Phase transition and triplet exciton mobility in anthracene—tetracyanobenzene charge-transfer single crystals

Triplet-Excitons in a mixed 1 : 1 anthracene—tetracyanobenzene single crystal are established by ESR-spectrocopy. These excitons (lifetime 0.5 msec) are characterized by a hopping frequency of v = 1.3 × 10⁶ K at 300K. The temperature dependence of the ESR line shape indicates a phase transition of the crystal at T_t = 205K. Above and below the phase transition the exciton mobility is nearly temperature independent.     

The polarized charge-transfer spectrum of crystalline anthracene-TNB complex

The intermolecular charge-transfer spectrum of the crystalline anthracene-TNB complex has been measured, using polarized light. The crystal spectrum differs from that of the solution to a much greater degree than is the case for ordinary aromatic systems. The transition is polarized along the intermolecular axis and is blue-shifted with respect to the solution. There is pronounced vibrational structure, including a prominent 250 cm^-1 interval. A Davydov splitting of approximately 200 cm^-1 is observed, and this is shown to be in reasonable agreement with the known crystal structure and with the symmetry assignment of the transition, which is A′ ← A′ for the complex in solution and A ← A in the crystal.     

The deuterium effect on electrochemiluminescence efficiencies of anthracene and phenanthrene

The effect of deuteration on the electrochemiluminescence (ECL) efficiencies of the mixed systems containing anthracene or phenathrene has been examined using the single light pulse in the double potential programme. Deuteration of anthracene or phenanthrene decreases the ECL efficiencies by factors of 1·2–16·0. This decrease appears to arise from the quenching of the triplets by radical ions in solution. The quenching factors are estimated by using Marcus theory of electron transfer reactions.     

The effect of gallium substitution on the superconductivity of the BiSrCaCuO system

Magnetic susceptibility, X-ray diffraction and resistivity measurements of the system Bi_1.4Pb_0.6Sr₂Ca_2−xGa_xCu₃O_y are reported for 0 x 2. The high-T_c 2223 phase with a T_c of 107 K for x = 0 exists for x 0.3. The low-T_c 2212 phase with a T_c of 75 K for x = 0 exists for the full range of x. The highest values of the critical temperature and the largest volume fraction of the low-T_c phase in compounds with Ga occur for x = 0.5 ± 0.1. The identification of CaO by X-ray diffraction for x 0.6 indicates that Ga replaces Ca in the compound.     

Heavy-atom effect on radiative and radiationless transitions in charge-transfer complexes

The quantum yields of fluorescence and phosphorescence and decay times of fluorescence were measured for a series of charge-transfer complexes of hexamethylbenzene (an electron donor) with various electron acceptors containing heavy (halogen) atoms and the rate constants of different radiative and radiationless transitions were determined. It was found that radiative T₁ → S₀ transition, i.e. the phosphorescence, is strongly enhanced by the heavy-atom effect. The mechanism of the heavy-atom enhancement of charge-transfer phosphorescence is discussed and it is concluded that increasing spin- orbit mixing of radiative T₁ → S₀ charge-transfer transition with singlet-singlet, locally excited transition of acceptor molecule is responsible for the observed effects.     

The effect of isotopic substitution on the thermodynamic properties of palladium-hydrogen alloys

An attempt has been made to predict the thermodynamic properties of the Pd-D and Pd-T systems from those of the Pd-H system. By considering only the isotopic dependence of the optic modes, a satisfactory interpretation of the infinitely dilute solutions is not possible. However, using only the isotopic dependence of the optic modes together with an empirical equation for the infinitely dilute solution yields calculated results for the concentrated solutions in both the single and two phase regions which are in excellent agreement with the experimental results for the deuteride and tritide systems.     

The effect of halogen substitution on the structure and electronic spectra of fluorone dyes

By means of the B3LYP density functional method with the use of the polarized continuum model PCM, we have performed quantum-chemical computations of the electronic absorption and fluorescence spectra of fluorone dianions: fluorescein, dibromofluorescein, eosin, erythrosine, and Rose Bengal in vacuum and methanol. We have revealed conformational features of the structure of fluorone dianions (charge redistribution, changes in the bond lengths and angles between bonds) second by the halogen substitution, the transition from the ground state to an excited state, and the change of the solvent (vacuum-methanol). Absorption and fluorescence wavelengths, constant dipole moments, transition dipole moments, and oscillator strengths have been calculated. We have showed that, upon halogenation of fluorones, the absorption spectra are redshifted and the Stokes shift decreases, which is qualitatively consistent with experimental results.     

Intense mass-separated beams of halogens and beta-delayed neutron emission from heavy bromine isotopes

Improved production yields of short-lived halogens were obtained from a ThO₂ target, irradiated with 600 MeV protons, in combination with a negative surface ionization source. Mass-separated samples were studied by decay spectroscopy. Production yields of radioactive isotopes of chlorine, bromine, iodine and astatine are presented. Half-lives and relative neutron emission probabilities were measured for the heavy bromine isotopes^89?92Br. Normalizing to earlier publishedP _n values for⁸⁹Br, the results are:⁸⁹Br (4.30±0.14s,P _n =13.6±0.8%),⁹⁰Br (1.92±0.06s,P _n =24.8±1.5%),⁹¹Br (0.53 ±0.03 s,P _n =30.1 ±2.1%), and⁹²Br (0.31 ±0.02 s,P _n =34.6±2.5%). Energy spectra ofβ-delayed neutrons were measured.     

The effect of sample tilt on an emission microscope

In an emission electron microscope with electrostatic optics, the sample is part of the optics. If the sample is tilted with respect to the optic axis, the image will shift by an amount depending on the energy of the emitted electrons. This shift could cause a loss of resolution in the direction of the tilt. In this paper, a simple model is presented for the image shift and the predictions of this model are tested against simulations.     

