The diffusion of the excitation energy in molecular crystals

A theory of the coefficient of diffusion of an exciton in molecular crystals is elaborated, based on the conception of the exciton as the excited state of the molecule of a crystal propagating through the crystal by chance transition from one molecule to another. The theoretical results are compared with the experimental data for an anthracene crystal.     

A theory of non-radiative transfer of excitation energy in solids

A theory of non-radiative transfer of excitation energy in solids is elaborated for the case of strong interaction of an atom of the sensitizer and an atom of the activator with the oscillations of the whole crystal lattice.     

Mechanisms of femtosecond energy transfer upon strong excitation in crystals

It is shown that the fast temperature-independent shift of the long-wavelength edge of fundamental absorption observed upon electron bombardment of alkali halide, alkaline-earth, and oxygen-containing crystals is caused by the impact mechanism. In the course of the impact shift of lattice ions, short-lived perturbation (mainly, of their valence shells) occurs. The perturbation fields in the vicinity of impact-shifted anions act for no more than 100 fs. As a result, the distribution of the density of states in the valence band of a crystal is distorted, and the long-wavelength shift of the fundamental-absorption edge may increase for less than 100 fs.     

Triplet-triplet transfer of electronic excitation energy in vapors of organic molecules

The rate constants of the triplet-triplet (T-T) transfer of energy of the electronic excitation obtained in the gas phase for two sets of donor-acceptor pairs with various spectral characteristics are compared with those predicted by the theories of nonradiative transitions. The rate constants K _TT and efficiencies β_DA of the T-T transfer are estimated from quenching of the phosphorescence of biacetyl by aromatic hydrocarbons (anthracene, 9-methylanthracene, 9,10-dimethylanthracene, 9-phenylanthracene, pyrene, and chrysene) and from quenching of the delayed fluorescence of benzophenone, 4-phenylbenzophenone, anthraquinone, carbazole, and naphthalene by biacetyl, as well as from the sensitization of phosphorescence of biacetyl by these compounds. The causes of the broad variations of β_DA in the gas phase from 0.35 (biacetyl-anthracene) to 10^?4 (biacetyl-chrysene) are discussed for the studied donor-acceptor pairs, which differ in the exothermicity of the process and in the configuration of the lowest triplet levels of the molecules participating in the transfer. The dependences of the transfer rate constants K _TT on the free energy change ΔG (the range from 0.4 to ?1.7 eV) are analyzed. It is shown that, in gas-phase systems, K _TT increase with the exothermicity of the transfer, attain a highest value, and then decrease in the range of negative ?ΔG. The observed differences in the experimental values of β_DA are discussed in terms of changes in the free energy ΔG of the process, in the energy of rearrangement of the nuclear configuration of the molecules during the transfer, and in the matrix elements of the interaction.     

Resonance transfer of molecular excitation energy: Enhancement in channels

An analysis is made of the influence of the boundary of the region in which resonant transfer of excitation energy takes place from one molecule to another, and it is pointed out that under appropriate circumstances boundary effects can lead to enhancement of energy transfer.     

The propagation of excitation energy in perturbed crystals

The process of the propagation of excitation energy in perturbed dielectric crystals with a weak bond between the atoms having zero permanent dipole moment is studied. It is shown that on certain assumptions this process of propagation can be regarded as the motion of a Frenkel exciton in the electrostatic field of the defect. Frenkel's exciton can be characterized in this case as a neutral polarizable particle having induced dipole moment equal to the change in the induced dipole moment of the crystal during the excitation of one of its atoms.

---

, . , . .

