Characterization of the excimers of poly(N-vinylcarbazole) using TRANES

Time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra (TRANES) are exploited to characterize the excimers of poly(N-vinylcarbazole) (PNVC) in toluene and water. The time-resolved study together with the steady state measurements, decipher the fluorescence of the system containing pendant carbazole moieties in the polymer backbone unambiguously. Isoemissive point in the TRANES establishes that there are only two excimers of PNVC, the partially overlapped excimer emitting at high energy and the sandwich type excimer emitting at lower energy, and they are formed exclusively in the photoexcited state. The study substantiates an equilibrium between the two excimers.     

Excimer emission from evaporated pyrene films

Emission spectra and decay times of fluorescence of pyrene thin films prepared by evaporation onto glass substrate at low temperatures were measured. The band positions of the excimer emission from the films evaporated onto substrates at temperatures below about 200 K, are found to the lower energy side of those of the crystal. The decay time of excimer emission in a film prepared by evaporation onto a substrate cooled with liquid nitrogen is about 100 ns at 85 K and is shorter than that of the crystal. These results are discussed in relation to an amorphous structure in the pyrene evaporated thin films. The energy transfer was also studied in evaporated pyrene films containing perylene as a dopant. The results are explained in terms of excimer exciton diffusion through the crystalline structure in the film.     

Structure in the emission from pyrene evaporated films

Emission spectra and decay times of fluorescence of pyrene thin films prepared by evaporation onto substrates at liquid nitrogen temperature were measured. Structure in the emission from films warmed slowly up to higher temperature is found to the higher energy side of the excimer emission band. The decay time is about 410 ns at temperatures between 110 K and 210 K. But the emission from the film warmed up to a temperature above 210 K shows only the excimer emission. These results are discussed in relation to an amorphous structure in the pyrene evaporated thin film.     

Luminescence of poly(N-vinylcarbazole) in the solution and solid state

Fluorescence spectra of poly(N-vinylcarbazole) have been studied as a function of various environments and temperature. Fluorescence spectra have been excited with UV light from 200 W Xenon lamp or with second-harmonics of a pulsed ruby laser. Polymer films and powders show exclusively excimer fluorescence at room temperature. At high intensities of excitation the observed fluorescence spectra have a weak quasiperiodic structure.     

Fluorescence and host-guest energy transfer in polymeric chains

Intrachain singlet electronic excitation energy transfer from host polyvinylcarbazole (PVCa) to guest vinylbenzocarbazole (VBCa) units has been studied through measurements of the quenching effect on PVCa fluorescence and observation of the sensitized guest VBCa emission in solutions at 293 and 77K for different intramolecular concentrations of guest VBCa units. A method of taking into account intrinsic traps of singlet excitons (excimer forming sites, EFS), ordinarily present in an impurity free polymer macromolecule, has been elaborated. The comparison of the experimental data with theory shows that the one-dimensional singlet excitation energy transfer in a polymeric chain involves motion of incoherent excitons randomly hopping to a nearest neighbor along the line under conditions when capturing occurs upon the first visit of a trap and trapping centers of different origin become indistinguishable as regards their capturing capability.     

Current-voltage characteristics of poly(diphenylenephthalide) thin films

Charge transfer near the threshold of polymer film transition (induced by a low uniaxial pressure) to the high-conductivity state is studied in an attempt to tackle the question of how the energy band structure of a wide-gap organic insulator varies near this threshold. The I-V characteristics of poly(diphenylenephthalide) films of various thickness versus uniaxial pressure are analyzed. The results obtained are treated within the model of space-charge-limited injection currents. The parameters of the injection model, such as the equilibrium concentration of electrons, electron mobility, the occupation of traps, etc., are estimated. It is concluded that deep traps due to an excess charge may appear in the energy gap of the polymer near the imref. This probably causes a narrow subband to arise, and charge transfer via this subband increases the charge carrier mobility and, hence, conductivity.     

Energy transfer in perylene-doped polyvinyl carbazole

Time resolved spectroscopy and fluorescence decay time measurements have been made on thin films of amorphous polyvinyl carbazole (PVK). The value obtained for the excimer fluorescence decay time in the undoped sample is less than that reported previously and a wavelength dependent double exponential decay is observed, which has been seen before only at low temperatures. The observed quenching of the excimer fluorescence decay times in the doped samples is evidence of direct energy transfer from excimers to perylene which is inconsistent with the model of energy transfer by exciton diffusion proposed to explain intensity quenching data. Several models for energy transfer in PVK are discussed and it is shown that the only model which predicts the observed time resolved spectroscopy results requires a long range resonant interaction strength much greater than that obtained theoretically. Several possible reasons for this are discussed.     

