Statistical analysis of spectra of single impurity molecules and dynamics of disordered solids: II. Manifestation of interaction of two-level systems with impurity molecules at different distances

Manifestations in the spectra of impurity chromophore molecules in a disordered solid matrix of the interaction of these molecules with two-level systems (TLSs) of the matrix are analyzed. The characteristic distances within the limits of which the interaction between TLSs and an impurity molecule leads, in the main, to either splitting or broadening of the spectrum, as well as the distance beginning with which this interaction is practically undetectable, are determined. Information about the effective value of the minimal distance between impurity molecules and TLSs, which is caused by nonzero sizes of the molecules and the TLSs, is presented. The analysis is performed by a statistical processing of experimental and model spectra of single tetra-tert-butylterrylene molecules in amorphous polyisobutylene at a temperature of 2 K, when the contribution of quasi-local low-frequency vibrational modes of the matrix to these spectra can be neglected.     

Low-temperature dynamics of amorphous polymers and evolution over time of spectra of single impurity molecules: I. Experiment

The optical dynamics of a doped amorphous system, tetra-tert-butylterrylene in amorphous polyisobutylene, has been experimentally studied by the spectra of single impurity molecules measured at temperatures of 2, 4.5, 7, and 15 K. The study of the temporal evolution of the fluorescence excitation spectra of the molecules under consideration made it possible to unambiguously establish the individual identity of the spectra of particular molecules and to analyze their multiplet structure. Repeated scanning of a selected spectral range with subsequent summation of the data made it possible to considerably reduce the errors that arise upon single scanning of the spectra of single molecules. The majority of the spectral trails detected were in agreement with the model of two-level systems. Jumps of spectral lines due to transitions in such systems were observed at all temperatures.     

Manifestation of tunneling TLS dynamics of a polymer matrix in single-molecule fluorescence blinking

Blinking (stochastic intermittence) of fluorescence is a quite common phenomenon that accompanies the emission of single quantum objects-organic chromophore molecules, quantum dots, and nanocrystals. It is demonstrated that fluorescence blinking of single organic molecules embedded into a polymer matrix including the occurrence of “grey” states is due to tunneling transitions in the two-level systems (TLSs) of the matrix. The repeated registration of fluorescence excitation spectra of single molecules (SMs) is used for our analysis. The statistics of fluorescence blinking of an SM is directly related to conformational changes (tunneling transitions in TLSs) in its immediate vicinity. Individual parameters of the corresponding elementary excitation are also determined.     

Statistical analysis of spectra of single impurity molecules and dynamics of disordered solids: I. Distributions of linewidths,moments, and cumulants

A review of recent results of studying the low-temperature dynamics of a disordered solid-state medium that were obtained by the method of single molecule spectroscopy (SMS) is presented. The studies were carried out with the use of the new approach of the statistical analysis of a large number of spectra of single molecules embedded into a medium under study as a microscopic spectral probe. The measurements were performed in the range 2–7 K. The use of SMS allowed us to avoid the information losses connected with the averaging over an ensemble of impurity molecules that is inherent in conventional spectral methods. For the example of the system studied (amorphous polyisobutylene doped with molecules of tetra-tert-butylterrylene), it was shown experimentally that, at low temperatures, the spectra of single impurity molecules in disordered media obey the Levy statistics. New microscopic information on the interaction of two-level systems and quasi-local low-frequency vibrational modes of the matrix with impurity molecules in a disordered medium was obtained.     

A single-molecule study of polycrystalline microstructure by fluorescence polarization spectroscopy

The structure of n-hexadecane polycrystalline matrices is examined with single molecule fluorescence spectroscopy at a temperature of 1.7 K. Some single chromophores exhibit spectral dynamics during experiments: their resonances either jump between two frequencies or split into two components. By means of polarization measurements, we investigate the molecular spectra in order to distinguish between the case of two spatially unresolved molecules and the case of chromophore — two level system (TLS) pair. For closely spaced molecules two-dimensional Gaussian function fitting is applied to extract the spatial coordinates of the molecules. For TLS-chromophore systems, parameters of the TLSs are estimated.     

K3C60在200K附近的取向相变机理

研究了K₃C₆₀单晶薄膜在200K附近的导带结构.样品温度为190K时,同步辐射角分辨光电子谱能够观察到[111]方向有规律的能带色散.而在220K附近色散不存在.这一实验结果与K₃C₆₀在200K存在取向相变相符合.用反铁磁Ising模型对实验结果进行了分析.结果表明,K₃C₆₀在200K的相变是由低温下的一维无序取向结构转变为200K以上的双取向结构畴与无序分子(约占40 关键词： 3C₆₀')" href="#">K₃C₆₀ 取向相变机理     

On the stability of parameters of molecular models in the parametric method of the theory of vibronic spectra

We have studied the stability of the system of parameters used in the parametric method of the theory of vibronic spectra of polyatomic molecules with respect to whether the initial structural-dynamic model of the ground state of a molecule is formed either by the fragmentary method or by direct quantum-chemical calculation. Modeling of excited states and spectra of the stilbene-h ₁₂ and stilbene-d ₁₂ molecules showed that, although the initial models of the ground states are significantly different, calculated changes in the geometry that occur upon excitation and electronic-vibrational spectra of the models are close and quantitatively agree with experiment. This indicates that the parameters of the method are stable and are applicable for performing predictive calculations of vibronic spectra based on models of the ground states created by different methods. We show that the fragmentary method has a considerable advantage, since models created by this method take into account the continuity of the structure in series of related compounds and calculations can be easily performed for molecules of arbitrarily high complexity. We show that direct quantum-chemical calculations are important in the case of molecules with unknown structural-dynamic models of fragments in the ground states.     