     

Triplet-triplet transfer of electronic energy upon nonequilibrium vibrational excitation of triplet molecules

The specific features of the triplet-triplet (T-T) transfer of electronic excitation energy in a gas phase upon nonequilibrium vibrational excitation of the triplet molecules of a donor were studied for an anthraquinone-diacetyl donor-acceptor pair using the time-resolved slow fluoresence of anthraquinone and sensitized phosphorescence of diacetyl. It is shown that in the gas phase, which allows regular control of the number of collisions, competition between the processes of T-T transfer and intermolecular vibrational relaxation is observed for nanosecond time resolution. The T-T transfer rate for the molecular system investigated exceeded the rate of intermolecular vibrational relaxation k_V in the triplet state T₁ of the donor. The effectiveness of the T-T transfer of energy by vibrationally excited molecules turned out to be higher than the effectiveness of transfer by thermalized ones, but even the highest of them was much less than unity. An increase in the equilibrium temperature of vapors led to a decrease in the effectiveness of transfer for both vibrationally excited and thermalized triplet molecules, thus indicating the importance of the collisional complex in the intermolecular process studied. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 474–479, July–August, 2000.     

Electronic excitation energy transfer between two single molecules embedded in a polymer host

Unidirectional electronic excitation energy transfer from a photoexcited donor chromophore to a ground state acceptor chromophore - both linked by a rigid bridge - has been investigated by low temperature high-resolution single molecule spectroscopy. Our approach allows for accurately accessing static disorder in the donor and acceptor electronic transitions and to calculate the spectral overlap for each couple. By plotting the experimentally determined transfer rates against the spectral overlap, we can distinguish and quantify F?rster- and non-F?rster-type contributions to the energy transfer.     

直角坐标系中的分子和分子晶体振动光谱分析

本文介绍在直角坐标系中用简单的弹性力常数模型分析振动光谱的方法,这种方法在处理孤立分子时与传统的 GF 矩阵法等效,而对于分子晶体振动的动力学计算具有一定的优越性。它利用统一的数学公式来描述晶体中分子内外的相互作用从而同时获得分子外部振动频率和内部模式在晶体场中分裂的信息。     

Luminescence and electronic excitation energy migration in stilbene crystals

Absorption and luminescence spectra are reported for various conditions. A new band occurs at the short-wave edge of the luminescence spectrum above 90 ° K, which probably represents the short-wave component of the exciton radiation. Reabsorption has a marked effect on the energy distribution in the luminescence spectrum.     

Ambiguities in the analysis of non-radiative energy transfer data in solid-state laser materials

We apply a recently developed model for non-radiative energy transfer in correlated systems to recently published data on energy transfer in garnets. We show that even in well-studied materials in which the analysis of the data appears straightforward, subtle deviations from theory and difficulty in quantitatively evaluating interaction parameters may be due to a non-random distribution of donors and acceptors in the crystal.     

Enhancement of Tb emission by non-radiative energy transfer from Dy in silicate glass

A study of energy transfer was performed in dysprosium-terbium-doped silicate glasses at room temperature. Enhancement of the Tb³⁺ emission and a decrease in the Dy³⁺ emission are observed as a result of energy transfer from Dy³⁺ ions to Tb³⁺ ions. The energy transfer efficiencies, transfer probabilities, as well as average donor-acceptor distances were also calculated. It is concluded that the energy transfer mechanism between Dy³⁺ and Tb³⁺ ion is mainly electric dipole-dipole in nature.     

Phonon-assisted excitation energy transfer in photosynthetic systems

The phonon-assisted process of energy transfer aiming at exploring the newly emerging frontier between biology and physics is an issue of central interest.This article shows the important role of the intramolecular vibrational modes for excitation energy transfer in the photosynthetic systems.Based on a dimer system consisting of a donor and an acceptor modeled by two two-level systems,in which one of them is coupled to a high-energy vibrational mode,we derive an effective Hamiltonian describing the vibration-assisted coherent energy transfer process in the polaron frame.The effective Hamiltonian reveals in the case that the vibrational mode dynamically matches the energy detuning between the donor and the acceptor,the original detuned energy transfer becomes resonant energy transfer.In addition,the population dynamics and coherence dynamics of the dimer system with and without vibration-assistance are investigated numerically.It is found that,the energy transfer efficiency and the transfer time depend heavily on the interaction strength of the donor and the high-energy vibrational mode,as well as the vibrational frequency.The numerical results also indicate that the initial state and dissipation rate of the vibrational mode have little influence on the dynamics of the dimer system.Results obtained in this article are not only helpful to understand the natural photosynthesis,but also offer an optimal design principle for artificial photosynthesis.