Nonradiative heterofission of guest molecule electronic excitations in doped organic molecular crystals

Formulae for the rate constants of the nonradiative heterofission of a guest molecule electronic excitation into one host molecule electronic excitation and one guest molecule electronic excitation in the doped organic molecular crystals are derived. The coherent and incoherent host exciton heterofission channels are considered. Formulae derived for the singlet host exciton heterofission channels are related to the measurable optical properties of the host and guest molecule of the crystal. The effects of the intramolecular vibration are discussed.     

Nonradiative destruction of triplet excitons by excess electrons in organic crystals

The nonradiative destruction of triplet excitons by excess electrons in organic crystals is examined theoretically within the framework of the first-order time-dependent perturbation theory and within framework of the diffusion theory. The numerical calculations have been carried out for the anthracene crystal and compared with experiment.     

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.     

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.     

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.     

Superliquid state of small-radius excitons in molecular crystals

The energy spectrum of non-ideal bosean gas of small-radius excitons in superliquid state in molecular crystals is theoretically investigated within the scope of Bogoljubov's theory. The conditions for the possibility of existence of the superliquid state are discussed. The critical temperature of the transition of the ideal bosean gas of the excitons to the condensed state are estimated.     

Indirect optical cooperative excitation of two-particle electronic excitations in doped organic molecular crystals

A theory on the indirect optical cooperative excitation of a two-particle electronic excitation composed of one host exciton and one guest-molecule one-particle electronic excitation through one electric dipole-moment active singlet guest molecule one-particle electronic excitation is developed by considering the nonradiative coupling bstween one- and two-particle electronic excitations.The complex dielectric tensor and the corresponding absorption coefficient are found out, which are expressed in terms of the Fourier-transform of the retarded Green function: The derived formulas are discussed in an explicit form for the one-dimensional model of the doped organic molecular crystal. Within the framework of ths one-dimensional model also the conditions for the existence of the host exciton-guest molecule one-particle electronic excitation bound states are established and discussed.     

Nonradiative heterofission of a singlet host exciton into a pair of triplet electronic excitations in doped aromatic hydrocarbon molecular crystals

The nonradiative heterofission process of a singlet host exciton into one triplet host exciton and one triplet guest molecule electronic excitation in some aromatic hydrocarbon molecular crystals is investigated theoretically. The formulae for the heterofission rate constants are derived and discussed and used for the numerical estimates. The conditions for the occurence of the heterofission of a singlet host exciton in the investigated crystals are discussed.     

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.     

Anomalous luminescence of impurity-bound excitons in lithium borate crystals doped with cerium ions

The spectra and relaxation kinetics of the anomalous (?? < 10 ns) luminescence of Li₆GdB₃O₉:Ce³⁺ crystals have been experimentally detected. The time-resolved vacuum ultraviolet spectroscopy study has shown that optical transitions at 6.2 eV, caused by the transfer of an electron from the 4f ¹ ground state of Ce³⁺ to autoionizing states near the conduction band bottom of a crystal, lead to the formation of an impurity-bound exciton with the hole component localized on the 4f state of Ce³⁺ and the electron localized on states of the conduction band bottom. It has been found that the decay of such an exciton in Li₆GdB₃O₉:Ce³⁺ occurs through radiative recombination, leading to fast luminescence at 4.25 eV. The energy threshold for the formation of the impurity-bound exciton has been determined. The distribution functions of elementary relaxations over the reaction rate constants H(k), which determine the relaxation kinetics and luminescence quenching processes, have been calculated.     

A note on triplet-triplet fission of singlet excitons in molecular crystals

The singlet exciton lifetime in the reaction of triplet-triplet fission due to purely electronic interaction is estimated in an ideal molecular crystal with favorable positions of singlet and triplet bands, for coherent plane wave form of exciton wave functions. An explicit form of the interaction of molecules in molecular crystal in second quantization reprezentation is presented, including exchange and charge-transfer terms. New commutation relations of molecular and exciton creation and annihilation operators are introduced.Na Slovance 2, Praha 8, Czechoslovakia.     

Structured emission of relaxing systems: molecular excitons in rare-gas crystals

We have computed the emission spectra of relaxing excitated-state systems in solids as a function of the rates of radiative decay (γ) from excited to ground state and non-radiative decay (γ) within the excited state. When these are comparable a pronounced dip is obtained in the central region of the spectrum, which then may have the appearance of two distinct, unrelated emission bands. The computations well reproduce the observed W and M₂ bands in Ar crystals. Accordingly, these arise from the initially excited and near repulsive regions of the A¹Σ⁺ emitting state, respectively.     

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.     

Diffusion equation for the quasi-incoherent motion of Frenkel excitons in molecular crystals

In a recent paper a master equation describing the quasi-incoherent motion of Frenkel excitons in molecular crystals has been derived within the Haken-Strobl model for the coupled coherent and incoherent motion of Frenkel excitons. Starting from this master equation and using only the translational symmetry of the crystal lattice, for crystals with one molecule in the unit cell a diffusion equation is derived. For crystals with several molecules in the unit cell instead of a diffusion equation one obtains a set of diffusion-like equations. These equations are solved explicitly for the case of two molecules in the unit cell and asymptotic expressions are discussed. It is shown that this asymptotic behaviour is again described by a diffusion equation.     

The role of Frenkel excitons in low-energy-electron diffraction theory for molecular crystals

In this paper a low-energy-electron diffraction theory for molecular crystals is established. The possibility of creating Frenkel excitons by the incident low-energy electrons is taken into account. The whole formalism is based on a set of diagram rules, which facilitates the evaluation of the differential scattering cross section. The influence of the bulk and surface excitons on the LEED intensities will be discussed in a series of subsequent papers.     

The interaction of excitons in the superliquid state with phonons in molecular crystals

The energy spectrum of excitons in the superliquid state has been investigated in interaction with phonons in molecular crystals. The corresponding branches of the energy spectrum of elementary excitations have been determined and the effect of the interaction with phonons on their stability has been investigated and discussed. The possibility of the resonance excitation of hydrons by means of phonons is discussed.     

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.     

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.     

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.     

Polarization in organic molecular crystals and charge-transfer salts

Polarization in insulators is a general phenomenon that extends over nanometer distances. Two special cases illustrate recent theoretical progress. Polarization energies of localized charges in organic molecular crystals exceed the bandwidth and redistribute the charge density. A systematic treatment of electronic polarization is summarized in the limit of zero intermolecular overlap for pentacene crystals or thin films on metallic substrates, with special attention to the transport gap for producing a separated electron–hole pair and the optical dielectric tensor of the crystal. When overlap cannot be neglected, the general formulation of polarization in extended insulators is in terms of the exact ground state's phase. This formulation is applied to organic charge-transfer (CT) salts whose correlated electronic structure is described by one-dimensional Peierls–Hubbard models. Near the Peierls instability, coupling to lattice modes generates large peaks in the dielectric response that is primarily due to lattice vibrations. Comparisons with experiment are mentioned for both organic molecular crystals and CT salts.