有机高分子中光生极化子的物理机制

有机聚合物中光生载流子（荷电极化子）产生的物理机制是一个非常重要的物理问题，但一直存在争议，文章介绍了作者对此问题的最新研究结果：荷电极化子和中性激子一样，是由光激发直接产生的，而不是激子解体的结果，其生成时间在100fs的量级．荷电极化子的量子产率约为25％，并且与激发能量无关，与实验观测的结果一致．     

杂质离子对有机共轭聚合物中极化子动力学性质的影响

基于一维紧束缚Su-Schrieffer-Heeger模型, 采用分子动力学方法, 讨论了杂质势的强度和杂质之间的距离对电子和空穴极化子动力学性质的影响. 研究结果表明: 1)当杂质势强度保持不变时, 两杂质离子之间的距离(d)在2-16个晶格常数变化时, 电子极化子的平均速度大于空穴极化子的平均速度, 这是由于电子、空穴极化子与杂质势的库仑作用不同而产生的差异, 同时极化子的平均速度随d的增加而增大; 若继续增加杂质离子之间的距离, 电子和空穴极化子的平均速度几乎保持不变, 仅有一些微小的振荡, 这是由于不同距离的杂质离子对电子和空穴极化子产生的势垒或势阱的叠加效果不同而引起的; 2)保持两杂质离子之间的距离不变时, 随着杂质势强度的增大, 电子和空穴极化子的平均速度均减小, 且空穴极化子的平均速度减小趋势更明显.     

Inelastic scattering of oppositely charged polarons in conjugated polymers

Within an one-dimensional tight-binding model, we investigate the inelastic scattering processes of oppositely charged polarons in conjugated polymers under the influence of an external electric field, by using a nonadiabatic evolution method. It is found that the polaron pair does not necessarily scatter into an entity(neutral exciton), but a mixed state composed of both polarons and excitons. The yield of the neutral exciton depends sensitively on the strength of applied fields. Additionally, effects of interchain coupling on the scattering processes are also discussed, which shows that the interchain coupling is of fundamental importance and facilitates the formation of the polaron-exciton.     

Dynamics of interchain delocalized polarons in polymers

Polaron dynamics in a system of highly ordered conjugated polymer chains is investigated based on the tight binding model. With the interchain coupling enhancing,it takes longer time for the electron added to the system to induce a localized polaron state. Beyond a certain strength of the interchain coupling,the electron evolves into a two-dimensional delocalized polaron state. Dynamical simulations suggest that the well-ordered organic molecule systems with two-dimensional polarons exhibit higher mobilitie...     

基态非简并聚合物中的极化子和双极化子动力学

研究了极化子和双极化子在基态非简并聚合物中的动力学.弱电场下，发现载流子为带电极 化子或双极化子，它们的晶格态与电荷态始终耦合在一起同步运动.极化子比双极化子的运 动速度快，存在饱和速度，且它们的饱和速度随聚合物非简并度的增加而降低；强电场下， 元激发的电荷态将脱离晶格态的束缚而快速运动，参与导电的不再是极化子或双极化子，而 是电子直接导电. 关键词： 极化子 双极化子 动力学模拟     

ESR and ENDOR studies of solitons and polarons in conjugated polymers

Nonlinear excitations such as solitons and polarons in conjugated polymers carry spins. In this case electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) provide unique methods to determine their wave functions. In this review article, the case of solitons in polyacetylene, CH_x, and polarons in an electroluminescent polymer, poly(paraphenylene vinylene) (PPV) are discussed as typical examples. High-resolution proton ENDOR spectra, obtained with stretch-oriented samples, yield the half extension of the excitations of 18 carbon atoms and 4 phenyl rings for CH_x and PPV, respectively. These extensions are well described by the theories in the case of finite electron correlation. In addition, light-induced ESR technique is shown to be useful in obtaining site-selective information of spin distribution in the case of PPV derivatives, as well as the excitation spectra of polarons.     

Effects of polarons on static order hyperpolarizabilities polarizabilities and second of conjugated polymers

According to the one-dimensional tight-binding Su-Schrieffer-Heeger model, we have investigated the effects of charged polarons on the static polarizability, axx, and the second order hyperpolarizabilities, γxxxx, of conjugated polymers. Our results are consistent qualitatively with previous ab initio and semi-empirical calculations. The origin of the universal growth is discussed using a local-view formalism that is based on the local atomic charge derivatives. Furthermore, combining the Su Schrieffer-Heeger model and the extended Hubbard model, we have investigated systematically the effects of electron-electron interactions on αxx and γxxxx of charged polymer chains. For a fixed value of the nearest-neighbour interaction V, the values of αxx and γxxxx increase as the on-site Coulomb interaction U increases for U 〈 Uc and decrease with U for U 〉 Uc, where Uc is a critical value of U at which the static polarizability or the second order hypcrpolarizability reaches a maximal value of αxx or γxxxx. It is found that the effect of the e-e interaction on the value of αxx is dependent on the ratio between U and V for either a short or a long charged polymer. Whereas, that effect on the value of γxxxx is sensitive both to the ratio of U to V and to the size of the molecule.     

