Volume 34, Issue 5

In a linear pulsed discharge of helium the spectrum of the atomic transition 4 3 D, F→2 3 P at → - 447.15 nm is measured time resolved by a multichannel analyzer. Pronounced structures of the line profile, reported by other authors could not be verified and helium molecular lines did not occur in the parameter range of this discharge below 100 Pa. The measured influence of spurious hydrogen might suggest the interpretation of the recently observed structures as being due to hydrogen molecular transitions.

The radioactivity of Greek lignites and their ashes was studied. It was found that the radionuclides in them belong to the uranium series. Fly ashes are relatively rich in radioactivity. Concentrations up to 7 · 10 -5 g/g and 12 pCi/g were measured for 238 U and 226 Ra, respectively. The associated hazards from radioactivity escaping from stacks of thermopower plants or when fly ashes are used as subtitutes of cement in concrete are evaluated.

It is well known that different types of crystal defects influence the static and dynamic behaviour of the dielectric polarization of Rochelle salt crystals. Particularly strong effects appear near the paraelectric-ferroelectric phase transitions. In this paper it is shown that a slightly modified relaxing defect cell model which has been developed [1] in order to explain the narrow central peaks observed near structural phase transitions adequately describes the interaction between the strongly temperature dependent relaxational motion of the order parameter and the relaxational motions of the various defect modes. This theory is shown to be equivalent to a phenomenological Landau Khalatnikov approach, which so far has only been used to characterize the macroscopic dielectric behaviour of Rochelle salt crystals with defects near the phase transitions, and which in a previous work [16], has been developed in order to give a general description of central peak phenomena caused by defects. For instance the so called very fast after effect and its influence on the phase transitions is discussed in more detail.

The assumption [1] that there exist two excited tautomeric forms of umbelliferone in ethanolaqueous solutions is compared with experimental results. Agreement is achived by assuming that the equilibrium between the two forms is dependent on the polarity of the solvent.

The bleaching and the laser properties of Rhodamine 6G and B in aqueous detergent solutions have been studied. The rate of bleaching depends on the concentration of the detergent. The relative quantum yields of the bleaching process for these solutions have been determined. It is found that the influence of detergents on the generation efficiency depends on the type of dye employed. From the results obtained it is concluded that the detergents influence the population of the triplet state of the lasing dye.

The absorption spectra of 9-methyl- and 9,10-dimethylanthracene in n-hexane and of anthracene in n-heptane at 77 K were measured spectrographically. From the comparison with calculated frequencies and intensities it can be shown that the observed intense vibronic lines correspond to the same totally symmetric vibrations as in the earlier measured fluorescence spectra. Compared with the fluorescence spectra deviations from the mirror symmetry were observed. In the case of anthracene and 9,10-dimenthylanthracene they result first of all from different intensities of the corresponding vibronic lines, and it can be shown that these differences are mainly due to the Duschinsky-Effect. For 9-methylanthracene the great frequency difference of one totally symmetric vibrations perturbs the mirror symmetry also.

Two-dimensional excitable media, for example the excitable version of the Belousov-Zhabotin-sky reaction, are capable of forming spiral-shaped self-sustaining rotating wave patterns (rotors). In order to explain Winfree's experimental observation of an irregular "meandering" of the rotor's core region, we present a numerical simulation of a continuous, two-variable excitable medium in two space dimensions. Two phenomena occur: 1) an irregular motion of the rotor's core; 2) a non-stationary peak inside the core region. Thus, "meandering" is obtained, together with a new phenomenon, the "peak". A sufficient condition for both phenomena is that the underlying (local) system be stiff, that is, admit two time scales for its approximate description. In this case, (1) the "reaching distance" of diffusion is small as compared with the core's radius, and (2) rotation symmetry of the core implies a gradation in local frequencies (increase toward the center), supposing radial decoupling. We propose that both constraints act together to induce a spontaneous breakdown of rotation symmetry under an increase of stiffness. Immediately (or soon) thereafter, the core is no longer synchronized by the wave circling around it; instead, excitation from the surrounding wave penetrates the core along a new path from time to time, causing a non-repetitive short-cut.

