Phosphorescence from the two triplet states of p-benzoquinone and toluquinone vapour

Molecular formulae for the isotropic and anisotropic scattering are derived by considering the internal field acting upon a molecule within a dense medium and its fluctuation caused by density fluctuations. The anisotropic scattering formula of classical theory is retained but modified by the factors of the internal field. The new formula for the isotropic scattering depends sensitively on the principal values of the optical polarizability tensor and on the parameters describing the anisotropic internal field. Assuming the internal field to be given by the semimacroscopic approach of the Onsager-Scholte model with an ellipsoidal cavity, comparison of the calculated isotropic Rayleigh factor with the experimental value allows a prediction of the degree of the cavity anisotropy for different liquid densities.     

Electron spin polarization of doublet state species due to interaction with excited triplet states in single crystals

Doublet states species trapped in crystalline solids show transient spin polarized EPR spectra if the crystal is illuminated by visible or UV light. The spin polarization is accounted for by the interaction of the doublet species with photoexcited triplet states. The mechanism of the process producing the spin polarization is examined and some experimental examples are discussed. The analysis of the time evolution of the transient variation of the EPR signal allows the measurements of the spin lattice relaxation time and in some cases of the diffusion rate of mobile triplet excitations in the crystal lattice.     

聚乙炔激子单线态和三线态的稳态性质与零磁场判据

在Su-Schrieffer-Heeger(SSH)模型基础上,计入弱电子关联的影响,研究聚乙炔单线态和三线态两类激子的行为.与单线态相比,三线态激子能级自旋简并丧失,能隙增大,电荷密度振荡局域分布以及自旋密度波产生,三线态激子吸收谱的相应特征峰发生~0.1 eV蓝移并且强度增加~40%.这些差异为区分两类激子提供了磁共振实验之外的判据.     

The state of the surface of martensitically transforming cobalt single crystals

The surface structure of single crystal cobalt has been monitored by low-energy electron-diffraction (LEED) in order to determine the existence of residual surface phases or “frozen in” surface embryos responsible for the nucleation of the martensitic transformation in cobalt. It has been determined that no such residual surface embryos exist and that the structural phase of the surface is equivalent to the respective allotropic phase of the bulk. The transformation at the surface is also observed to be strongly first-order with significant temperature hysteresis.     

Instability of the lowest triplet state of benzene under environmental perturbations : zero-field splitting and radiative properties of the spin components

Coupling is considered between the lower ³ B _1u and ³ E _1u states of benzene under the combined influence of substituents and the crystal field. A model is derived in which the value of the E.S.R. parameter E, the orientation of the zero-field axes relative to the molecular frame and the radiative properties of the upper two spin components of the lowest triplet state are interpreted in terms of an electronic wavefunction.

With the model the reduced intensity of the 0-0 band relative to the 8₁ ⁰ band in the phosphorescence spectra of a number of benzene-like systems is explained. Further the radiative properties of the zero-field spin components of toluene in a benzene host are interpreted; in this system the angle between the in-plane zero-field axes and the molecular axes appears to be 74° (±6°).     

白藜芦醇顺反异构体及第一三重激发态结构的理论研究

在B3LYP/6-311++G(d,p)水平对白藜芦醇顺反异构体及第一三重激发态进行了结构优化、频率计算和自然键轨道(NBO)分析。在MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p)水平比较了白藜芦醇顺反异构体的能量。反式白藜芦醇整个分子呈现平面结构,顺式白藜芦醇两苯环之间存在约30º扭角。第一三重激发态中两苯环几乎处于互相垂直的关系,C7-H5与C8-H6键也是几乎互相垂直的关系。顺式和反式白藜芦醇C7-C8的σ键成键情况分别为sp1.53-sp1.53和sp1.59-sp1.60,C7与C8各自提供p轨道形成π键,即形成C7=C8双键。三重态中,C7-C8的成键情况为sp1.92-sp1.89,没有p-p π键,C7、C8均还有一个2p轨道未参与杂化,NBO分析证实C7、C8的各自剩下的2p轨道均几乎独立形成了高能量的反键轨道,分别垂直于单羟基和双羟基苯环,C7-C8键长明显长于白藜芦醇顺反异构体。顺式白藜芦醇比反式白藜芦醇的自由能高约1.3-2.5kcal/mol,反式构型是热力学稳定构型。TD/DFT方法,B3LYP/6-311++G(d,p)水平计算得反式和顺式白藜芦醇最强紫外吸收峰分别在330nm和319nm。     

白藜芦醇顺反异构体及第一三重激发态结构的理论研究

在B3LYP/6-311++G(d,p)水平对白藜芦醇顺反异构体及第一三重激发态进行了结构优化、频率计算和自然键轨道(Natural Bond Orbital,NBO)分析.在MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p)水平比较了白藜芦醇顺反异构体的能量.反式白藜芦醇整个分子呈平面结构,顺式白藜芦醇两苯环之间存在约30o扭角.第一三重激发态中两苯环几乎处于互相垂直的关系,C7-H5与C8-H6键也是几乎互相垂直的关系.顺式和反式白藜芦醇C7-C8的σ键成键情况分别为sp1.53-sp1.53和sp1.59-sp1.60,C7与C8各自提供p轨道形成π键,即形成C7=C8双键.三重态中,C7-C8的成键情况为sp1.92-sp1.89,没有p-pπ键,C7、C8均还有一个2p轨道未参与杂化,NBO分析证实C7、C8的各自剩下的2p轨道均几乎独立形成了高能量的反键轨道,分别垂直于单羟基和双羟基苯环,C7-C8键长明显长于白藜芦醇顺反异构体.顺式白藜芦醇比反式白藜芦醇的自由能高约1.3-2.5 kcal/mol,反式构型是热力学稳定构型.含时密度泛函方法(Time-Dependent Density Functional Theory,TD-DFT)方法,B3LYP/6-311++G(d,p)水平计算得反式和顺式白藜芦醇最强紫外吸收峰分别在330 nm和319 nm.     