K_3C_(60)在200K附近的取向相变机理

研究了K3C6 0 单晶薄膜在 2 0 0K附近的导带结构 .样品温度为 190K时 ,同步辐射角分辨光电子谱能够观察到[111]方向有规律的能带色散 .而在 2 2 0K附近色散不存在 .这一实验结果与K3C6 0 在 2 0 0K存在取向相变相符合 .用反铁磁Ising模型对实验结果进行了分析 .结果表明 ,K3C6 0 在 2 0 0K的相变是由低温下的一维无序取向结构转变为2 0 0K以上的双取向结构畴与无序分子 (约占 40 % )的混合     

Angle-resolved UV photoelectron spectroscopy of ethylene and benzene on nickel

A review of results obtained by Angle-Resolved UV-Photoelectron Spectroscopy (ARUPS) using linearly polarized synchrotron radiation is presented for two model systems, ethylene/Ni and benzene/Ni. It is shown that for these systems detailed conclusions concerning adsorbate/substrate and adsorbate/adsorbate interactions can be derived from ARUPS spectra using symmetry selection rules, and in combination with model calculations. In particular, electronic structure, bonding, orientation and symmetry of the adsorbates in dilute and saturated layers will be discussed. It is shown that at high adsorbate coverages lateral interactions in the adsorbate layer play a dominant role. Steric effects in densely packed layers can lead to a reorientation of the molecules as compared to the orientation of the single molecules. The ARUPS spectra of well ordered, densely packed layers exhibit significant (up to 2 eV) dispersion of the various adsorbate bands and allow detailed conclusions on two-dimensional adsorbate band structures.     

Partial inversion in small molecules: New potentialities for the spectroscopic diagnostics of gases

The formation of vibrational-rotational absorption spectra of small molecules in vibrationally non-equilibrium conditions, with the possibility of manifestation of partial population inversion of levels and negative absorption, is studied. Specific features of the partial inversion in linear and top molecules are analyzed. It is shown that water vapor is the best object among atmospheric gases for the realization of this effect. Formulas for the integrated line intensities and the absorption coefficient of two-and three-level nonequilibrium vibrational systems with a rotational structure are obtained. It is shown that the partial-inversion effect can be used to study weak vibrational-rotational transitions. Concrete calculations for the (000)-(010)-(020) bands of water molecules at different temperatures are made. The frequencies are determined for which negative absorption is the strongest.     

Calculation of the vibronic spectra and structure of methylstyrene in the excited state by the parametric method

Calculations of the excited-state structure and of the absorption and radiation spectra of trans-β-methylstyrene are performed by the parametric method for models of the first and second approximations. Only three of the parameters, being constant in a series of related compounds, are used for molecular fragments. In the calculation, the main regularities observed in electronic spectra are reproduced, and an analysis and interpretation of their fine vibrational structure are carried out. The use of the model of the second approximation with a single additional parameter allows more accurate reproduction of angular deformations of the molecule under excitation and of the characteristic changes observed in the low-frequency spectral region in the series of diphenyl polyenes, stilbene, styrene, and methylstyrene molecules. It is shown that the parameters of the second approximation (just as of the first one) possess the transference property in the homological series of molecules. In modeling complex molecules in excited states and their vibronic spectra, the parametric method is more efficient than the ab initio one. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 148–153, March–April, 2000.     

Non-equilibrium entangled steady state of two independent two-level systems

We determine and study the steady state of two independent two-level systems weakly coupled to a stationary non-equilibrium environment. Whereas this bipartite state is necessarily uncorrelated if the splitting energies of the two-level systems are different from each other, it can be entangled if they are equal. For identical two-level systems interacting with two bosonic heat baths at different temperatures, we discuss the influence of the baths temperatures and coupling parameters on their entanglement. Geometric properties, such as the baths dimensionalities and the distance between the two-level systems, are relevant. A regime is found where the steady state is a statistical mixture of the product ground state and of the entangled singlet state with respective weights 2/3 and 1/3.     