Superinelastic electron transfer: electron trapping in H2O ice via the N-2((2)Pi(g)) resonance

We present measurements on the trapping of 0-3 eV electrons in H2O ice films covered with a submonolayer of N2 molecules. At the energy of the N-2((2)Pi(g)) shape resonance, the absolute cross section for electron trapping in ice is approximately 5.5 x 10(-16) cm(2), similar to that for vibrational excitations of gaseous N2 via the resonance. This result, indicating that nearly all electrons from autoionization of N-2((2)Pi(g)) are transferred to electron traps in ice, is explained by superinelastic electron transfer from N-2((2)Pi(g)) into preexisting traps in polar ice, leaving N2 in high vibrational excited states.     

乙醇-水团簇分子形成激基缔合物及荧光发射机理研究

紫外光照射具有特殊结构的长链式乙醇-水团簇分子时,处于激发态和基态的分子形成了分子间激基缔合物,并发射荧光.根据实验结果分析和能量转移理论可知,激发态单分子和激基缔合物间形成了电子迁移洛合物并发生了能量转移.根据Mulliken理论对电子迁移洛合物进行量子力学处理,得出了团簇分子在基态和激发态能量E^b_N和E^b_E以及由于电子迁移而引起的静电相互作用能E^s;根 关键词： 荧光光谱 激基缔合物 电子迁移 乙醇-水团簇     

Laser time-resolved spectroscopy: Investigations of energy transfer in Eu3+ and Er3+ doped YVO4

Host-sensitized energy transfer in YVO₄ crystals doped with Eu³⁺ and Er³⁺ ions was investigated using laser excited time-resolved spectroscopy techniques. The results are consistent with a model in which the energy is transferred by a single step process from self- trapped excitons to activators at low temperatures, and by thermally activated exciton hopping at high temperatures. The rate of the latter process is found to be strongly affected by the trapping step with activator-induced host traps playing an important part in the trapping process. Time-resolved spectroscopy techniques are shown to be expecially useful in separately determining the characteristics of migration and trapping processes in exciton energy transfer. The parameters describing these processes are determined for the systems investigated using both diffusion and generalized random walk theories.     

Study of the trapping states in ZnS single crystals grown from gallium melt

The trapping levels in zinc sulphide single crystals grown from gallium melt have been investigated using thermoluminescence techniques. The observed peak at 175° K consists of two overlapping components at 173 and 200° K respectively. Thermal activation energies and frequency factors were calculated for both traps. The dependence of glow curve shape on excitation conditions is caused by the retrapping by non-filled 200° K traps of electrons freed from 173° K traps in the course of the glow curve run. In addition to the results on “pure” crystals, measurements were made on samples grown with chlorine, oxygen and copper impurities, as well. Although no positive identification of the chemical nature of the 173 and 200° K trapping centers has been possible, we find that our results are not inconsistent with a previously suggested model in which the traps are identified as complex defects. Comparison is also made with trap spectra observed earlier in gallium-doped zinc sulphide samples prepared by the usual methods.     

Excimer emission from glassy films of some aromatic hydrocarbons

The fluorescence emission spectra and fluorescence decay times of amorphous films prepared by evaporation of a number of aromatic hydrocarbons on to a substrate cooled with liquid nitrogen were measured. The fluorescence of such films appears to originate from sites where the interaction between neighbouring molecules is of the same kind as that present in the well-known excimer state. The density of such sites is found to be so high as effectively to prevent energy migration. Some interesting exceptions to this behaviour are noted and comparisons drawn with experimental data reported for excimer formation in solution and in the crystalline phase.     

Excimer emission of crystalline naphthalene

Excimer emission of naphthalene has been observed in microcrystals obtained by grinding single crystals mechanically and in evaporated films. The emission spectrum of the excimer in the microcrystal was equal to that in the evaporated film. The decay times of both excimer emissions were about 140 ns at 77 K. The temperature dependence of these decay times has been interpreted by the distribution of the excimers in thermal equilibrium between two vibrational states.     

Thermal transport in melting and recrystallization of amorphous and polycrystalline Si thin films

The liquid-solid interface motion and temperature history of thin Si films during excimer laser annealing are observed by in situ experiments combining time-resolved (~1 ns) electrical conductance, optical reflectance/transmittance at visible and near-IR wavelengths and thermal emission measurements. For laser energy densities causing partial melting, the maximum temperature remains close to the melting point of amorphous silicon (a-Si). When complete melting occurs, substantial supercooling is observed, followed by spontaneous nucleation. These phase transformations are consistent with the recrystallized poly-Si morphologies. It is also found that the melting of poly-Si occurs close to the melting point of crystalline silicon. This temperature is higher than the melting point of a-Si by about 100-150 K.     

