Molecular excitons in alkali cyanides

The emission bands of X irradiated Alkali Cyanides were measured at 4.2 K. This emission is associated to the radiative recombination of molecular (Frenkel type) excitons, for the first time observed in these ionic molecular crystals. The nature of the emitting state is discussed and identified as a long lived triplet state.     

Optical detection of triplet excitons in GaS

Optically detected magnetic resonance has been used to investigate exciton recombination in the layered semiconductor GaS. Five triplet exciton resonances have been observed all with the same g-value of g_ex6 = 2.006 ± 0.002 but with different zero field splittings:

$\text{D}{}^{\mathrm{I}}\text{= + 0.013 cm}{}^{\mathrm{?1}}\text{, D}{}^{\mathrm{II}}\text{= + 0.024 cm}{}^{\mathrm{?1}}\text{, D}{}^{\mathrm{III}}\text{= + 0.025 cm}{}^{\mathrm{?1}}$

,

$\text{D}{}^{\mathrm{IV}}\text{= + 0.075 cm}{}^{\mathrm{?1}}\text{, D}{}^{\mathrm{V}}\text{= + 0.010 cm}{}^{\mathrm{?1}}$

. The resonances from the high energy wing are remarkably narrow and we believe that this may be the first observation of resonance from free indirect excitons.     

Non-radiative annihilation of singlet excitons on triplet excitons in organic molecular crystals

The per sec propability is calculated of the non-radiative annihilation of singlet excitons on triplet excitons in organic molecular cyrstals. The influence of the non-radiative annihilation of singlet excitons on triplet excitons on the quenching of crystal fluorescence caused by the radiative annihilation of singlet excitons is studied theoretically on the basis of formulae derived.Na Slovance 2, Praha 8, Czechoslovakia.     

Optical absorption by neutral bound excitons

In this note we determine the oscillator strengths for the dipole absorption of neutral bound excitons in direct gap semiconductors, using our previously obtained 35-term Page and Fraser type wave function, and taking into account the detailed electronic structure as well as the electron-hole exchange interaction.The envelope part of the oscillator strengths varies considerably with the electron-hole mass ratio $\text{σ =}\text{m}{}^1{}_{\mathrm{e}}\text{m}{}^1{}_{\mathrm{h}}$, and is maximum for the (D⁰, X)- complex when σ = 0.4. For typical σ-values (σ? 0.1–0.2), . But when σ approaches zero, the overlapping of the electron and the hole envelope wave functions of the (A⁰,X)-complex decreases progressively so that the oscillator strength also decreases and tends to zero.In the case of zinc-blende materials (T_d) and positive spin-orbit coupling at $\text{k}\text{= 0}$, we confirm that the line strength for transitions to or from $\text{J =}\text{1}\text{2}\text{(Γ}{}_6\text{)}$ or $\text{J =}\text{5}\text{2}\text{(Γ}{}_7\text{+ Γ}{}_8\text{)}$ level of the (A⁰, X)-complex is equal to one quarter of the line strength to or from the $\text{J =}\text{3}\text{2}\text{(Γ}{}_8\text{)}$ level.In the case of CdS, where our computed values are only in qualitative agreement with the experimental values, we discuss the use of the phenomenological result of Rashba.     

Organic electroluminescence channel avoiding triplet excitons

Polarons and bipolarons are main carriers in conducting polymers. It is shown that a bipolaron can open a channel of electroluminescence, which does not involve a triplet exciton, and can enhance the efficiency of electroluminescence. The dynamics of this channel is simulated.     

A comparative analysis of the spectral characteristics of triplet self-trapped excitons and F 2 centers in alkali halide crystals

A comparative analysis of the spectral characteristics of self-trapped excitons (STE) and F ₂ centers in the states with the same spin multiplicity is carried out. Based on the analysis, a criterion for the separation of the triplet-triplet (T-T) absorptive transitions in the electronic and hole components of the STE in any alkali halide crystal is proposed. It is concluded that inhomogeneities in the form of a homological cation or anion impurity in the nearest coordination shells of the spatial position of the STE, rather than hole, affect the spectral position of the T-T transitions in the electron component of the STE.     

Symmetry reduction in the phase transitions of alkali cyanides

For the Landau type phase transitions of alkali cyanides the symmetry of the order parameter and general relevant structural changes including orientational distribution of the CN molecules have been found. All phase transitions are driven by the representation of either the elastic dipol or the elastic quadrupol of the CN molecule and this leads to either ordered or disordered head-tail distribution at CN molecules. It is shown that the monoclinic phaseAa would be the result of condensation of at least two irreducible representations ofFm3m space group and that the CN molecules in this phase can be still disordered with respect to head-tail orientation.     

Dipolar configuration in the ordered state of alkali cyanides

The electrostatic ground state energy of the fully ordered state in KCN and NaCN is evaluated. The main contribution is shown to arise from an indirect interaction of CN^- dipoles via induced dipoles due to lattice displacements. The resulting effective interaction has a maximum at wave vector $\text{q}\text{= (0,}\text{2π}\text{b}\text{, 0)}$, corresponding to an antiferroelectric state such that the dipole at the centre of the orthorhombic unit cell is antiparallel to the dipoles at the corners. The predicted configuration agrees with neutron scattering data.     

Faraday rotation of excitons in alkali halides

The Faraday rotation is the long wavelength tail of excitons has been measured with pulsed magnetic fields. When the data are analyzed in terms of a rigid shift of the index of refraction they yield the exciton gyromagnetic factor of KCl, KBr, KI and RbI which is found to be 9.86 ± 0.04.     

Transformation of charge transfer excitons into molecular excitons in molecular organic crystals

Formulae for persec probabilities of the nonradiative and radiative transformation of charge transfer excitons into molecular excitons in molecular organic crystals are derived and discussed. Numerical calculations are performed for solid naphtalene and anthracene.     

Contributions to the theory of singlet exciton decay into pairs of triplet excitons in organic molecular crystals

Probability of singlet exciton decay into pairs of triplet excitons in organic molecular crystals is calculated. The dependence of the calculated probability on the separation of created pairs of triplet excitons, on temperature as well as on variations of equilibrium positions of molecules caused by changes of the state of electron excitations in the crystal is discussed. In conclusion there is given the estimation of the order of the probability value.Na Slovance 2, Praha 8, Czechoslovakia.     

Line-shape of light absorption by excitons in one-, two- and three-dimensional molecular structures

Within the framework of the dipole approximation the line-shape of light absorption for exciton transitions between narrow bands in one-, two- and three-dimensional periodic structures are calculated. Exciton damping due to lattice imperfections is accounted for as a frequency independent parameter. The obtained analytical expressions allow analysis of the line-shape for different space dimensions of structure depending on bandwidth difference, damping and temperature.     

Importance of annealing on the order-disorder phase transition in alkali cyanides

Abstract

Contrary to monocrystals powdered KCN displays, after grinding and without annealing, an intermediate monoclinic phase between the high-temperature pseudocubic and the low-temperature orthorhombic phases. The consequence of annealing is the total disappearance of this intermediate phase. A further effect of the annealing process is the reduction of the cubic distortion at the transition. In this paper, we analyse this effect on the intermediate rhombohedral phase observed in the (KCN) _x (KBr)_1?x mixed system.     

Energy transfer by singlet and triplet excitons in carbazole-containing polymers

Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C^3′](acetylacetonate)iridium [Btp₂Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.