Ultrafast experiments on intermolecular electron transfer in the benzene - bromine atom charge transfer complex

Ultrafast pump - probe measurements have been made on the benzene - bromine atom charge transfer (CT) complex in CCl₄ and cyclohexane solutions. Ultrafast optical excitation of the CT band of the complex yields an ion pair, which is comprised of a benzene cation and a bromide anion. The rate of charge recombination between the bromide and the benzene cation in the ion pair has been observed to be much faster than the rate of diffusion apart. The charge recombination rate is accelerated at high benzene concentrations as a result of the formation of a benzene dimer cation - bromide ion pair which undergoes much faster charge recombination than the benzene cation - bromide ion pair.     

The effect of deuterium substitution in the amino group on the internal-rotation barrier of acetamide

Peptide molecules XCO-NYY′ are characterized by low potential barrier V₃ to internal rotation of a methyl group substituted for X and/or Y. A most conspicuous example is acetamide, for which V₃ was previously reported to be 25.043857(19) cm⁻¹[8]. The present study intended to clarify why V₃ is so low in acetamide, by examining the effect of the out-of-plane bending or inversion of the amino group on the molecular structure through deuterium substitution for amino hydrogens. The potential barrier V₃ in acetamide was found to decrease by 2.630, 2.986, and 5.532 cm⁻¹, when H′s at cis, trans, and both positions in the amino group were replaced by deuterium atoms, respectively. The reduction was proportional to the effective mass of the out-of-plane bending mode of the amino group (hereafter referred to as the amino inversion), which was in turn ascribed to the change in electronic resonance character of the peptide linkage. The amino inversion is coupled with the CH₃ internal rotation, producing an interaction term proportional to τ sin 3α, where τ and α denote the amino inversion and methyl internal rotation angles, respectively. This coupling term, when the inversion is treated by second-order perturbation, yields a V₆ term in the internal-rotation potential function of the methyl group, in agreement with the finding of Ilyushin et al. [8], who derived an unusually large V₆ term of −10.044874(73) cm⁻¹. It is quite interesting that even a small perturbation such as deuterium substitution causes a substantial change in electronic structure of the peptide linkage.     

The structure of liquid bromine

The atom-atom correlation function for liquid bromine, derived from the X-ray diffraction studies reported in the preceding paper, is compared with the results of Monte Carlo computer simulations and reference site interaction model (RISM) calculations for various bromine-like model molecules. The models used included the two-centre Lennard-Jones interaction model, both by itself and with idealized point quadrupole and quadrupole-induced dipole interaction terms in the computer simulations, and two and three-centre hard-sphere models in RISM. None of these models lead to atom-atom correlation functions which are in good agreement with that obtained from experiment. This suggests that additional information about the interactions between bromine molecules will be needed before the structure of liquid bromine can be completely understood. Finally, by comparing the results of our computer simulations with RISM results for analogous models, we obtain some insight into the accuracy of the reference site interaction model.     

Excited state charge-transfer effect on the phosphorescence emissions of 4-aminopyridine at 77 K

Absorption and phosphorescence spectral characteristics of 4-aminopyridine (AMP) have been studied in polar and non-polar solvents at 77 K. Absorption data provide evidence that the excited singlet π,π¹ states of the molecule possess substantial charge-transfer (CT) character. The 4-AMP molecule is nonfluorescent in all the solvents used in the investigation but phosphoresces strongly in ethanol glass at 77 K. The phosphorescence emissions have a lifetime and negative polarization characteristic of π,π¹ origin. From the observations that in acidic ethanol glassy medium the phosphorescence quantum yield somewhat decreases and the lifetime moderately increases as compared to that in neutral ethanol glass but the negative polarization persists, it has been concluded that all the coupling schemes ¹π,π¹(CT)^$\text{SO3}$π,π¹, ¹π,π^{1$\text{SO3}$}n,π^{1$\text{vib3}$}π,π¹ and ¹π,π^{1$\text{vib1}$}n,π^{1$\text{SO3}$}π,π¹ are responsible for the phosporescence characteristics of 4-AMP molecule. The substatial reductionsin the values of the phosphorescence quantum yield and lifetime of 4-AMP in non-polar rigid matrix at 77 K have been attributed to enhanced nonradiative process arising from increased vibronic interactions between the closely spaced ³n,π¹ and ³π,π¹ states of the molecule in such environment.     

The effect of Fe and Ru substitution on the superconductivity in MgCNi3

We report the effects of partial substitutions of the VIIIa elements: Fe (3d) and Ru (4d) in the intermetallic perovskite superconductor MgCNi₃. In both the MgCNi_3−xRu_x and MgCNi_3−xFe_x systems, the superconducting critical temperature (T_C) decreases monotonically as x is increased. T_C decreases more slowly in the case of Ru, suggesting that the 3d electrons in Fe are magnetically pair breaking. This is supported by the normal state magnetic susceptibility measurements (χ), which shows that χ increases with Fe and decreases with Ru doping. No ferromagnetism is observed in either system.     

Ion-electron emission: The effect of oxidation

We have studied electron emission from Al and Mo surfaces by 4–60 keV Ar⁺ ions under ultrahigh-vacuum conditions and as a function of oxygen exposure. We found that the dependence of the electron yield γ on oxygen exposure, for a given metal, is similar to that of φ, the work function of the surface. Comparison of the results for Al and Mo shows, however, that the magnitude of the change in γ cannot be attributed solely to a change in φ. We propose that the effect of oxygen exposure on electron emission induced by ions, and by extension by photons and electrons, is related to a change in the escape probability of the electrons over the modified surface barrier, and to the emission of electrons from the oxide layer in the presence of the surface dipole field.