Dynamical properties of photogenerated polarons in one-dimensional dimerized Mott Insulators

We study physical properties of photogenerated electron-lattice coupled states, polarons, in one-dimensional (1D) Peierls-Hubbard model with classical lattice distortion by means of the density matrix renormalization group method. The numerical results show novel midgap peaks in optical response spectra of polarons for large on-site Coulomb interaction U. These midgap peaks originate from charge-transfer excitations within the dimer in polarons.     

Effects of polarons on static polarizabilities and second order hyperpolarizabilities of conjugated polymers

According to the one-dimensional tight-binding Su-Schrieffer-Heeger model,we have investigated the effects of charged polarons on the static polarizability,α xx,and the second order hyperpolarizabilities,γ xxxx,of conjugated polymers.Our results are consistent qualitatively with previous ab initio and semi-empirical calculations.The origin of the universal growth is discussed using a local-view formalism that is based on the local atomic charge derivatives.Furthermore,combining the Su-Schrieffer-Heeger model and the extended Hubbard model,we have investigated systematically the effects of electron-electron interactions on α xx and γ xxxx of charged polymer chains.For a fixed value of the nearest-neighbour interaction V,the values of α xx and γ xxxx increase as the on-site Coulomb interaction U increases for U U c and decrease with U for U U c,where U c is a critical value of U at which the static polarizability or the second order hyperpolarizability reaches a maximal value of α max or γ max.It is found that the effect of the e-e interaction on the value of α xx is dependent on the ratio between U and V for either a short or a long charged polymer.Whereas,that effect on the value of γ xxxx is sensitive both to the ratio of U to V and to the size of the molecule.     

Dynamics of carriers photogenerated in a dot-in-a-well nanostructure

We review our recent results on the investigation of carrier dynamics of semiconductor nanostructures, i.e., dot-in-a-well. We introduced a technique based on the measurement of time-resolved differential photoluminescence spectra induced by subpicosecond pump and probe laser pulses. In this technique, the temporal resolution was achieved by adjusting the temporal delay between the pump and probe pulses. Using such a new technique, we measured the exciton decay times and then studied the dependences of integrated photoluminescence intensity and photoluminescence linewidth on temperature in InAs quantum dots embedded in InGaAs/GaAs quantum wells. We gave consistent interpretations to our experimental results.     

二维链间扩展的极化子动力学研究

基于扩展的SSH模型,研究了有序耦合聚合物链系统中的极化子动力学,包括极化子的形成过程及其在外场下的输运.发现,当聚合物链间的耦合较强时,注入到系统中的电子会诱发二维链间扩展的极化子态,分布在多条聚合物链上.另外,动力学模拟表明,与一维链内定域极化子相比,在相同的电场强度下二维极化子具有更大的运动速度,这与实验结果一致. 关键词： 链间扩展极化子 链间耦合     

Segmented conjugated polymers

Segmented conjugated polymers, wherein the conjugation is randomly truncated by varying lengths of non-conjugated segments, form an interesting class of polymers as they not only represent systems of varying stiffness, but also ones where the backbone can be construed as being made up of chromophores of varying excitation energies. The latter feature, especially when the chromophores are fluorescent, like in MEHPPV, makes these systems particularly interesting from the photophysics point of view. Segmented MEHPPV-x samples, wherex represents the mole fraction of conjugated segments, were prepared by a novel approach that utilizes a suitable precursor wherein selective elimination of one of the two eliminatable groups is affected; the uneliminated units serve as conjugation truncations. Control of the compositionx of the precursor therefore permits one to prepare segmented MEHPPV-x samples with varying levels of conjugation (elimination). Using fluorescence spectroscopy, we have seen that even in single isolated polymer chains, energy migration from the shorter (higher energy) chromophores to longer (lower energy) ones occurs — the extent of which depends on the level of conjugation. Further, by varying the solvent composition, it is seen that the extent of energy transfer and the formation of poorly emissive inter-chromophore excitons are greatly enhanced with increasing amounts of non-solvent. A typical S-shaped curve represents the variation of emission yields as a function of composition suggestive of a cooperative collapse of the polymer coil, reminiscent of conformational transitions seen in biological macromolecules.     

Local-field anisotropy in conjugated polymers

The experimental values of the Lorentz tensor components L _j have been obtained for uniaxial optically positive and negative films of conjugated polymers with thicknesses of tens of nanometers. The correlation between the components L _j and the measurable parameter η has been revealed and the analytical dependence L _j (η) has been derived in agreement with the experiment.     

Polarons in organic conjugated polymers

An unified treatment of the electronic structure of organic conjugated polymers based on a renormalization approach is presented. The changes in the electronic structures of trans-polyacetylene, poly-(p-phenylene), polypyrrole and polythiophene, brought about by the presence of polarons in these systems, are studied. For all systems, two localized states, which can be interpreted as the symmetric and antisymmetric combinations of soliton wavefunctions, are created at the central gap.