Excitation Energy Transport in a Concentrated System of Flavomononucleotide in Polyvinyl Alcohol Films

Excitation energy transport mechanism of flavomononucleotide polyvinyl alcohol films is studied. Excitation wavelength and temperature dependences of fluorescence spectra and quantum yields for different concentrations of the dye are investigated. These measurements together with those of absorption reveal that dimers are imperfect traps for excitation energy and that the energy transfer can occur both in the forward and in the reverse direction. It was found that, contrary to liquid solutions, the dimerization constant of flavomononucleotide in polyvinyl alcohol films does not depend on temperature and that the irregularities observed can be explained by the temperature-dependent changes in the quantum yield of the monomer–dimer system and the effect of inhomogeneous orientation broadening of energy levels.     

Energy transfer in europium doped yttrium vanadate crystals

The absorption, fluorescence and excitation spectra and fluorescence lifetimes were obtained at temperatures from 8 K to room temperature, for single crystals of yttrium vanadate doped with several concentrations of europium. A model is proposed to explain the luminescence properties of YVO₄ crystals based on transitions within the VO^3-₄ ion. The host sensitized energy transfer in YVO₄: Eu³⁺ is explained by amodel based on thermally activated exciton migration. The model predicts the observed temperature and concentration dependences for the fluorescence intensities and lifetimes. The diffusion coefficient, diffusion length, exciton hopping time and trapping rate are determined and compared with similar data obtained on other systems.     

Electronic energy transfer in benzophenone adlayer

The extent to which energy transfer occurs in electronically excited organic adlayer films on dielectric surfaces is investigated. Migration and subsequent trapping of the energy in the film are observed by pumping the singlet state of an organic adlayer of benzophenone and by monitoring the phosphorescence and fluorescence lifetimes. To observe the effects of adsorption, benzophenone was chosen as the adlayer because the energies of its well characterizedn, carbonyl states are remarkably sensitive to solvent interactions. Upon excitation with a nitrogen laser, the perturbation on the electronic states of benzophenone by the substrate caused the emergence of the normally absent fluorescence from the adlayer traps at the interface between the surface of the dielectric substrate and the adlayer. Energy transfer to this interface was observed as a function of film thickness. On the surface of a single crystal of an organic crystal, naphthalene, energy transfer from the adlayer to the substrate was observed, whereas such transfer was not energetically possible with the other dielectric surfaces.     

Luminescence and energy transfer in TbAl3B4O12

An investigation of the luminescence properties of TbAl₃B₄O₁₂ in the temperature region 1.4–300 K is reported. Laser site-selection and time-resolution techniques were used.The results show that energy migration among Tb³⁺ ions on the regular crystallographic sites occurs. The transfer characteristics at room temperature are consistent with a diffusion-limited transfer process to Mo³⁺ quenching centres. The diffusion constant and the critical transfer distance for Tb³⁺ → Mo³⁺ transfer are derived. The rate of diffusion increases for decreasing temperature. At about 60 K a transition from diffusion-limited transfer to trapping-limited transfer occurs. This behaviour is due to the variation in the diffusion constant with temperature. In the temperature region below 60 K transfer to Tb³⁺ traps is observed. The intensity of the emission from the traps increases exponentially with decreasing temperature. However, the overall transfer rate to Mo³⁺ and traps remains roughly constant. A simple model including time-independent transfer rates and back transfer from traps to intrinsic Tb³⁺ ions is proposed to explain the results. The rates of transfer to Mo³⁺ and traps are obtained.     

An anomalous emission excited near the absorption edge of pyrene crystals

Emission spectra and decay times of the fluorescence excited at the absorption edge region in pyrene crystals were measured. At liquid nitrogen temperature, the fluorescence under the excitation at 390 nm is considered as the excimer emission and its band peak shifts a little to the short wavelength in comparison with that of the excimer emission under the excitation at 360 nm. The emission decay times under the excitation at the 390 nm and 360 nm are about 155 ns and 180 ns, respectively, at liquid nitrogen temperature. The former decay time changes its value abruptly near 127 K. This abrupt change of the decay time may be due to the phase transition in pyrene crystals.     

Time-resolved fluorescence of cationic dyes covalently bound to poly(methacrylic acid) in rigid media

Atactic poly(methacrylic acid) labeled with acridine and Nile blue (NB) were studied by photophysical techniques in bulk solid state and in solution-cast films over different surfaces (glass, ITO, and polymethylmethacrylate). In the systems with both dyes, energy transfer from acridine to NB occurs with an efficiency depending on the type of substrate (solid or film). The films are more disordered fluorescent rigid media than the bulk chromophoric or bichromophoric polymers, and this effect is ascribed to inhomogeneous distribution of the dyes in the film. This effect enhances dye bimolecular interactions and increases the energy transfer rates between acridine donor and NB acceptor. Bimodal distributions of donor fluorescence lifetimes are observed.