Electronic couplings in organic mixed-valence compounds: the contribution of photoelectron spectroscopy

We show that the electronic coupling in strongly coupled organic mixed-valence systems can be effectively probed by means of gas-phase ultraviolet photoelectron spectroscopy (UPS). Taking six diamines as examples, the UPS estimates for the electronic couplings H(ab) are compared with the corresponding values determined from the intervalence charge-transfer absorption bands and from electronic structure calculations. Similar trends are observed for the H(ab) values estimated from UPS and optical spectra; this provides support for the applicability of Hush theory to strongly coupled organic mixed-valence systems. The UPS electronic couplings are found to be somewhat smaller than those from optical spectroscopy, which is attributed to the role of vibronic coupling to symmetrical modes; when corrected for this vibronic coupling, the UPS H(ab) estimates confirm that triarylamine-based mixed-valence systems are close to the class-II/class-III borderline.     

Many-particle rapidity correlations in light-ion interactions at the CERN intersecting storage rings

Many-particle rapidity correlations have been studied for αα,dd, andpp interactions at the CERN Intersecting Storage Rings using the Split-Field Magnet detector, and basing the investigating on the distributions of rapidity gaps between particles. In the framework of a simple cluster model, it is found that all the data samples have similar features and that the events contain clusters of two different types.     

Probing charge transport in pi-stacked fluorene-based systems

The molecular parameters governing charge transport along a pi-stacked fluorene chain in poly(dibenzofulvene) are studied by a joint experimental and theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. We specifically investigate the electronic couplings between fluorene moieties as well as the intramolecular reorganization energies, for both holes and electrons. Our results indicate that a pi-stacked fluorene chain favors hole transport over electron transport. The values for electronic couplings and reorganization energies estimated here are compared with those derived recently for pentacene.     

Manifestation of hexagonal-like structure in the second-order nonlinear effects in the GaN nanocrystallites

Contribution of the hexagonal-like structural components to the photoinduced second harmonic generation (SHG) in GaN large-sized nanocrystallites (with sizes about the 10–30 nm) incorporated into the polyvinylalcohol photopolymer matrices is revealed. The SHG measurements were done using pulsed Nd:YAG laser beam (λ=1.06 μm; pulse duration τ=15–50 ps, laser power about 30 MW) as a fundamental ones and a picosecond nitrogen pulsed laser (P=10 MW; λ=0.377 μm; pulse time duration τ=10–25 ps) as a photoinducing one. We have found that with increasing pumping power density the SHG output signal increases and achieves its maximum value for the power density about 2.6 GW/cm² per pulse. The maximal output photoinduced SHG signal was achieved for parallel directions of the pumping and fundamental beam polarizations. The maximal values of the second-order nonlinear susceptibilities were equal to about 1.09 pm/V. We have observed an increase of the output SHG below 30 K and for pump-probe delaying time about 18 ps. Substantial contribution to the SHG of wurtzite-like (hexagonal) structural fragments is shown.     

ZnSe:Co—nonlinear optical absorber for giant-pulse eye-safe lasers

ZnSe:Co²⁺ crystals were grown by the Bridgeman technique. Optical absorption measurements showed non-uniform distribution of Co²⁺ ions along the as-grown crystals. Using Pfann formula the distribution coefficient of Co²⁺ ions between crystal and melt was estimated to be 0.5. Transmission dependence on laser power at λ=1535 nm was investigated for different Co²⁺ ions concentration. Maximal final transmission was found for ZnSe samples containing 1.6×10¹⁹ cm⁻³ of Co²⁺ ions. Obtained ZnSe:Co²⁺ samples were used as saturable absorbers to generate giant-pulse eye-safe laser radiation. KIGRE QE-7S rod placed in resonator cavity was used as active element.     

Studies of potential inversion in an extended tetrathiafulvalene

Electrochemical oxidation of the extended tetrathiafulvalene 9,10-bis(1,3-dithiole-2-ylidene)-9,10-dihydroanthracene (2) was studied in N,N-dimethylformamide. A single, two-electron oxidation peak occurs, and on the return sweep of a cyclic voltammogram, a two-electron reduction peak is seen. The oxidation of 2 to its cation radical and dication occurs with potential inversion (i.e., removal of the second electron occurs more easily than removal of the first). The extent of potential inversion was estimated by cyclic voltammetry to be 0.28 V by analysis of the process in terms of concerted structural change and electron transfer. Failure to detect the cation radical by EPR of an equimolar mixture of neutral 2 and the dication is consistent with this value. The inner reorganization energy of the cation radical was determined by gas-phase photoelectron spectroscopy (PES) to be 0.31-0.35 eV. Calculations, consistent with earlier experimental data, show rather large changes in structure associated with the oxidation processes. These large structural changes contrast with the relatively small inner reorganization energy found by PES. This observation prompted an analysis of voltammetry in terms of two-step processes, with structural change either preceding or following electron transfer. Agreement of simulations based on this mechanism with experimental voltammograms was equally as good as with the concerted mechanism. Notably, the two-step mechanism produced more realistic values of the transfer coefficient and electron-transfer rate constant for the first step of oxidation.     

Phase-field-crystal models for condensed matter dynamics on atomic length and diffusive time scales: an overview

Here, we review the basic concepts and applications of the phase-field-crystal (PFC) method, which is one of the latest simulation methodologies in materials science for problems, where atomic- and microscales are tightly coupled. The PFC method operates on atomic length and diffusive time scales, and thus constitutes a computationally efficient alternative to molecular simulation methods. Its intense development in materials science started fairly recently following the work by Elder et al. [Phys. Rev. Lett. 88 (2002), p. 245701]. Since these initial studies, dynamical density functional theory and thermodynamic concepts have been linked to the PFC approach to serve as further theoretical fundamentals for the latter. In this review, we summarize these methodological development steps as well as the most important applications of the PFC method with a special focus on the interaction of development steps taken in hard and soft matter physics, respectively. Doing so, we hope to present today's state of the art in PFC modelling as well as the potential, which might still arise from this method in physics and materials science in the nearby future.     

Mean field theory for a reversibly crosslinked polymer network

We present a mean field theory for melts and solutions of reversibly crosslinked polymers. In our model, crosslinks are considered as local bonds between two monomers. For a blend of A+B+AB polymers, we assume reversible crosslinks between the copolymers AB with a crosslink strength z and interaction weights ω(A) and ω(B) for monomers of type A and B, respectively. The usual mean field model for polymer blends without reversible crosslinks is recovered if z vanishes. With or without crosslinks, the A+B+AB blend can form a lamellar phase with A and B rich regions. If reversible crosslinks are enabled and ω(A) differs strongly from ω(B), the lamellar nanophase separation of A and B monomers is accompanied by a similar segregation of crosslinked and noncrosslinked polymers. If ω(A) and ω(B) are equal, crosslinked copolymers are well mixed with the homopolymers. For a homopolymer solution with reversible crosslinks between the polymers, our calculations show that polymers and solvent molecules are separated macroscopically if the Flory-Huggins interaction parameter and the crosslink strength are suitably high or if the volume fraction of polymers or the chain length are suitably low.     

Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo2(4+) compounds and their bicyclic guanidinate precursors

A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo2(4+) units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo2(TEhpp)4 complex which has a broad first ionization band centered around 4.27 +/- 0.03 eV and an ionization onset at the very low energy of 3.93 +/- 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (eta5-C9Me7)2Co and (eta5-C5Me5)2Cr.