Impurities In Molecular Crystals Research Articles

Singlet exciton dynamics of guest impurities in molecular crystals

The relaxation dynamics of excited guest singlet states in high-concentration p-terphenyl: pentacene crystals was investigated using picosecond transient grating techniques. The temperature and concentration characteristics of the grating decays were rationalized to arise from the competition between resonant transport and trapping of the excitation energy by lower-lying sites.

Trapping of excitons with multiphonon emission at impurities in molecular crystals

Rates of exciton trapping at isotopic impurities acting as deep trap centres in molecular crystals are derived. The excess exciton energy is emitted in the form of phonons. Rates are calculated for doped naphthalene crystals and results are discussed for possible applications.

Exciton trapping with coherent emission of phonons at impurities in molecular crystals

The process of exciton trapping at impurities with deep trap depths is studied in molecular crystals assuming that the excess exciton energy can be emitted in the form of coherent phonons. We represent the state of emitted phonons by coherent states and derive rates of exciton trapping for isotopic impurities. The calculation is done with acoustic as well as optical phonons. In naphthalene crystals with impurities of trap depth 350 cm -1 rates are calculated in terms of the coherent state parameter |α|. For 1.1 ⩽ |α| ⩽1.8 it is found that the rate of acoustic phonon emission is 10 16 s -1 and of optical phonon emission is 10 18 s -1. Such high rates suggest that a laser-like new source of intense and coherent phonon energy can be designed.

Exciton trapping by emitting two phonons at impurities in molecular crystals

Exciton trapping by emitting two phonons at impurities in molecular crystals

Exciton cluster states of isotopic impurities in molecular crystals

Infrared spectra of isotopic mixed crystals of chloroform have been recorded over a wide concentration range. We have studied the fine structure of the chloroform ν 4 (E) band, near 1216 cm −1, in isotopic mixed crystals. Absorption lines of monomer, dimers are observed and identified from their characteristic concentration dependance. The exciton coupling constants of guest pairs are determined.

Spin‐lattice relaxation theory of excited triplet‐state impurities in molecular crystals

AbstractThe processes of spin‐lattice relaxation of the excited local triplet states in molecular crystals at low temperatures are investigated. The coupling of the translational and rotational motion of the molecule (TRC relaxation mechanism) is chosen as the major phenomenon, leading to the coupling of the spin system with acoustic vibrations. The general expressions for the spin‐lattice relaxation time of the triplet centre in molecular crystal with impurities are derived taking into account the change of force constant due to the impurities. The angular, field, and temperature dependences of SLR time are calculated for an isotopic impurity in naphthalene crystal. The results of calculation are in accordance with available experimental data.

The theory of photon echoes for strongly coupled electron–phonon systems

We calculate the photon echo decay for a strongly coupled electron–phonon system, without making unrealistic assumptions about excitation pulse durations. We find that asymptotically, the photon echo decay is exponential, and we provide a formal expression for the decay rate 1/T2 which is independent of pulse duration. In contrast to the usual results, our expression for 1/T2 is nonperturbative in the electron–phonon interaction. We calculate the dependences of the echo amplitude on pulse power and duration, finding that the strong electron–phonon coupling modifies the simple two level system result. We also focus on the relationship between the zero phonon line shape in the homogeneous optical absorption spectrum, and the photon echo decay rate. We find that the zero phonon line shape is Lorentzian, with width 1/πT2. We show that the usual optical Bloch equation approach to analyzing time-domain experiments, which normally is only applicable if the electron–phonon coupling is weak, can be extended to the case of strong coupling. These strong coupling Bloch equations are adequate for analyzing the time dependence, but not the pulse power and duration dependences, of time-domain experiments. Our results indicate that within the Condon approximation, electron–phonon coupling cannot explain the very low temperature anomaly of recent photon echo experiments on impurities in molecular crystals.

Theory of photon echoes from a pair of coupled two level systems: Impurity dimers and energy transfer in molecular crystals

We present the theory of the photon echo and stimulated photon echo from a four level system that results from a pair of weakly coupled two level systems. This model describes an impurity dimer in a molecular crystal. We include the effect of relaxation processes, focusing on energy transfer between delocalized states. We find that both the photon and stimulated echoes contain information about static level splittings and dynamic energy transfer processes. Moreover, these observables show a dramatic dependence on the excitation power. We discuss implications of our results for recent experiments on concentration dependent dephasing of impurities in molecular crystals. Our analysis casts considerable doubt on the intermolecular interaction interpretation of these experiments.

Guest excitation energy in dilute organic mixed crystals with chemically distinct guests

Philpott's theory of vibronic transitions of substitutional impurities in molecular crystals has been examined. The lowest energy guest transition in a number of mixed crystals has been calculated and compared with experiment. Satisfactory agreement is achieved. In some cases consideration of charge overlap of the guest and nearest host gives better agreement.

Phonon Structures of Optical Absorption Bands of Radicals Produced by Irradiation in Naphthalene Single Crystals

Abstract The electronic spectra of impurities in molecular crystals are usually characterized by the presence of phonon sidebands.1–5 For a forbidden transition, the coupling with phonons may enhance the transition probability and cause phonon sidebands, which may become more intense with increasing temperature.6 For an allowed transition, sideband structures are the manifestation of the coupling between electrons and local phonons.3 If the lattice distortion around the impurity is small, the sidebands display the density of lattice phonon states. In this case the relative intensity of the sidebands to the zero-phonon line is a measure of the strength of the electronic coupling of the impurity to the surrounding molecules. If the lattice distortion is large, coupling to the local phonon modes would affect the sideband structures. Studies of electron-phonon coupling have provided important information on the impurity state in the crystal.

Exciton interactions between substitutional impurities in molecular crystals

A compact secular determinant for the exciton energies of a crystal containing substitutional impurities is derived using the dipole approximation for intermolecular interactions. In distinction to previous work all host and impurity transitions from the ground state are included and the number and nature of the impurities is arbitrary. The first feature is important for proper consideration of crystal field polarization effects. It is shown that the effective long range interaction between impurities has factors that do not appear the corresponding interactions between interstitial impurities. The differences are smallest in the limit of deep traps.

Impurity States in a Linear Molecular Crystal

The assumption of parallel impurity and host transition moments made in the existing theory of localized electronic states of impurities in molecular crystals is shown to be unnecessary for certain one- and two-dimensional model crystals. The dependence of localized impurity-state energy and radius on the angle between host and impurity moments is calculated for a linear crystal.

Intermolecular forces from optical spectra of impurities in molecular crystals

A procedure is outlined by which one can get information about the effects of electronic excitation on the interactions between molecules. The absorption and emission spectra of impurities in molecular crystals, and specifically the mean positions and widths of the lines, are required as functions of temperature. A theory of M. Lax is used to relate mean position and width to intermolecular forces. The preliminary treatment of this problem by McCarty and Robinson is evaluated and extended: their procedure is expected to be valid for very narrow lines only.