Role of the Medium in Radiationless Conversion of the Energy of the Excited S 1 State in Equilibrium Systems with Intramolecular Charge Transfer

Using the Onsager model, the effect of molecular orientation interactions in solutions on the rate constant of radiationless conversion of the energy of the excited S ₁ state has been calculated for equilibrium systems with intramolecular charge transfer. The rate constant of the radiationless conversion of energy S ₁ — X _n is represented as a function of the polarity of the medium f() = ( – 1)/(2 + 1), where is the permittivity of a solvent. The results obtained are compared with the Marcus theory and are also used to explain the well-known literature data on the influence of a solvent on the lifetime of the S ₁ state of some carotenoids.     

Anomalous isotopic effect in multibubble sonoluminescence of aqueous solutions of terbium chloride

An anomalously low isotope effect has been discovered in 20-kHz sonoluminescence of terbium chloride solutions in H₂O-D₂O mixtures. The intensity of luminescence in the characteristic 488-and 545-nm lines of the Tb³⁺ ion, which are observed against the solvent continuum (230–700 nm), increases with the content of D₂O to a maximum value of 4.0 ± 0.4, whereas the isotope effect in photoluminescence of the same solutions reaches 10 ± 1.0. The result is explained using the model of nonexponential decay of sonoluminescence of Tb³⁺ ions. These ions, which are formed in an excited state in cavitation bubbles, first undergo radiative and radiationless deactivation in the gas phase. However, some excited ions enter the solution bulk, because the excitation lifetime is longer than the average bubble lifetime. At the first stage, the isotope effect is small, because the density of the gas phase is low and quenching by solvent molecules is weak. At the second stage, the isotope effect coincides with the effect in usual photoluminescence in the solution. The total decrease in the effect in sonoluminescence depends on the weight of the “gas” stage of deactivation of excited Tb³⁺ ions.     

Biphotonic photochemistry of benzophenones in dimethylsulphoxide: a flash photolysis EPR study

Benzophenone and its derivatives in dimethylsulphoxide (DMSO) exhibit biphotonic photochemistry under 355 nm laser photolysis. Flash photolysis electron paramagnetic resonance experiments demonstrate that a single laser pulse is capable of producing and exciting benzophenone triplets, which can sensitize dimethylsulphoxide and subsequently lead to photodecomposition. In decafluorobenzophenone, electron transfer is the dominant process of the highly excited triplet state. Despite the rapid radiationless decay of 2-hydroxybenzophenone (2OHbenzophenone) in non-polar solvents, radical signals are observed from the photoexcitation of 2OHbenzophenone in DMSO. This is attributed to the sufficiently rapid excitation of the triplet state, which competes with the radiationless decay process, aided by the unique solvent properties of DMSO. It is concluded, in contrast to literature data, that the excited triplet state of DMSO is reactive, and can produce methyl radicals that show triplet mechanism polarization via the biphotonic photoexcitation of benzophenone.     

When can a classical electron accelerate without radiating?

A classical point electron radiates when it accelerates. However, there are classical electron models with extended charge distributions which can accelerate and/or deform without radiating. Can a model be contrived that will undergo radiationless motion while accelerating (on the average) over a distance large compared to its size? The answer is no: we prove that the “center” of the electron is always closer than the electron “diameter” to a fictitious point undergoing constant-velocity motion, if the electron's motion is radiationless.     

SBN晶体中Eu~(3+)激发态~5D_0的无辐射跃迁

通过测量~5D_0→~7F_2发射带的荧光寿命与晶体温度的依赖关系,研究了Eu~(3+)激发态~5D_0的无辐射跃迁.结果表明,~5D_0→~7F_2带的温度淬灭效应主要是由于电荷转移态的最低能量太低,~5D_0态易被热激活至电荷转移,然后通过电荷转移无辐射弛豫至~7F_2态.     

Bombardment-induced photon emission from GaAs as a function of target temperature

The results of experiments on bombardment-induced photon emission from a GaAs single crystal as a function of target temperature are described. The target temperature was varied in the range 200–500°C. The line emission from excited Ga atoms sputtered by argon ions of 8 keV energy was measured. The variation of the sputtering yield as a function of the target temperature does not affect the intensity of photon emission. One can assume that this effect indicates a two-particle mechanism of excitation that occurs at primary steps of the collision cascade inside the solid. The annealing of the surface layer as a factor that influences the sputtering process is also considered. The evolution of the simple cascade model and radiationless relaxation are proposed to explain the experimental data.     

Radiative and nonradiative transitions in atomic systems

In the nonrelativistic limit, radiationless (Auger) transitions are mediated by the Coulomb interaction between two charged particles. It is shown that the standard relativistic generalization or the nonrelativistic Auger interaction yields matrix elements which include both radiationless and radiative transitions. The separation of terms responsible for radiationless transitios is presented.     

