electrolytes and sensitized separately by six triphenylmethane dyes (rose bengal, rhodamine B, crystal violet, ethyl violet, fast green fcf, and brilliant green) have been analyzed. Impedance measurements indicated that these p-GaP(100) photoelectrodes operated under depletion conditions with an electric field of ∼8.5 × 10(5) V cm(-1) at the p-GaP/solution interface. The set of collected wavelength-dependent quantum yield responses were consistent with sensitization occurring specifically from adsorbed triphenylmethane dyes. At high concentrations of dissolved dye, the measured steady-state photocurrent-potential responses collected at sub-bandgap wavelengths suggested unexpectedly high (>0.1) net internal quantum yields for sensitized hole injection. Separate measurements performed with rose bengal adsorbed on p-GaP surfaces pretreated with (NH(4))(2)S verified efficient sensitized hole injection. A modified version of wxAMPS, a finite-difference software package, was utilized to assess key operational features of the sensitized p-GaP photocathodes. The net analysis showed that the high internal quantum yield values inferred from the experimental data were most likely afforded by the internal electric field present within p-GaP, effectively sweeping injected holes away from the interface and minimizing their participation in deleterious pathways that could limit the net collection yield. These simulations defined effective threshold values for the charge carrier mobilities (≥10(-6) cm(2) V(-1) s(-1) and ≥10(-1) cm(2) V(-1) s(-1) at dopant densities of 10(18) and 10(13) cm(-3), respectively), hole injection rate constants (≥10(12) s(-1)), and surface trap densities (10(12) cm(-2)) needed to attain efficient hole collection with the quality of p-GaP materials used here. The cumulative experimental and modeling data thus provide insight on design strategies for assembling new types of dye-sensitized photocathodes that operate under depletion conditions.

Wavelength Dependence of Quantum Yield for CO2 Fixation and Photochemical Efficiencies of Photosystems I and II

Plants are continuously exposed to changes in the light spectrum, both short term and long term, due to changes in weather, the sun angle, and sun shade conditions. Of course, the absorption of incident light by a leaf is strongly wavelength dependent because different leaf pigments have different

The photorelease of nitrogen monoxide (NO) from pentacyanonitrosyl coordination compounds of group 8 metals

The photodetachment of NO from [M(II)(CN)5NO]2- with M = Fe, Ru, and Os, upon laser excitation at various wavelengths (355, 420, and 480 nm) was followed by various techniques. The three complexes showed a wavelength-dependent quantum yield of NO production Phi(NO), as measured with an NO-sensitive electrode, the highest values corresponding to the larger photon energies. For the same excitation wavelength the decrease of Phi(NO) at 20 degrees C in the order Fe > Ru >> Os, is explained by the increasing M-N bond strength and inertness of the heavier metals. Transient absorption data at 420 nm indicate the formation of the [M(III)(CN)5H2O]2- species in less than ca. 1 micros for M = Fe and Ru. The enthalpy content of [Fe(III)(CN)5H2O]2- with respect to the parent [Fe(II)(CN)5NO]2- state is (190 +/- 20) kJ mol(-1), as measured by laser-induced optoacoustic spectroscopy (LIOAS) upon excitation at 480 nm. The production of [Fe(III)(CN)5H2O]2- is concomitant with an expansion of (8 +/- 3) ml mol(-1) consistent with an expansion of the water bound through hydrogen bonds to the CN ligands plus the difference between NO release into the bulk and water entrance into the first coordination sphere. The activated process, as indicated by the relatively strong temperature dependence of the Phi(NO) values and by the temperature dependence of the appearance of the [Fe(III)(CN)5H2O]2- species, as determined by LIOAS, is attributed to NO detachment in less than ca. 100 ns from the isonitrosyl (ON) ligand (MS1 state).

Investigation of the damage as induced by 1.7MeV protons in an amorphous/crystalline silicon heterojunction solar cell

Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×1012 cm−2 protons. Good agreement has been obtained between measured and simulated data using DIFFIN,1 a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.

Light-induced oxidative changes in a model dairy spread. Wavelength dependence of quantum yields.

