Methyl rotation in NMP-TCNQ

Comparisons of continuous wave NMR in NMP-TCNQ and the analog in which the methyl group is deuterated are used to show that methyl reorientation occurs in NMP-TCNQ at all temperatures (T > 1.5°K). Methyl reorientation is suggested to contribute to the nuclear spin-lattice relaxation, and its effects on the specific heat are discussed.     

Dependence of aggregate strength, structure, and light scattering properties on primary particle size under turbulent conditions in stirred tank

The steady-state size and structure of aggregates produced under turbulent conditions in stirred tank, for primary particle diameter, d(p), equal to 420 nm and 120 nm, were studied experimentally for various values of the volume average shear rate, G, and solid volume fraction, phi, and compared with data for d(p) = 810 nm. To exclusively investigate the effect of dp, polystyrene latexes with same type and similar density of surface charge groups (sulfate) were used. The mass fractal dimension, d(f), obtained by image analysis, was found to be invariant of d(p) and G, with a value equal to 2.64 +/- 0.18. Small-angle static light scattering was used to characterize the cluster mass distributions by means of the root-mean-square radius of gyration, R(g), and the zero-angle intensity of scattered light, I(0), whose steady-state values proved to be fully reversible with respect to G. The absolute values of R(g) obtained for similar phi and G proved to be independent of d(p), and for all studied conditions, R(g) was proportional to G-1/2. At very low phi, a critical aggregate size for breakage was obtained and used to evaluate the aggregate cohesive force, as a characteristic for the aggregate strength. The aggregate cohesive force was found to be independent of aggregate size, with similar values for the investigated dp. Due to large d(p) and high d(f), the effect of multiple light scattering within the aggregates was found to be present, and by relating the scaling of R(g) with I(0) to d(f), the corresponding correction factors were evaluated. By combination of the independently measured aggregate size and structure, it is possible to experimentally determine the relation between the maximum stable aggregate mass and the hydrodynamic stresses independent of the multiple light scattering present for large d(p) and compact aggregates.     

Site representation for charge transfer excitations in molecular crystals

A phenomenological site hamiltonian is constructed for molecular crystals whose lowest electronic excitations involve an electron transfer, as in many donor-acceptor and free-radical crystals. When only the lowest chargetransfer states are retained, a fermion representation of the site hamiltonian is obtained which leads to a typical many-body problem. The model reduces in special cases to the Hubbard and the Pariser-Parr-Pople models. However, instead of a one-electron, frozen core model, the site representation includes core rearrangements and polarization.     

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.     

Beitr?ge zur Gasanalyse. III. Die Bestimmung- von ?thylen

Ohne Zusammenfassung     

Phase transition to the conducting state in a system of charge-transfer excitons at a donor-acceptor interface

We discuss the phase transition to the conducting state in a system of 2D charge-transfer excitons (CTEs) at a donor-acceptor interface. The phase transition arises due to strong dipole-dipole repulsion between CTEs which stimulates the population of free carriers in higher energy states even at low temperature. We use the computer simulations with the random distribution of excitons, with finite lifetime explicitly taken into account. The critical concentration of CTEs and their energy distribution are calculated. We also discuss the possibility of observing the predicted phenomena. Fiz. Tverd. Tela (St. Petersburg) 41, 781–784 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Spin solitons in organic charge-transfer salts

Organic charge-transfer (CT) salts that crystallize in face-to-face stacks of π-electron donors (D) and acceptors (A) are one-dimensional electronic systems with strong coupling to lattice and molecular vibrations. Peierls–Hubbard models of CT salts resemble π-electron theory of conjugated polymers, although transfer integrals t along mixed DA stacks are considerably smaller. Strong D and A yield Peierls systems with dimerized ground states of ion radicals, D⁺ and A⁻. Topological spin solitons separate regions of opposite dimerization, similarly to solitons in trans-polyacetylene, but are thermally accessible in ionic CT salts due to small t. Salts with still smaller t have Peierls transitions at T_P < 300 K to undimerized stacks. A Peierls–Hubbard model, H_CT, describes both types of salts and estimates of t are consistent with T_P < 300 K in some salts and spin solitons in others with higher T_P. Electron paramagnetic resonance (epr) spectra of single crystals provide direct evidence for spin solitons and one-dimensional electronic states. Spin solitons and H_CT resolve longstanding conflicts between vibrational and magnetic data that indicate dimerized stacks in several prototypical CT salts, while structural data point to undimerized stacks with large thermal ellipsoids. The low energy scale, availability of single crystals and diversity of CT salts offer opportunities for detailed modeling.     

Beitr?ge zur Gasanalyse. V Weitere Anwendungen des Druckverfahrens

Ohne Zusammenfassung