Solvent and temperature dependence of the phosphorescence decay time of hexacyanochromate (III)

The temperature dependence of the ²E_g - ⁴A_2g phosphorescence of tetrabutylammonium hexacyanochromate (III) dissolved in various solvents has been measured within the liquid range of these solvents. In this limited temperature range, the reciprocal decay times obey Arrhenius relationship. From the absence of a solvent isotope effect, in ethanol and methanol and their monodeuterated analogues it is concluded that hydrogen bonds between the solvent and the complex ion do not provide promoting or accepting modes for the radiationless deactivation. The room temperature decay times have been found to correlate with the solvatochromic shift of the ⁴A_2g ? ⁴T_2g absorption bands and with Dimroth's E_T parameter. According to the selection rules for non-radiative decay a combination of CN stretching with either a Cr-C-N bending or a Cr-C stretching vibration is postulated as the promoting mode.     

Theory of Direct Photoisomerization of Stilbene

A microscopic model for describing the direct trans-cis photoisomerization of stilbene is developed and an interpretation of the experimental results found from time-resolved fluorescence and absorption spectroscopy of stilbene in solution is given. According to this model the photoisomerization of trans-stilbene is proposed to involve the radiationless transition from the trans singlet state S₁(trans), prepared by direct optical excitation, into the perp singlet state S₁ (perp) and the radiationless decay of this state into the quasi-isoenergetic trans triplet state T₄ (trans).     

Neutralization and excitation in low-energy ion-surface collisions

A model is presented which describes neutralization and excitation occurring in low-energy ion-surface collisions. Important processes that determine experimentally measurable quantities such as the charged fraction ?⁺ and photon yield γ are assumed to be Auger neutralization, electronic excitation and radiationless deexcitation. The model yields ?⁺ and γ in good agreement with experiment data, although log ?⁺ and log γ are found to exhibit nonlinear dependences on the inverse velocity of the incident ion beam.     

Effect of temperature on the fluorescence emission of 2H-chromen-2-one derivative in non-polar and polar solvents

The effect of temperature (293-333 K) on the fluorescence emission of 4-(5-methyl-3-phenyl-benzofuran-2-yl)-6-chloro-2H-chromen-2-one (MPBClC) is recorded in non-polar (1,4-dioxane, and toluene), and polar (butanol and DMSO) solvents. It is found that there is no shift in the position of fluorescence maxima, but the intensity decreases with increase in temperature, which depends on the polarity of the solvent. A mechanism of fluorescence quenching with rise in temperature is discussed in terms of the relative location of lowest ¹(ππ^?) and ³(nπ^?) states, and the energy difference between them. The change in temperature brings about a change in the probabilities of radiative and non-radiative transition. The radiationless deactivation of excited-state in the absence of quencher is temperature-dependent and its thermal activation energy has been determined.     

Solvent and nonlinear effects on the charge renormalization of nanoparticles within a molecular electrolyte model

A general analysis of the effect of the molecular structure of a polar solvent on the effective interactions among suspended charged nanoparticles (macroions up to 30 nm in size) is performed using a simple molecular model for the solvent in the supporting electrolyte. The solvent molecules are modeled as small rods with end point charges of opposite sign and equal magnitude, whereas the small ions are assumed to be point-like. We compare the renormalized charges of the effective pair potentials (EPPs) among the spherical nanoparticles, obtained after contracting the supporting electrolyte, with those obtained from a similar model, which does not include the solvent molecules. The parameters of both models have been adjusted to give the same screening length. The comparison shows that the renormalized charges are overestimated when the molecular structure of the solvent is neglected. This is in agreement with the image charge effect induced by the different permittivities inside and outside the nanoparticles for the model with explicit solvent molecules; an effect that is missing in the model without solvent molecules. A new numerical method allows us to explore macroion diameters much larger than the solvent molecular size.     

The shape of doppler-broadened spectral lines produced by light atom reflection from metals: I. Theory

A model for predicting the shapes of Doppler-broadened spectral lines due to de-excitation by photon emission of excited neutral atoms backscattered from solids is presented. The distribution of backscattered particles is calculated on the basis of single Rutherford collisions within the solid accompanied by energy loss due to electronic stopping during passage of the particle through the solid. The effect of the energy dependence of the stopping power, radiationless de-excitation of the escaping neutral atom and the probability of the particle leaving the surface in the desired excited state are incorporated into the calculation. The broadening of the spectral line is determined by combining the Doppler shift formula with the energy- and angular distribution of the backscattered particles.     