Phase transition and triplet exciton mobility in anthracene—tetracyanobenzene charge-transfer single crystals

Triplet-Excitons in a mixed 1 : 1 anthracene—tetracyanobenzene single crystal are established by ESR-spectrocopy. These excitons (lifetime 0.5 msec) are characterized by a hopping frequency of v = 1.3 × 10⁶ K at 300K. The temperature dependence of the ESR line shape indicates a phase transition of the crystal at T_t = 205K. Above and below the phase transition the exciton mobility is nearly temperature independent.     

Direct imaging of anisotropic exciton diffusion and triplet diffusion length in rubrene single crystals

We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.     

The lowest triplet state of Zn porphin

The dynamics of populating and depopulating the phosphorescent triplet state of Zn porphin in an n-octane matrix is investigated by microwave induced delayed phosphorescence experiments at 1·2K. Values for the absolute rates of decay of the three spin levels are given and also the relative steady-state populations, radiative decay rates and probabilities for entry through intersystem crossing. Further, it is established that the phosphorescence is inplane polarized. The results are compatible with the decay modes expected for a lowest vibronic state that deviates from D ₄ symmetry by combined action of the Jahn-Teller effect and crystal field anisotropy.     

Stability of the single-domain state in single crystals of triglycine sulphate

The stability of the single-domain state in single crystals of triglycine sulfate is investigated using the Barkhausen effect. Barkhausen jumps are detected upon invariable macroscopic polarization. The parameters of the observed Barkhausen jumps are estimated and compared.     

Features of the excitonic optical reflection curves of GaAs crystals

The excitonic optical reflection curves are investigated for GaAs crystals subjected to a surface-sensitive electron bombardment. The observed features of the curves can be explained on a qualitative level by using an extremely simple square-well approximation of the subsurface excitonic potential. Fiz. Tverd. Tela (St. Petersburg) 39, 610–612 (April 1997)     

Superposition-model investigations of the zero-field splittings and their stress and temperature dependence for Mn in guanidinium aluminum sulfate hexahydrate

The zero-field splittings of sites I and II for Mn²⁺ in guanidinium aluminum sulfate hexahydrate (GASH) are calculated from the superposition model by using the normal intrinsic parameter ₂(R₀) obtained from Mn²⁺ ions in many oxygen octahedra. The results are in good agreement with the experimental findings. Based on this, the local compressibilities and local thermal expansion coefficients in the vicinity of the impurity ion Mn²⁺ have been studied from the stress and temperature dependence of zero-field splitting and are found to be half of the host ones. The reasonableness of these local values is discussed.     

The spin-lattice relaxation mechanism in the phosphorescent triplet state

A new mechanism is proposed for spin-lattice relaxation (SPL) in the phosphorescent triplet state of a molecule that has no inversion point. The mechanism is based on the fact that the internal spin-orbital interaction in a molecule allows the transition between spin sublevels of the triplet induced by the variable crystal field of the lattice.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 27–31, May, 1978.     

Color centers in the ionized and in the triplet state —Z 2 centers in ytterbium-doped KCl crystals

Z ₂ centers in Yb²⁺-doped KCl crystals are investigated by optical absorption experiments and by ESR experiments using the optically excited triplet state. Two mainZ ₂ center configurations have been found. One of them shows (100) axial symmetry and is optically orientable in the crystal. The other one possesses (110) symmetry elements. The measured fine structure constants are explained partly by dipole-dipole interaction of the two electronic magnetic moments partly by spin orbit interaction due to the Ytterbium impurity. The structure of the optical absorption spectra is attributed to as ²–s d charge transfer transition affected by the combined effect of crystal field and spin orbit interaction.     

Properties of an excitonic insulator in the superconducting state

The properties of an excitonic insulator with embedded (nondissipative) current are studied using the self-consistent field approximation, in which the wavefunction of the system has the form of the known Bardeen-Cooper-Schrieffer trial function with time-dependent coefficients; the equations for these coefficients are derived. Such a formulation holds for the homogeneous case (in the absence of a coordinate dependence). We consider two problems: (i) time evolution of the system in the case when an embedded current exists at the initial instant; and (ii) the response of the system to an abrupt perturbation (the vector potential changes jumpwise from zero to a certain finite value). In both cases, the state of the system depends on time, but some characteristics (e.g., undamped current) tend to a constant value. For a weak perturbation, the system behaves as an insulator. If the perturbation is not small (on the order of the gap in the spectrum), nonlinear effects lead to substantial differences: a certain part of the embedded current is preserved in the former case, while the initial current in the latter case acquires a certain addition.     

The lowest triplet state of the biexciton may be bound

Using Haken's hamiltonian and a simple variational wavefunction, the (1_σg, 1_σu)³Σ_u state of the biexciton in a polar crystal is found to be stable with respect to dissociation into two ground-state excitons, for ceratain range of (realistec) material parameters, unlike the corresponding state in the H₂ molecule.     

Investigation of the excitonic structure in the photoconductivity spectra of CdS crystals

The changes in the low-temperature photoconductivity spectra of CdS crystals under the action of external influences are investigated. The high sensitivity of the fine (excitonic) structure of the spectra to changes in the conditions for the recombination of nonequilibrium carriers near the surface and in the bulk of the semiconductor is demonstrated. Fiz. Tverd. Tela (St. Petersburg) 40, 941–943 (May 1998)