A construction for a versatile absorption cell is presented, which may be used for a large variety of double resonance experiments in the radiofrequency, microwave and millimeter wave region. Some measurements of collision induced transitions are reported.

Conductometric measurements on solid lithium, sodium and potassium propanoates and X-ray powder diffraction measurements on the sodium and potassium salts have been performed over temperature ranges including solid state transitions. The temperature dependence of the transport and lattice parameters of the different crystalline phases and the changes of the same parameters in correspondence with phase transitions have been put into evidence.

An approximate force field for the in-plane vibrations of condensed aromatics with only five parameters is investigated. Calculated frequencies are given for naphthalene, anthracene and coronene. The results are compared with values from other, more elaborate methods, mainly the Califano-Neto force field. The five-parameter approximation is found to be useful as an aid in assignments of experimental frequencies. It appears to give amazingly good results considering its great simplicity and small number of parameters. New experimental infrared and Raman data for anthracene and coronene are given. Especially some new features in the Raman spectrum of anthracene are reported in some details.

Electron energy distribution functions (EDF) of molecular H 2 have been calculated by numerically solving the Boltzmann equation including all the inelastic processes with the addition of superelastic vibrational collisions and of the hydrogen atoms coming from the dissociation process. The population densities of the vibrational levels have been obtained both by assuming a Boltz-mann population at a vibrational temperature different from the translational one and by solving a system of vibrational master equations coupled to the Boltzmann equation. The results, which have been compared with those corresponding to a vibrationally cold molecular gas, show that the inclusion of superelastic collisions and of the parent atoms affects the EDF tails without strongly modifying the EDF bulk. As a consequence the quantities affected by the EDF bulk, such as average and characteristic energies, drift velocity, 0-1 vibrational excitation rate are not too much affected by the inclusion of superelastic vibrational collisions and of parent atoms, while a strong influence is observed on the dissociation and ionization rate coefficients which depend on the EDF tail. Calculated dissociation rates, obtained by EDF's which take into account both the presence of vibrationally excited molecules and hydrogen atoms, are in satisfactory agreement with experimental results.

A helix structure can be induced in nematic phases by adding chiral compounds. In general the helix pitch increases with temperature. It is shown that by using two suitable doping agents, each of which would give different senses of helix rotation when used individually, a particular temperature behaviour can be obtained. Depending on the concentrations, a temperature independent pitch, for example, or a helix inversion can be achieved. The results of experiments using the nematic phase N 4 (Merck) show good agreement with the theoretically predicted temperature dependence of the pitch.

A Green's function perturbation technique is presented which is appropriate for studying the chemisorption behavior of ordered atomic layers on a metal surface. The phase shift technique is used to determine the change in the electronic density of states due to chemisorption. The method is quite general and can be used to study the electronic structure of arbitrary ordered overlayers. Numerical results are given for p(2x2), c(4x2), p(2x1), c(2x2) and p(1X1) overlayers on a (001) surface of a tight-binding fee metal. It is shown that the effects of long-range order and the binding geometry of the adsorbate layer are of great importance for the electronic structure of ordered overlayers.

Experiments have been performed using time-resolved spectroscopy techniques in a four-level double resonance scheme. The time-resolved variations of radiation probing the populations of a K-type doublet of H 2 CO have been studied as pump radiation suddenly switched on and off induces population changes between levels of another K-doublet. Non-exponential behavior with characteristic times longer than the relaxation time associated with pressure broadening has been exhibited. A theoretical treatment of this effect, using a generalized density matrix equation, is proposed. It gives reasonable agreement with the observed time dependences.

Microwave spectra of trideuterated and perdeuterated N-nitroso-dimethylamine have been investigated. On the basis of these measurements an improved structure has been calculated.

Some Europium(III) dibenzoylmethide and benzoylacetonate complexes have been subjected to Mössbauer studies at liquid nitrogen temperature. An attempt is made to discuss covalency in these complexes in terms of 6s, 4f and/or 5d orbital participation in the bond formation. Small linewidths are correlated to less symmetric structures. Resonance effects reflect the polymeric nature of these chelates.