Non-radiative transitions as Förster''s energy transfer to solvent vibrations

New experimental evidence is given to the approach suggested by the authors to non-radiative transitions in rare earth and transition metal ions as radiationless energy transfer from ion to ligand and solvent molecule vibrations. Using non-radiative transitions in Yb³⁺ as an example the participation of combinations of vibrations in the electronic energy dissipation was investigated. Dy³⁺ in solutions was used to show the role of anharmonicity in non-radiative transitions. It was proved that the semi-empirical calculations of non-radiative transition rates by Förster's formula gave also good results for Mn²⁺ ions luminescence which is an example of “strong” vibronic interaction. The suggested approach permits to explain the anomalous low probability of the ⁵D₁−⁵D₀ non-radiative transition in the Eu³⁺ ion.     

Electronic states and transition probabilities of the Au center in KCl

The emission spectra and lifetimes of the Au^— center in KCl crystals were measured. The Au^— concentration was measured by counting the activity of a radioactive tagged sample to determine the oscillator strength of the electronic transitions. The lifetime of the A'emission is calculated from the oscillator strength and is close to the experimental value of 103 ns at 4 K. The lifetimes and intensities of the Au^— emission bands C', B', A' and were measured between 4 and 500 K. The A'emission consists of a fast and slow decaying component. Time-resolved emission spectroscopy improved the measurements of the emission. From the temperature dependence of the lifetimes and intensities it is found that the radiating states of the A' and emission ³T_1u and ³A_1u are coupled by radiationless transitions. These transitions and the radiationless transitions from the excited state of the B' emission must be described as transitions between weakly coupled states similar to the radiationless transitions in rare earth ions. The excited state of an absorption band at 236 nm is responsible for the C' emission. Both the C' emission and the 236 nm absorption are phonon-induced transitions. The most probable explanation for the C' excited state is that the C' as well as the B' excited states come from the atomic state ³P₂ which is split into ³T_2u and ³E_u by the crystal field. The splitting energy is 0.9 eV and implies that one should look for the same effect in other s² centers.     

Nuclear excitation via free-bound muon atomic transitions

We have examined the probability of the excitation of the giant nuclear dipole resonance through the radiationless capture of a free muon into a bound orbit. Our calculation predicts that the probability is extremely small, about 4 × 10^?5 per stopping muon in lead in the surface transition model.     

Temperature dependences of spectral and kinetic characteristics of photoluminescence of alkali and alkaline-earth metal chromate crystals

Based on the results of temperature studies of spectral and kinetic characteristics, a study is made to determine the influence of the structure of different types of luminescence centers (LC) formed in chromate crystals on the processes of relaxation and radiationless deactivation of excitation energy. It is shown that "isolated" LC are characterized by simple temperature quenching of luminescence above the barrier. It is established that several excited states take part in the radiation of complex LC. The possibility of radiationless transitions between them with a change in spin is due to spin-lattice relaxation, which, as calculations showed, is described for different types of LC by the "van Vleck" or "defect" mechanisms. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 182–187, March–April, 2000.     

Adaptive resolution simulation of an atomistic DNA molecule in MARTINI salt solution

We present a dual-resolution model of a deoxyribonucleic acid (DNA) molecule in a bathing solution, where we concurrently couple atomistic bundled water and ions with the coarse-grained MARTINI model of the solvent. We use our fine-grained salt solution model as a solvent in the inner shell surrounding the DNA molecule, whereas the solvent in the outer shell is modeled by the coarse-grained model. The solvent entities can exchange between the two domains and adapt their resolution accordingly. We critically asses the performance of our multiscale model in adaptive resolution simulations of an infinitely long DNA molecule, focusing on the structural characteristics of the solvent around DNA. Our analysis shows that the adaptive resolution scheme does not produce any noticeable artifacts in comparison to a reference system simulated in full detail. The effect of using a bundled-SPC model, required for multiscaling, compared to the standard free SPC model is also evaluated. Our multiscale approach opens the way for large scale applications of DNA and other biomolecules which require a large solvent reservoir to avoid boundary effects.     

Hydrogen bonding,polarity and matrix effects on the spectroscopic and photophysical properties of some N- and 3-substituted derivatives of carbazoles

Fluorescence spectra and photophysical properties of N-cyano and N-carboethoxy carbazole derivatives have been compared to the equivalent 3-substituted compounds in fluid polar and unpolar solutions and in a low temperature rigid matrix at 77 K. The 3-carboethoxy derivatives have been shown to form an excited state complex (exciplex) with ethanol in fluid solutions. On the other hand the photophysical properties of the N-substituted derivatives are much affected by the solvent polarity in fluid solutions in that the radiationless decay rate constants show a marked increase in these media. A tentative explanation has been given in terms of the relative energy gap existing between the ¹L_b and the ¹L_a states of these molecules.