Light-induced formation of lipid peroxides in a water-in-oil emulsion based on purified rape-seed oil (80%) was found to increase with decreasing wavelength and to have the (apparent) quantum yields (1.1 +/- 0.1) x 10(-)(3) for 436 nm, (2.6 +/- 0.1) x 10(-)(3) for 405 nm, and (4.5 +/- 0.4) x 10(-)(3) for 366 nm irradiation, as determined after 12 h of exposure to monochromatic light of an approximate intensity of 10(18) quanta.min(-)(1).mL(-)(1) and related to total light absorption. Riboflavin (0.8 ppm) had no effect on lipid peroxidation, but photodegraded with a quantum yield ((1.5 +/- 0.3) x 10(-)(5) for 436 nm, (1.7 +/- 0.2) x 10(-)(5) for 405 nm and (1.39 +/- 0.09) x 10(-)(5) for 366 nm irradiation) independent of irradiation wavelength. beta-Carotene was only photodegraded to a minor extent, but protected riboflavin against photodegradation and the lipids against peroxidation for 436 and 405 nm irradiation (reduction in quantum yield three times for 4.5 ppm beta-carotene for lipid oxidation and more for riboflavin degradation), but not for 366 nm irradiation, where beta-carotene has an absorption minimum.

Mehrfachbindungen zwischen hauptgruppenelementen und Übergangsmetallen. 158. Photochemische und photophysikalische Eigenschaften des epoxidierungskatalysators CH 3ReO(O 2) 2·H 2O

The photolysis of CH 3ReO(O 2) 2·H 2O in methylene chloride yields, like the thermolysis, molecular oxygen in the triplet spin state. The quantum yield Q Ph of photolysis shows a remarkable dependence the wavelength, increasing from 0.12 at 365 nm to 1.0 at 248 nm. One single excited state is responsible for this behaviour. The wavelength-dependent quantum yield profile corresponds in a first approximation to the ratio between the LMCT-band and the total absorption spectrum was made on a mathematical basis using symmetrical Gauss curves. This is the first time that a fluorescence and phosphorescence emission of an alkyl transition-metal complex of d 0-configuration has been detected, thus allowing for the determination of both the S 1 and the T 1-energy levels. The quantum yield of the fluorescence ( Q F ) is below 10 −3; that of the phosphorescence is below 0.04.

Analysis of reaction kinetics by Fourier transform infrared spectrometry: Laser-induced photochemistry of 1,2-benzoquinones

Photokinetic studies of an intermolecular [4 + 2] photocycloaddition starting from 3,4,5,6-tetrachloro-1,2-benzoquinone ( o-chloranil) and of CO elimination from 3,5-di- tert-butyl-1,2-benzoquinone (DBQ) are reported. Both reactions were initiated by exciting the lowest energetic 1(π-π) * [and also the 1(n-π) * state in the case of DBQ] with argon ion laser emission lines. The kinetic analyses were mostly carried out with the use of time-dependent IR intensity alterations in the range of C=O stretching modes. The reactions have a non-uniform character which reveals the existence of intermediates. The intermediate of the dimerization could be established by the wavelength-dependent quantum yield related to the initial compound. The laser-induced reaction can be described by a two-photon process in which the first photon excites the starting compound while the second photon interacts with the intermediate. In the case of the CO elimination, the bis-ketene intermediate could be identified by recording the asymmetric stretching vibration at 2173 cm −1 in the vicinity of the vibration of free carbon monoxide at 2134 cm −1.

Unusual substituent effect by the α-acetyl group on the type II photoreaction of valerophenones: strong wavelength dependence of quantum yields

α-Acetyl-γ-methylvalerophenone ( 1a) and α-acetylvalerophenone ( 1b) show wavelength-dependent type II photoreactivity. Their quantum yields of disappearance are 0.01 at 313 nm and unity at 366 nm. The wavelength dependence was analysed on the basis of the kinetics by considering the internal filter effect caused by the enol form of both the product and the starting material. The enol form of the starting material was found to play a much more important role in the wavelength dependence than the endol form of the product. α-Acetylα-methylvalerophenone ( 1c), which has no enolizable hydrogen, also undergoes the type II reaction. However, the quantum yields for the disappearance of 1c are low at both 313 and 366 nm. This is probably due to the effective reverse hydrogen transfer to yield the ground state starting material from the 1,4-biradical intermediate or the effective recombination of radicals produced via rapid α cleavage.

