一维分子晶体中的极化子

本文从量子化的Holstein模型出发,运用相干态展开法和能量极小原理,通过实施连续近似得到了一维分子晶体模型处于基态的极化子满足的非线性薛定谔方程及其定态孤子解、基态能量、晶格位移,其结果与Holstein T[1].从半经典理论所得的结果完全一致,因此相干态展开方法在处理与极化子有关的其他物理问题中是一种非常有效的方法.     

Structural stability of nano-sized crystals of HMX: A molecular dynamics simulation study

The interaction potential energy and heat of sublimation of nanoparticles of HMX crystal polymorphs are studied by using molecular dynamics methods with a previously developed force field [Bedrov, et al., J. Comput.-Aided Mol. Des. 8 (2001) 77]. Molecular dynamics simulations of nanoparticles with 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 molecules of HMX are carried out at 300 K. The intermolecular, intramolecular and total interaction energies per mole for the nanoparticles are calculated at 300 K. Then, we have calculated sublimation enthalpy of HMX crystal polymorphs with different sizes. For the all sizes, the β-HMX is found to be the most stable phase, due to having the least total interaction energy. Also, α-HMX is more stable than δ-HMX. An increase in the sublimation enthalpy with the size of the nanoparticle can be seen.     

Self-energy in conducting molecular crystals

I have calculated the self-energy, i.e. the effect of the electron-phonon interaction on the electronic energy levels, to second order of perturbation theory for conducting 1-d molecular crystals. The resulting level shifts should have important effects above ～100K on the density of states and properties dependent on it. Evaluating the shifts for tetrathiafulvalene tetracyanoquinodimethane, (TTF) (TCNQ), with the known phonon frequencies and coupling constants, I can explain the “anomalous” temperature variation of the magnetic susceptibility.     

Electrostatic interactions in molecular crystals

Lattice dynamics calculations for nitrogen and carbon dioxide have been carried out for empirical intermolecular potentials which incorporate accurate representations of the molecular charge distributions. In the case of nitrogen, the higher order moments are found to make large negative contributions to the harmonic librational frequencies. The differences between nitrogen and carbon dioxide and the implications for other crystals are discussed.     

Nonradiative relaxation processes in molecular crystals

Internal conversion is the dominant relaxation channel from higher lying excited states in molecular crystals and involves the transfer of energy from the electronic system to the lattice. In this work, we present results from simulations of the nonradiative relaxation process with an emphasis on both intra- and interband transitions. We find the internal conversion process to be strongly nonadiabatic and the associated relaxation time in the case of large energy excitations to be limited by the transitions made between states of different bands.     

Anomalous light absorption in molecular crystals

The method of successive approximations is used to obtain a solution for an infinite chain of nonlinear coupled equations for the Fourier transform of the retarded two-time Green’s function in a system in which the electron vibrational interactions of the lattice photons and of the intramolecular photons are included in the terms of the Hamiltonian that are diagonal in the electron operators. A unique subsequence, which provides a finite contribution to the integral intensity of the absorption spectrum, is extracted from the infinite series and summed. The resulting formulas are used to classify the excited states of a crystal and interpret the cases of anomalous light absorption, when the absorption intensity becomes inversely proportional to the Franck-Condon overlap integral. Zh. éksp. Teor. Fiz. 112, 1021–1040 (September 1997)     

Torsional lattice vibrations in molecular crystals

Atom-atom interaction potential energies derived from various sources have been used to calculate the torsional lattice vibration frequencies of crystalline benzene. The results are compared with available experimental data. The usefulness of this type of model potential is discussed.     

Singlet triplet-triplet-triexcitons in organic molecular crystals

A theory on the formation of triexcitons consisting of one singlet exciton and two triplet excitons is developed by considering exciton-exciton dynamical interactions. The conditions for the existence of the singlet-triplet-triplet-triexciton bound states are established and discussed.     

Habit of long-chain molecular crystals

The quantitative analysis of the temperature dependence of the heat capacity of molecular crystals with chains of different lengths was performed using the theory of diffuse first-order phase transitions. The same chemical structure of the “core” of molecular crystals of {CH₃(CH₂)_nCH₃} normal paraffins, {COH(CH₂)_nCOH} diols, {CH₃(CH₂)_nCOH} normal alcohols, and {CH₃(CH₂)_nCOOH} saturated carboxylic and {COOH(CH₂)_nCOOH} dicarboxylic acids enabled the comparative analysis of phase transition parameters.     

