Manifestations de fréquence spatiale dans les spectres de dichroïsme et de biréfringence des gaz

A new version of the pump and probe technique at low excitation level is reported, measuring ultrafast dichroism at variable probe pulse frequencies. The method allows to distinguish rotational relaxation times in different electronic states and also gives information on vibronic relaxation of dye molecules. Experimental results on phenoxazone 9 and rhodamine 6G presented.     

Configuration vibronic mixing for a neutral vacancy in silicon and diamond

Configuration vibronic mixing is considered for a fully symmetric Jahn-Teller electronic term with orientation-degenerate terms (due to the distortion direction) including a correlation correction in a single-open-shell approximation. The approach is nonempirical and involves only linear vibronic coupling. The adiabatic potential is a multiwell one, because the different configurations involved in the exact Jahn-Teller term have different vibronic coupling with a lattice distortion. The stabilization energy, the frequencies of local lattice vibrations, the vibronic coupling parameter, and the energy barriers to migration and to distortion-axis reorientation are estimated for a neutral vacancy in silicon and diamond with allowance made for configuration vibronic coupling. The estimates agree with the results obtained by different experimental and theoretical methods for a wide range of properties associated with the Jahn-Teller effect.     

Spectroscopic effects of conical intersections of molecular potential energy surfaces

A simple vibronic coupling model involving two electronic states and two vibrational modes is considered. The model is based on harmonic diabatic potentials and linear coupling of the diabatic electronic states. It is shown that the adiabatic electronic potential energy surfaces exhibit, in general, a conical intersection. The well known E × E and E × B Jahn-Teller problems are contained as special cases. Using numerical methods the optical absorption spectrum is calculated exactly. Extremely complex vibronic spectra are obtained when the conical intersection occurs within the Franck-Condon (FC) zone. The exact vibronic spectra are compared with spectra calculated in the adiabatic and FC approximation. The genuine spectroscopic effects of conical intersections are revealed by a comparison with the results of standard one-dimensional vibronic coupling calculations. The presence of a conical intersection limits the applicability of the adiabatic and FC approximations much more strongly than in the one-dimensional case. The upper adiabatic electronic state is strongly affected by non-adiabatic coupling even when the point of intersection lies outside the FC zone. The relevance of these results for the calculation of molecular electronic spectra is briefly discussed.     

Improvement of the parametric method of the theory of vibronic spectra of polyatomic molecules: Absorption spectra and the structure of butadiene, hexatriene, and octatetraene in the excited state

The structure of the excited states and absorption spectra of butadiene, hexatriene, and octatetraene are calculated by the parametric method of the theory of vibronic spectra using models of the first-and second-order approximations. It is shown that these molecular models adequately reflect the molecular structure and allow one to predict quantitatively the shape and fine vibrational structure of the absorption spectra. When passing to the second-order approximation, only two additional (angular) parameters are used. These parameters are transferable in the series of polyenes. Compared to the first-order approximation model, the second-order approximation model more accurately takes into account the angular deformations of polyenes upon their excitation and describes the intensity distribution in the vibrational spectrum, including weak lines. In addition, the calculations also quantitatively predict spectral variations in the molecular series. The parametric method is more efficient for modeling polyatomic molecules in the excited states and their vibrational spectra compared to other semiempirical and ab initio methods.     

De-excitation process and hot luminescence in the FA(type I) center in KCl:Na

The de-excitation process of F_A(type I) centers in KCl:Na has been investigated by measuring the hot luminescence spectrum from optically excited F_A centers with time-resolved spectroscopy. The experimental results are analyzed by using a model that describes a time evolution of the phonon wave packet during the vibronic relaxation process from the Franck-Condon state to a relaxed excited state. From the analysis of the experimental data, information on the vibronic mixing between 2p and 2s states, whose magnitude varies during the relaxation process, and the adiabatic potential energy curves of 2s and 2p states are extracted. The present results are compared with the already known ones of the F_A(type II) centers.     

Nuclear spin-lattice relaxation in transition metal compounds with local tetragonal symmetry

We report on the various contributions to the total spin-lattice relaxation rate in metallic materials with local tetragonal symmetry. The analytical formulae are given in the tight-binding approximation. The calculations show the relation between various partial electron densities of states and corresponding contributions to the relaxation rates. The presented formulae can be used to compare theoretically calculated electron band structure parameters with those obtained from NMR spin-lattice relaxation time measurements.     