The nature of the primary photochemical events in rhodopsin and isorhodopsin

The nature of the primary photochemical events in rhodopsin and isorhodopsin is studied by using low temperature actinometry, low temperature absorption spectroscopy, and intermediate neglect of differential overlap including partial single and double configuration interaction (INDO-PSDCI) molecular orbital theory. The principal goal is a better understanding of how the protein binding site influences the energetic, photochemical, and spectroscopic properties of the bound chromophore. Absolute quantum yields for the isorhodopsin (I) to bathorhodopsin (B) phototransformation are assigned at 77 K by using the rhodopsin (R) to bathorhodopsin phototransformation as an internal standard (phi R----B = 0.67). In contrast to rhodopsin photochemistry, isorhodopsin displays a wavelength dependent quantum yield for photochemical generation of bathorhodopsin at 77 K. Measurements at seven wavelengths yielded values ranging from a low of 0.089 +/- 0.021 at 565 nm to a high of 0.168 +/- 0.012 at 440 nm. An analysis of these data based on a variety of kinetic models suggests that the I----B phototransformation encounters a small activation barrier (approximately 0.2 kcal mol-1) associated with the 9-cis----9-trans excited-state torsional-potential surface. The 9-cis retinal chromophore in solution (EPA, 77 K) has the smallest oscillator strength relative to the other isomers: 1.17 (all-trans), 0.98 (9-cis), 1.04 (11-cis), and 1.06 (13-cis). The effect of conformation is quite different for the opsin-bound chromophores. The oscillator strength of the lambda max absorption band of I is observed to be anomalously large (1.11) relative to the lambda max absorption bands of R (0.98) and B (1.07). The wavelength-dependent photoisomerization quantum yields and the anomalous oscillator strength associated with isorhodopsin provide important information on the nature of the opsin binding site. Various models of the binding site were tested by using INDO-PSDCI molecular orbital theory to predict the oscillator strengths of R, B, and I and to calculate the barriers and energy storage associated with the photochemistry of R and I for each model. Our experimental and theoretical investigation leads to the following conclusions: (a) The counterion (abbreviated as CTN) is not intimately associated with the imine proton in R, B, or I. The counterion lies underneath the plane of the chromophore in R and I, and the primary chromophore-counterion electrostatic interactions involve C15-CTN and C13-CTN. These interactions are responsible for the anomalous oscillator strength of I relative to R and B. (b) The presence of a small activation barrier (~0.2 kcal mol-1) in the 9-cis - 9-trans excited-state surface is associated with the location of the counterion as well as the intrinsic photophysical properties of the 9-cis chromophore. The principal difference between the 1 1-cis -c 1 -transphoto reaction surface and the 9-cis - 9-trans photoreaction surface is the lack of effective electrostatic stabilization of distorted 9 = 10 conformations due to incomplete charge polarization. (c) Hydrogen bonding to the imine proton, ifpresent, does not involve the counterion. We conclude that water in the active site, or secondary interactions with the protein (not involving the CTN), are responsible. (d) All photochemical transformations involve one-bond photoisomerizations.This prediction is based on the observation of a very small excited state barrier for the I -- B photoreaction and a negative barrier for the R - B phototransformation, coupled with the theoretical prediction that all two-bond photoisomerizations have significant S, barriers while one-bond photoisomerizations have small to negative S, barriers.(e) Rhodopsin is energetically stabilized relative to isorhodopsin due to both electrostatic interactions and conformational distortion, both favoring stabilization of R. The INDO-PSDCI calculations suggest that rhodopsin chromophore-CTN electrostatic interactions provide an enhanced stabilization of -2 kcal mol-1 relative to I. Conformational distortion of the 9-cis chromophore-lysine system accounts for -3 kcal mol-1. (f) Energy storage in bathorhodopsin is-60% conformational distortion and 40% charge separation. Our model predicts that the majority of the chromophore protein conformational distortion energy involves interaction of the C,3(-CH3)=CI4--C,5=N-lysine moiety with nearby (unknown) protein residues. (g) Strong interactions between the counterion and the chromophore in R and I will generate weak, but potentially observable charge-transfer bands in the near infrared. The key predictions are the presence of an observable charge-transfer transition at 859 nm (1 1,640 cm- 1) in I and an analogous, but slightly weaker band at 897 nm (11,150 cm-1) in R. Both transitions involve the transfer of an electron from the counterion into low-lying l theta* molecular orbitals.