Kinetics of exciton-exciton annihilation in molecular crystals

The theoretical investigation of the time dependence of the concentration of incoherent excitons formed by pulsed pumping is carried out. The basis of this investigation is a system of an infinite number of coupled equations for multiparticle distribution functions. Using the Kirkwood method of uncoupling, this system is reduced to a system of differetial equations for one- and two-particle distribution functions. The algorithm for calculating these equations by means of an electronic computer is developed.A simple phenomenological differential equation for exciton density has been obtained. The solution of this phenomenological equation coincides with good accuracy with the solution obtained by numerical integration of the above-mentioned system of differential equations for one- and two-particle distribution functions.     

Incoherent singlet-triplet biexcitons in organic molecular crystals

The conditions of the existence, the energy spectrum of internal motion and the dynamics of propagation as a whole of incoherent singlet-triplet biexcitons in organic molecular crystals is studied theoretically. It is assumed that the group velocity of the propagation of singlet excitons is very high with respect to that of triplet excitons.     

On the thermoluminescence build-up in molecular crystals

Measurements were performed on the thermoluminescence build-up of phenanthrene crystals UV excited at liquid N₂ temperature. Kinetic investigations show that the passage of electrons from excited molecules to traps takes place by means of a direct process, without the intervention of the conduction band states. Arguments are advanced to point out the possible role of Frenkel excitons in this process.     

Polaron and molecular states of a spin–orbit coupled impurity in a spinless Fermi sea

We investigate the polaron and molecular states of a fermionic atom with one-dimensional spin–orbit coupling(SOC)coupled to a three-dimensional spinless Fermi sea. Because of the interplay among the SOC, Raman coupling and spinselected interatomic interactions, the polaron state induced by the spin–orbit coupled impurity exhibits quite unique features. We find that the energy dispersion of the polaron generally has a double-minimum structure, which results in a finite center-of-mass(c.m.) momentum in the ground state, different from the zero-momentum polarons where SOC are introduced into the majority atoms. By further tuning the parameters such as the atomic interaction strength, a discontinuous transition between the polarons with different c.m. momenta may occur, signaled by the singular behavior of the quasiparticle residue and effective mass of the polaron. Meanwhile, the molecular state as well as the polaron-to-molecule transition is also strongly affected by the Raman coupling and the effective Zeeman field, which are introduced by the lasers generating SOC on the impurity atom. We also discuss the effects of a more general spin-dependent interaction and mass ratio. These results would be beneficial for the study of impurity physics brought by SOC.     

Charged Frenkel biexcitons in organic molecular crystals

It is known that the energy of the lowest electronic transition in the neutral molecules of anthracene, tetracene, and other polyacenes is blue-shifted in comparison with the corresponding transition energy in univalent molecular ions. This effect in a molecular crystal may be responsible for the attraction between a molecular (Frenkel) exciton and a charge carrier. Due to this attraction, a bound state of Frenkel exciton and free charge (charged Frenkel exciton) may be formed [5]. As we demonstrate below, the same mechanism can be responsible for the formation of a charged biexciton (bound state of two Frenkel excitons and a charge carrier). A one-dimensional lattice model is used which corresponds to J aggregates and is also a good approximation for quasi-one-dimensional crystals. Calculations are performed for molecular crystals like tetracene, where the exciton band at low temperature is much narrower than the band of the charge carrier.     

Nonlinear molecular scattering of light in crystals

Nonlinear molecular scattering of light in crystals KDP and LiIO₃ is studied experimentally and theoretically. Spectral distributions of scattered radiation intensity are obtained. Experimental results are compared with theory.     

Ultrafast relaxation in quasi-one-dimensional organic molecular crystals

We present a comprehensive study of ultrafast relaxation properties of optical excitations in thin films of quasi-1D stacked organic materials PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide) over five decades of time. Pump-probe experiments reveal excitonic intraband relaxation time constants of 65 fs for MePTCDI and 100 fs for PTCDA. The initial time-resolved luminescence anisotropy is consistent with the exciton model of Davydov-split states. The subsequent decay of the anisotropy can be explained with a thermally activated exciton hopping process. A full understanding of the pump-probe experiments calls for an explanation beyond the models presently available.