Resonant inelastic scattering of an X-ray photon by a linear molecule

The double differential cross section of resonant inelastic scattering of a linearly polarized X-ray photon by a spatially oriented HF molecule in a gas phase is theoretically described in the energy range of the ionization threshold of the deep molecular orbital 1σ. The effects of radial relaxation of the wave functions of the core and excited scattering states in fields of the formed core vacancies, the vibronic effects, and the effects of Auger and radiative decays of the vacancies are taken into account in the nonrelativistic Hartree-Fock approximation. The results of the calculations are predictive and agree well with the results of the experimental measurements of the Kα emission spectrum of the HF molecule at an incident photon energy of 2 keV, which considerably exceeds the energy of the 1σ ionization threshold.     

A theoretical analysis on the vibronic spectra with mass analyzed threshold ionization spectroscopy

We have theoretically studied the absorption vibronic spectra with the resonance two-photon (R2P) and non-resonance two-photon (NR2P) mass analyzed threshold ionization (MATI) spectroscopy. The theory uses the inverse Born-Oppenheimer approximation (IBOA) to establish a proper basis set. To analyze the MATI vibronic spectra, we have calculated the Franck-Condon factors involved in the vibronic transitions. Several experimental spectra are analyzed using this theory with emphasis on the importance of the resonance intermediate states. The long vibrational progression in a MATI spectrum can be partly attributed to the result of including the anharmonic correction in the calculated Franck-Condon factors. The experimentally observed isotope effect is also analyzed.     

在群论框架下电子三重态与声子耦合的理论研究

本文基于绝热近似和群论导出了电声耦合系统的哈密顿量的一般形式,讨论了电声耦合系统中的电子算符和活跃的声子模式. 应用幺正平移变换和能量最小化方法,进一步计算了正四面体群下T*(e+t₂)杨-泰勒系统中的激发态能量,从对称性的角度分析了T₁电子态的能级分裂以及晶格体系的对称性破缺,得出了对称性的破缺方式和电声耦合系统密切相关的结论. 结果表明:通过群论与对称性分析完全可以定性地解释由于电声耦合所造成的简并电子 关键词： 电声耦合 杨-泰勒畸变 活跃的杨-泰勒声子模式 电声耦合哈密顿量     

First-principles calculation of alloy scattering in Ge(x)Si(1-x)

First-principles electronic structure methods are used to find the rates of intravalley and intervalley n-type carrier scattering due to alloy disorder in Si(1-x)Ge(x) alloys. The required alloy scattering matrix elements are calculated from the energy splitting of nearly degenerate Bloch states which arises when one average host atom is replaced by a Ge or Si atom in supercells containing up to 128 atoms. Scattering parameters for all relevant Delta and L intravalley and intervalley alloy scattering are calculated. Atomic relaxation is found to have a substantial effect on the scattering parameters. f-type intervalley scattering between Delta valleys is found to be comparable to other scattering channels. The n-type carrier mobility, calculated from the scattering rate using the Boltzmann transport equation in the relaxation time approximation, is in excellent agreement with experiments on bulk, unstrained alloys.     

捕光复合物中起源于电子-振动耦合的长时间相干

We theoretically investigate the evolutions of two-dimensional, third-order, nonlinear pho-ton echo rephasing spectra with population time by using an exact numerical path integral method. It is shown that for the same system, the coherence time and relaxation time of excitonic states are short, however, if the couplings of electronic and intra-pigment vibra-tional modes are considered, the coherence time and relaxation time of this vibronic states are greatly extended. It means that the couplings between electronic and vibrational modes play important roles in keeping long-lived coherence in light-harvesting complexes. Particularly, by using the method we can fix the transition path of the energy transfer in bio-molecular systems.     

二维合金系统的有序动力学研究

采用主方程方法在对概率近似下对三元fcc结构晶体在(001)方向的一个层面的有序无序动力学过程进行了研究.计算了长程序参量(LRO)和短程序参量(SRO)随时间的演化过程.在系统从完全无序态弛豫到平衡态的过程中存在各种瞬态有序相.瞬态相有着不同的特点,如序参量(LRO,SRO)瞬态相曲线形状不同等.产生它们的原因有不同的原子特征迁移时间、原子间最近邻和次近邻相互作用差异、短程相关性和长程相关性的差异对弛豫过程的影响. 关键词： 对概率近似 瞬态有序相 长程序参量(LRO) 长程相关性     

The influence of geometry on the vibronic spectra of quantum aggregates

A study is presented of the localisation of excitonic states on extended molecular aggregates composed of identical monomers arising, not from disorder due to statistical energy shifts of the monomers, induced by environmental interactions (Anderson localisation), but rather simply due to changes in the orientation and geometrical arrangement of the transition dipoles. It is shown further that such small changes nevertheless can have a drastic effect on the shape of the vibronic spectrum of the aggregate. The vibronic spectra are calculated using the ‘coherent exciton scattering’ (CES) approximation whose derivation we generalise to be applicable to aggregates of arbitrary size and geometry.     