In this paper a solid state NMR relaxation study and inelastic neutron scattering (INS) experiments (the latter in the solid and liquid phase) are reported for a molecule containing methyl groups in different surroundings, namely 1,4-dimethoxy-2,6-dimethylbenzene. The results are compared with each other and with those of a liquid state NMR study reported earlier. It is found that in this way both the shapes and magnitudes of the various hindering potentials can be determined. The result is that the methyl groups 1, 2 and 4 possess a threefold cosine-shaped potential with heights given by 7.7, 9.0 and 15.0 kJ/mol according to the solid state NMR measurements (from the INS study the values 7.5, 7.5 and 16.0 kJ/mol were obtained). For methyl group 6 a mixture of a three-and six-fold symmetry had to be taken into account in order to explain the experiments. Except for methyl group 4 the liquid state NMR results deviate from those obtained with the other two techniques. A possible cause for this discrepancy is discussed. For the molecular structure it is concluded that the reorientations of the methyl groups 1 and 2 are influenced by mutual hindering and that the angle between the plane through the C 4 -O-C bond and the aromatic plane is small, whereas for the C 1 -O-C bond this angle is estimated to be about 70°.

A Dirac-like equation for the photon is considered by using a four-by-two matrix wave function. Conservation laws and transformation properties under Lorentz transformations are considered.

An accurate photographic method for measuring intensities on a fluorescent screen is described. Simultaneously with the fluorescent screen a calibrated 7-step grey scale is recorded on the film. Using a scanning photometer the intensity distribution can be digitized and by comparison with the grey scale converted to electron current density. As an example LEED-intensities from a Si (lll)-surface obtained with the photographic technique are compared with spot-photometer measurements.

Computer Simulation of the fluorescence decay of an anthracene single crystal shows that strong reabsorption leads to nonexponential decay curves.

In 1974 a theory of the radiative recombination including the Coulomb interaction and the exciton recombination was published by Schlangenotto et al. Now, a quantitative evaluation of this theory was carried out for Si: it shows a strong enhancement of the recombination probability, compared with the theory of van Roosbroeck and Shockley, and yields a remarkable agreement with the measurements.

After a short description of the evaluation of the calorimetric measurements, we give the results for the enthalpies of mixing in the liquid system formic acid + acetic acid. The molar excess enthalpy H̄ E has been determined as a function of the mole fraction x of acetic acid at 18 °C, 20 °C, 25 °C, 30 °C, and 40 °C. The function H̄ E (x) is always positive and nearly symmetric (with a maximum at about x = 0.5) and increases with increasing temperature. A three-parameter fit of the function H̄ E (x) has been achieved for each temperature.

The "differential" method of Achar, Brindley and Sharp (ABS) and the "integral" one of Coats and Redfern (CR) are applied to the dehydration of BaBr2-H20 and HCOOLi-H20 and to the thermal decomposition of Pb(SCN)2, and the kinetic parameters so obtained are compared with those deduced isothermally. The results of the ABS method, at the lowest heating rate (q = 1.2 K min — '), agree well with the isothermal ones, whereas the CR method leads to satisfactory results only for the Pb(SCN)2 thermal decomposition (q = 1.2 K min~*). A possible explanation is given.

9,9′-Dianthryl disulphide is the first example studied for a diaryl disulphide whose uv/vis absorption spectrum exhibits a distinct vibrational structure whereas the formerly investigated spectra of diphenyl disulphide and the isomeric dinaphthyl disulphides are characterized by the complete lack of vibrational structure. Different interpretations have been given in the literature for this particular property of the diphenyl disulphide and dinaphthyl disulphide spectra. The long wave-length region of the dianthryl disulphide spectrum is interpreted with the assumption of three electronic transitions. Two of them are localized in the anthracene or dithio anthracene chromophore, respectively, whereas the third transition is assumed to be non-localized including the whole system with the S-S bridge acting as a conjugative (butadienoid) link. This assignment is consistent with a p π -model without p π /d π -participation in agreement with results of photoelectron spectral investigations on disulphides reported in the literature.