Molecules-in-molecules fragment-based method for the evaluation of Raman spectra of large molecules

We present the first implementation of the evaluation Raman spectra of large molecules using the molecules-in-molecules (MIM) fragment-based method (MIM-Raman). Molecular fragments and associated overlapping subsystems are constructed by cutting the C–C bonds in the large molecule based on the connectivity information and a number-based fragmentation scheme. After saturating the dangling bonds with hydrogen link-atoms, independent energy and Raman frequency calculations are performed on each subsystem. Subsequently, link-atom-related forces, Hessian and polarisability derivative matrix elements are projected back onto the corresponding host and supporting atoms through the Jacobian projection method. In the two-layer model (MIM2), the long-range interactions, absent in the single layer model (MIM1), are taken into account through a second layer at a lower level of theory. The MIM-Raman method is benchmarked on a set of large linear and cage molecules. The MIM extrapolated energy and Raman spectra are compared with the full calculations at B3LYP/6-311G(d,p) or B3LYP/6-311+G(d,p) levels of theory. The benchmark analysis of 21 molecules at MIM2 show an accuracy improvement of 85% in energies, 74% in Raman frequencies and 66% in intensities over MIM1. The implementation and benchmark analysis validates the MIM-Raman model for exploring Raman spectra of large molecules in the future.     

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction ¹⁹⁷Au + ¹⁹⁷Au at 1000 MeV per nucleon. Pairs of ^3,4He and ^6,7Li isotopes and pairs of ^3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τ_lab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.     

共轭聚合物单分子构象和能量转移特性研究

利用基于宽场显微光学系统的单分子散焦成像技术测量了不同构象poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT)共轭聚合物单分子的光物理与动力学特性.通过分析共轭聚合物单分子的荧光轨迹和对应的发射偶极取向变化识别共轭聚合物单分子发光单元,发现延伸构象下的单分子呈现多发色团发光特性,而折叠构象下的单分子保持高效链间能量转移,呈现单个发色团发光特性.共轭聚合物单分子构象对能量转移效率的影响可用于研究基于共轭聚合物的光电器件和分子器件.     

Echo spectroscopy of two-level systems in a Y2SiO5: Pr3+ crystal

The results of an experimental investigation of low-temperature optical spectra and phase relaxation of electronic excitations of Pr³⁺ impurity ions in a Y₂SiO₅ crystal are reported. It is established that at low temperatures spectral lines are broadened by a mechanism that is uncharacteristic for crystals and is due to the interaction of impurity ions with two-level systems. The constants characterizing the interaction of Pr³⁺ impurity ions with phonons and two-level systems are found. Zh. éksp. Teor. Fiz. 115, 704–715 (February 1999)     

Soft-potential model and homogeneous width of spectral lines of impurity centers in molecular amorphous media

A study is made to analyze the possibility of using the soft-potential model in optical investigations of disordered molecular systems with impurities. A procedure is suggested for calculating the temperature dependence of the homogeneous width of a phononless line in amorphous media with impurities within the soft-potential model. A calculation is performed of the temperature dependence of the width of a phononless line (optical dephasing) in an amorphous system of polymethyl methacrylate (PMMA) with an addition of tetratert-butylterrylene (TBT) using the parameters of this system known from the literature. Calculations are performed of the contributions to the width of a phononless line due to the interaction of an impurity with tunneling two-level systems, with thermally activated barrier crossings in double-well potentials, and with quasilocal modes of the matrix. The model calculation results are compared with the experimental data on the photon echo for TBT/PMMA, measured by us in the temperature range from 0.3 to 20 K. It is found that the soft-potential model describes qualitatively correctly the temperature behavior of the homogeneous width of a phononless line. In the temperature range of T < 2 K, where the main contribution to optical dephasing is associated with tunneling two-level systems, the predicted values of phononless line width agree well with the experimental data. At higher temperatures, some difference is observed between the prediction and experimental data, which may be due to the effect of impurity on the formation of quasilocal oscillation of the matrix.     

Mechanisms of formation of near-edge fine structure of <Emphasis Type="Italic">K</Emphasis> X-ray absorption spectra of metallic Cu,Ni, Co (HCP and FCC phases), and Cr

The K X-ray absorption near-edge fine structure spectra of metallic Cu, Ni, Co (hexagonal close-packed and face-centered cubic phases), and Cr have been studied. The experimental spectrum of cobalt has been measured in the high-temperature face-centered cubic phase. The calculations have been performed by the full multiple scattering method in the cluster approximation using the semiempirical muffin-tin potential. The spectra have been calculated for clusters containing from 13 to 935 atoms. It has been demonstrated that the spectra cease to depend on the cluster size for clusters containing about 400 atoms. It has been established that the intensity ratio of two peaks located at the fundamental edge of all the spectra under investigation sharply changes with a variation in the atomic potential strength. The mechanisms responsible for the formation of the near-edge structure of the spectra have been revealed using the results of the model calculations. The studies resulted in good agreement of the calculated spectra with the experiment.     

Molecular orientation entanglement and temporal Bell-type inequalities

We make a detailed study of Bell-type inequalities based on correlations between measurements of continuous observables performed on trapped molecular systems. We show that, in general, when an observable has a continuous spectrum which is bounded, one is able to construct non-locality tests sharing common properties with those for two-level systems. The specific observable studied here is molecular spatial orientation, and it can be experimentally measured for single molecules, as required in our protocol. We also provide some useful general properties of the derived inequalities and study their robustness to noise. Finally, we detail possible experimental scenarios and analyse the role played by different experimental parameters.