The model of ultrafast light-induced insulator-metal phase transition in VO2

Ultrafast light-induced insulator-metal phase transitions (PT) in VO₂ thin films was studied with use of a pump-probe technique. The theoretical and experimental study of PT kinetics shows that the PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of PT is proposed. The main channel for the ultrafast light-induced PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. During this process an equilibrium local distortion occurred. According to the proposed model the experimental observation of the drastic temperature- and pump power- dependent relaxation processes could be interpreted.     

Predissociation and the vibrational spectrum of molecular oxygen in an intense laser field. A Schumann-Runge band interval

The probabilities of predissociation and vibronic transitions between the states of the oxygen molecule in the Schumann-Runge band in the presence of a strong laser field are examined. The interaction of the molecule with the laser field is described using the rotating wave approximation. The predissociation probabilities for the avoided crossing of two adiabatic molecular terms are calculated within the framework of the Landau-Zener model. The energies of the vibrational states in the laser field are determined by diagonalization of the adiabatic Hamiltonian in the harmonic oscillator basis set. The predissociation thresholds are determined and the Franck-Condon factors are calculated as functions of the frequency and intensity of the external electromagnetic field.     

A multi-frequency EPR spectroscopy approach in the detection of boson peak excitations

The influence of boson peak (BP) excitations on low-temperature spin-lattice relaxation rate of a paramagnetic center embedded in a glassy matrix is investigated in the context of multi-frequency electron paramagnetic resonance (EPR) detection. In the theoretical analysis, the transition rate of spin one-half in the presence of a phonon field is calculated within the approximation of Fermi's golden rule. Several phonon densities of states are compared, among which one originating from a model of quasi-localized vibrations has been introduced into electron spin relaxation formalism for the first time. The respective frequency dependencies of spin-lattice relaxation rates are predicted which should lead to observable effects of BP modes if a multi-frequency study at very low temperatures is performed.     

Response signal in a femtosecond pump-probe experiment in a condensed medium with account of the memory induced by the relaxation medium

For a model molecular system with one vibrational degree of freedom and three electronic states coupled by pump and probe laser pulses in a condensed medium, the response signal in a femtosecond pump-probe experiment is calculated. The potential curves of all three states are described by the Morse potential. Calculations are performed using two qualitatively different approaches to describing the medium-induced relaxation: with memory of the relaxation process and without memory (Markovian approximation). The temporal evolution of the vibrational wave packet in the intermediate electronically excited state is described using a master equation for the density matrix of the molecular system, which is derived within the framework of the Nakajima-Zwanzig formalism. It is demonstrated that, at short delay times, when the proposed approach is applicable, taking into account memory effects can substantially change the form of the pump-probe experiment signal in comparison with the signal calculated in the Markovian approximation.     

The effective Hamiltonian for the Renner-Teller effect

A derivation of the effective vibronic Hamiltonian for a linear triatomic molecule in a Π electronic state within the harmonic approximation is presented. In addition to the well-established vibrational and Renner-Teller coupling terms, there is a small term which has been overlooked before. The form of this term is derived and the parameter involved, g_K, is related to other observable quantities. Several experimental examples are presented which demonstrate that the extra term is needed to reproduce the vibronic levels in Π electronic states, the contribution being of the order of a few cm^?1.     

Autowave instability in refractive crystals

The intensities of vibronic lines are experimentally measured in fluorescence and fluorescence excitation spectra of jet-cooled anthracene. An original method is developed for calculating geometrical parameters of benzene hydrocarbons in the ground and excited electronic states. Using these parameters, the intensities of vibronic lines in fluorescence and absorption spectra of anthracene are calculated in the Franck-Condon approximation taking into account the mixing of all the twelve normal coordinates of totally symmetric vibrational modes. After correction for the quantum yield of fluorescence, good agreement is obtained between the calculated line intensities in the absorption spectrum and the measured line intensities in the fluorescence excitation spectrum. Based on these data, a new assignment of the lines in the fluorescence excitation spectrum corresponding to totally symmetric modes 7 and 8 is suggested.