The Dependence of Quantum Yield on Wavelength and Growth Irradiance

The wavelength dependence of photosynthetic quantum yield was measured in a leaf disc oxygen electrode using narrow-band interference filters. Photorespiration was suppressed by measuring oxygen evolution in air containing ~ 1% CO2. Rates of oxygen evolution were determined as a function of absorbed irradiances between 0 and 100 �mol quanta m-2 s-1 and the slope was taken as the quantum yield. The wavelength-dependence previously observed in many species was confirmed for pea and spinach leaves. The maximum quantum yields obtained here and by others are close to 0.111 mol O2 mol-1 quanta for red light. Given the wavelength dependence of quantum yield and the spectral distribution of light, the quantum yields for various white lights were calculated to be around 85% of the red maximum. The quantum yields in white light were the same for plants grown at different irradiances. It is argued that the wavelength dependence of quantum yield reflects the distribution of quanta between the two photosystems with the quantum yield dropping at wavelengths strongly absorbed by chlorophyll b as this is mainly associated with photosystem II.

Comparison of the action of some merocyanine dyes as solar photovoltaic elements and as photographic sensitizers

Experimental photovoltaic (PV) cells have been constructed from a combination of semitransparent aluminum (alumina surface), polycrystalline organic dye, and silver backing electrodes. Working with merocyanine dyes familiar in photographic sensitization, these cells have given open-circuit voltages of 1.1 V and short-circuit current quantum efficiencies of 15 to 30%. The fill factors, which govern efficiency under load, are on the order of 0.25, and the operating lifetimes are not yet adequate. Individual dyes are compared as photovoltaic sensitizers and as silver bromide photoconductivity sensitizers. The electronic thresholds for Al absolute value Al/sub 2/O/sub 3/ dye and AgBr dye appear to be approximately equal. Since the dye packing structures are different (being three-dimensional crystals in the PV cells and close-packed monolayers on the AgBr surface), the absorption spectra differ grossly in form. The patterns of wavelength-dependent quantum yields also differ in the two applications. Field-dependent ionization sets an important limitation on many of the dyes in PV cells, as does the familiar drop of AgBr sensitization as monolayer coverage is exceeded. Ultimately one hopes for a synthesis of understanding of these two limitations, and even for a common solution. 8 figures, 1 table.

Photochemistry of substituted cycloheptatrienes XIII: The dependence of quantum yields for sigmatropic hydrogen shifts on the excitation energy

The partial quantum yields in a substituted cycloheptatriene reversible system were measured to determine the dependence on the excitation energy. The quantum yields decrease significantly with increasing excitation wavelength. This is in accordance with the prediction from orbital symmetry considerations for a sigmatropic hydrogen shift. The chemical deactivation from higher excited states is able to compete with internal conversion processes. Furthermore the wavelength dependence of the partial quantum yields is interpreted with two different pericyclic minima in the excited state. The participation of conformers as a possible origin for the wavelength dependence of quantum yields is excluded.

Photochemistry of substituted cycloheptatrienes XIII: The dependence of quantum yields for sigmatropic hydrogen shifts on the excitation energy

The partial quantum yields in a substituted cycloheptatriene reversible system were measured to determine the dependence on the excitation energy. The quantum yields decrease significantly with increasing excitation wavelength. This is in accordance with the prediction from orbital symmetry considerations for a sigmatropic hydrogen shift. The chemical deactivation from higher excited states is able to compete with internal conversion processes. Furthermore the wavelength dependence of the partial quantum yields is interpreted with two different pericyclic minima in the excited state. The participation of conformers as a possible origin for the wavelength dependence of quantum yields is excluded.