Coherent external and internal phonons in quasi-one-dimensional organic molecular crystals

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( N-N'-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.     

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.     

Optical properties of quasi-one-dimensional molecular crystals with different polarizations

Quasi-one-dimensional organic compounds in which the acceptor TCNQ molecules form dimers are studied. Site Orbitals describing the electron in half of the TCNQ molecule are introduced. Hubbard's model is generalized so as to take into account the Coulomb repulsion between two electrons at one site. It is shown that the optical excitations of the electron are asssociated with intramolecular vibrations both when the incident light is polarized along the TCNQ chains and when the incident radiation is polarized transverse to the chains. The conductivity of the system with different polarizations is calculated taking into account the electron-vibrational interaction.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 74–78, February, 1986.     

Quadrupole relaxation enhancement--application to molecular crystals

A general theory of field dependent spin-lattice relaxation for nuclei of the spin quantum number 1/2 (1H, 19F, 13C) caused by dipole-dipole interactions with neighboring quadrupolar nuclei (nuclei possessing a quadrupolar moment) is presented. The theory is valid for arbitrary motional conditions and should be treated as a quadrupolar counterpart of the paramagnetic relaxation enhancement theory. When the energy level splitting of the dipolar spin (I=1/2) matches one of the transition frequencies of the quadrupolar nuclei one can observe a local enhancement of the dipolar spin relaxation (referred to as "quadrupolar peaks"). To see such effects the dynamics modulating the spin interactions has to be relatively slow. This brings the system beyond the validity range of perturbation approaches and requires the stochastic Liouville equation to be applied. The presented theory describes the quadrupolar relaxation enhancement (QRE) for an arbitrary spin quantum number of the quadrupolar nuclei and includes the asymmetry of the quadrupolar coupling. It has been applied to interpret the shape of magnetization curves (amplitude of 1H magnetization versus magnetic field) for the molecular crystal [C3N2H5]6[Bi4Br18] ([C3N2H5]-imidazolium). The magnetization curves show several dips (local minima) attributed to 1H-14N quadrupolar relaxation enhancement effects. In addition, as a limiting case a perturbation approach to QRE has been presented and its validity conditions have been discussed.     

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.     

Librational relaxation of molecular impurities in host crystals

A study is made of phonon-assisted transitions between closely spaced librational levels of an impurity molecule in a host crystal. The contributions to librational relaxation of direct, of Orbach-type and of Raman processes are evaluated as a function of temperature. It is found for a model case of NH(³ π) in solid Ar, that below T = 16 K, the direct process dominates, while for T > 20 K relaxation takes place mainly by the Raman mechanisms. The calculation predicts a nanosecond time scale for the relaxation at 4 K, and a picosecond time-scale for the relaxation at T = 24 K.     

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.     

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.     

Ultrafast relaxation processes in semiconductors

A brief review of theoretical studies concerning ultrafast relaxation processes in direct-gap polar semiconductors is presented. Comparison with measurement of time-resolved optical spectra in GaAs is shown.     

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.     

Excess-carrier hopping-current relaxation in quasi-one-dimensional trapping systems

We derive field and time dependencies of injected carrier mobility for quasi-one-dimensional systems characterized by hopping controlled by trapping of carriers at distributed (exponentially in energy and randomly over space) localized states. We find that the field as well as interchain jumps provide a macroscopic time scale to the process, causing the asimptotic power exponent for the relaxation current long-time tail to change its one-dimensional value to a three-dimensional one.     

Application of photon echoes to electronic relaxation in molecular crystals

The possibility of using photon echoes to elucidate the pathways of intramolecular electronic relaxation in molecular crystals is discussed.     

Electron localization in quasi-one-dimensional organic conductors

The theory of electron localization in quasi-one-dimensional organic conductors is reviewed in detail. The effective diagram technique to take into account strong scattering and commensurability effects is developed using the Berezinsky method. The singularities of the electron density of states, of the localization length and of the static dielectric constant near the middle of the electron band and near other rational points of the band are discussed. A transition similar to the Anderson transition is found near the band edge. The effects of the electron-phonon interaction are taken into account and exact results for the hopping conductivity are obtained. The experimental data on the electrical and optical properties of TCNQ salts with strong structural disorder and of other quasi-1d organic conductors are reviewed and analysed.     

Origin of temperature-independent electron mobilities in organic molecular crystals

Recently the almost-temperature-independent mobility was observed for electrons in anthracene, naphthalene, and As₂S₃ in the direction with a narrow bandwidth. It is attributed to the phonon-induced electron hopping between planes along which the electron motion is coherent in a two dimensional energy band. The mobility increases abruptly below about the Debye temperature due to the increasing contribution of the electron transfer without phonon participation, as observed in naphthalene.     

Nonradiative heterofusion of excitons in doped organic molecular crystals

The nonradiative heterofusion of a host exciton with a guest molecule electronic excitation resulting in a higher electronic neutral single excited state is examined theoretically in the doped organic molecular crystals. The heterofusion process is treated as a nonradiative spontaneous multi-lattice phonon transition of the crystal from the electronic double excited initial state to the electronic single excited final state under the action of the interaction between the host molecules and the guest molecule. Two heterofusion channels are considered: the first one resulting in a higher electronic excited state of the guest molecule and the second one resulting in a higher host exciton state. The corresponding rate constants are calculated and discussed within the framework of the first order time dependent perturbation theory.     

Electronic correlations in oligo-acene and -thiopene organic molecular crystals

From first-principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The electronic polarization of the molecules that surround the charged oligomer reduces the bare Coulomb repulsion between the holes by approximately a factor of 2. The effects of relaxing the molecular geometry in the presence of holes is found to be significantly smaller. In all cases the effective hole-hole repulsion is much larger than the valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.     

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.