Processes of electronic energy transfer between dye molecules of different types in water-polyelectrolyte solutions

Processes of electronic energy transfer between dye molecules of different types in water-polyelec-trolyte solutions were studied. The efficiency of these processes was found to depend extremally on the concentration of polyelectrolyte as well as the length of the polyion. A fractal distribution of the interacting dye molecules is ascertained from the experimental data. The dependences of the fractal dimension of the water-polyelectrolyte dye solutions on the polyelectrolyte concentration and polyion length is determined.     

Polarization and threshold energy variation of distributed feedback lasing of oxazine dye in zirconia waveguides and in solutions

Distributed feedback lasing characteristics of oxazine dye in zirconia waveguides and in solutions were investigated. Oxazine-dye-doped waveguides were characterized by ellipsometry. Intensity modulation and polarization modulation were used to generate laser action. Tunable narrow line width laser action was achieved for the first, second, third and fourth Bragg orders. It was observed that the threshold pump energy decreased with the decrease of the Bragg order. The degree of polarization of the laser output also changed from 0.8 at the third Bragg order to 0.1 at the first Bragg order. Wide-band tuning of the distributed feedback laser output from 700 nm to 870 nm was achieved for the first-order action during intensity modulation. PACS 42.55.Mv; 42.70.Jr; 42.79.Gn     

Nonradiative triplet-singlet transfer of electronic excitation energy between dye molecules in the vicinity of the silver-film surface

Nonradiative triplet-singlet transfer of electronic-excitation energy between molecules of organic dyes (erythrosine and methylene blue) in a polymer film that is deposited on the surface of a silver layer is experimentally studied. It is demonstrated that the energy-transfer efficiency in such a system is lower than the efficiency in the samples without metal layers. The results of the proposed mathematical model are in qualitative agreement with the experimental data.     

Collective spontaneous emission of dye molecules and lasing

The threshold pump power density for lasing in dye solutions is found to depend on the photon energy of pumping radiation. An increase in the pumping photon energy can significantly lower the threshold pump power of dye lasers. For an ethanol solution of rhodamine 6G with a concentration of 4×10¹⁸ cm^?3, the threshold power density for pumping radiation with a wavelength of 532 nm is 20-fold higher than for pumping radiation with a wavelength of 347 nm. This phenomenon is associated with the competition of collective spontaneous emission, which can lead to the efficient deactivation of excited molecules in femtosecond times, and the dephasing of excited molecules due to the intramolecular nonradiative processes of absorbed-energy conversion. An increase in the dephasing rate with the increasing energy of exciting photons lowers the efficiency of collective spontaneous emission and increases the concentration of dephased excited molecules responsible for lasing.     

Thermal rotations and relaxation of vibrational energy of excited dye molecules in solution

Measurements of polarization and decay time of fluorescence of dyes in solution as a function of excitation wavelength were performed. The rate of thermal rotational depolarization was found to be independent of vibrational energy excess. To explain the observed influence of excitation wavelength on the emission anisotropy, a change in the magnitude of torsional vibrations, in addition to possible mixing of states, is proposed.     

Processes of electronic excitation energy transfer between luminescent dye molecules in microheterogeneous systems based on polymer micellar solutions

The processes of electronic excitation energy transfer (EEET) between different-type dye molecules in polymer micellar solutions with different polyelectrolyte concentration have been analyzed. The possibility of regulating the EEET efficiency between dye molecules due to change of the structure of polymer micellar complexes has been established. The results presented are explained from the viewpoint of the fractal character of distribution of interacting particles. The experimental dependence of the fractal dimension of the distribution of dye molecules in polymer micellar solutions on the polyelectrolyte concentration has been obtained.     

Efficiency of intramolecular electronic energy transfer in coumarin bichromophoric molecules

Intramolecular energy transfer in bichromorhoric molecules consisting of two coumarins linked by a variable number of methylene groups (n = 3, 4, 8, 12), was studied with special attention to the effects of n, temperature, viscosity and solvent nature on the efficiency of transfer. The validity of model compounds for the donor and acceptor moieties was carefully examined. Fluorescence polarization experiments revealed that a non-randomicity of mutual orientation of donor and acceptor can only be small, if any. Dual fluorescence from donor and acceptor was observed in propylene glycol whatever the value of n and at any temperature ranging from -60 to 60°C. Fluorescence from the donor was not detectable in dimethylformamide owing to the very low quantum yield of the donor in this solvent. Transfer efficiencies were determined by three steady-state methods and were found to be more dependent on n in dimethylformamide than in propylene glycol. Incomplete energy transfer is clearly shown in the case of dimethylformamide as a solvent. The effects of temperature are weak in the -60, + 60°C range. The results are discussed in light of theories of energy transfer.     

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.     

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.     

染料分子电子激发态振动失相过程的非相干光时延四波混频研究

本文对噁嗪(Oxagine)、甲酚紫(cresyl violet)染料分子的吸收带和吸收边进行了非相干光时延四波混频(TDFWM-IL)的详细研究,得到了有关失相参量的数据,并利用非相干光时延四渡混频的多能理论解释了实验结果和失相参量的物理意义。     

The cross sections and their temperature dependence of collisional energy transfer from high vibrationally excited ethylene molecules to electronic ground state Rubidium atoms

The cross sections and their temperature dependence of collisional energy transfer from the high vibrationally excited C_2H_4 to the electronic ground sate of Rb (V-E) have been studied by the method of IR laser pump combined with fluorescence probe. The fluorescence of rudidium atoms due to V-E energy transfer was detected. The measured rate constants of this V-E energy transfer is about 10~(-10)cm~2molec~(-1)s~(-1) and the corresponding cross section varies from 60 (?)~2 to 90 (?)~2. The cross sections decrease with increasing temperature over the temperature range of 393-683 K.     

Ultraviolet emissions from Gd ions excited by energy transfer from Ho ions

Ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ ion were investigated in Y_1.838−xGd_xYb_0.16Ho_0.002O₃ (x=0, 0.16, 0.4, 1, 1.4) bulk ceramics under 976 nm laser diode (LD) excitation. The UC emissions centered at 309 and 315 nm are assigned to the transition of ⁶P_5/2→⁸S_7/2 (Gd) and ⁶P_7/2→⁸S_7/2 (Gd). The ⁶P_J levels of Gd³⁺ ions are populated by an energy transfer (ET) process from ⁸S_7/2 (Gd)+(³P₁, ³L₈, ³M₁₀) (Ho)→⁶P_J (Gd)+⁵I₈ (Ho). A four-photon ET UC process was confirmed by the dependence of the ⁶P_7/2 level emission intensity on the pumping power. We found that the intensity of the UC emissions increased with Gd³⁺ ion concentration and peaked at 8 mol%, then starts to decrease until the Gd³⁺ ion concentration reached 70 mol%. The variation in the UV emission intensity is the result of the competition between the ET process and concentration quenching effect. Theoretical calculations based on steady-state equations validated the proposed UC mechanisms.     

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.