Band 26, Heft 12

The meaning of a recently proposed formalism for quantization of interacting fields is discussed by studying the consequences of the time-dependent unitary transformation which is essential for this approach. It turns out that non-relativistic quantum electrodynamics in dipole approximation may serve as a useful, although rather singular, example for this method. In the relativistic case a different point of view is suggested in order to avoid inconsistent interpretation. It is further possible to give arguments for a reasonable choice of the unitary transformation concerned

This paper deals with the polarisation of nuclei in molecular crystals due to their hyperfine interaction with optically excited triplet states and excitons, a phenomenon which is termed optical nuclear polarisation (ONP). A theory is presented which extends the mechanisms of dynamic nuclear polarisation (Overhauser effect) to the case of spin systems containing triplet states with S = 1 and nuclei with I = 1/2. In this mechanisms the optical electron polarisation (OEP) caused by symmetry selection rules for intersystem crossing to and from the magnetic triplet sublevels is assumed to be transferred to the nuclear magnetic substates by efficient hyperfine relaxation transitions. The adiabatic fast passage is used to detect the nuclear polarisation. The advantages and conditions of this technique as compared to other nmr techniques are discussed. ONP results at room temperature are given as a function of the external field H 0 , its orientation with respect to the crystalline axes, the intensity and frequency of the exciting light and the concentration and types of guest molecules. In phenazine crystals relative polarisation factors up to 50 are found with marked orientation dependencies even in fields as high as 11 kG. These results can be interpreted in principle in terms of the presented theory. However, in low fields (0 - 200 G) where ONP caused by hyperfine relaxation vanishes, large polarisations are found in doped crystals of fluorene and anthracene. The maximum absolute polarisation in fluorene doped with acridine is 3.6 · 10 -4 at H 0 = 80 G corresponding to a relative polarisation factor of ~ 10 4 . The effect of doping is discussed.. Reference is made to a possible ONP mechanism 3 which is able to produce large nuclear polarisations at low fields

Stark broadening of the neutral helium lines λ 5015 Å and λ 3889 Å was compared with the width of H ß in a quasi-stationary plasma produced in helium-hydrogen mixtures of various composition. Electron densities derived from the broadening of H ß differ from those determined from helium line profiles by a factor about 1.7. The electron densities varied from 0.8 -5 • 10 16 cm -3 in the H ß -scale; the temperatures in the range 18 000 -26 000 °K were found from helium-line measurements and from the underlying continuum. Special care was taken for the time-constancy of the plasma emission and for demixing effects

Investigations of the diffusion of tritium in transition metals have been carried out by using "time-lag" methods. The β-radiation, originating from tritium decay within a thin layer near the external surface of the metal, was taken as the signal to follow the change of tritium concentration with time in this layer. No passage of tritium atoms through the phase boundary is necessary for this method; the influence of energy barriers, which might retard this passage, can therefore not interfere. Measurements with palladium at temperatures between 16.5 °C and 50 °C and vanishing hydrogen content in the Pd yielded: D 0 T = (10.5 ± 3.0) • 10 -3 • exp{- (6350 ± 300)/RT} cm 2 s -1 , (R T in cal/mole). The observed isotope effects in the diffusion of H, D and T in palladium are discussed. At 25 °C diffusion coefficients of tritium in Pd/Ag-alloys of different compositions (0 - 70 atom-% Ag) were determined. They first decrease slowly with rising silver content up to 30% and then drop sharply to very low values at higher Ag-contents. Measurements with tantalum at 25 °C yielded: D T = (0.50 ±0.25) • 10 -6 cm 2 s -1

The abundance and isotopic composition of all noble gases are reported in the Wellman chondrite, and the abundance and isotopic composition of xenon and krypton are reported in the gases released by stepwise heating of the Tell and Scurry chondrites. Major changes in the isotopic composition of Xe result from radiogenic Xe 129 and from variations in the isotopic mass fractionation pattern in the different temperature fractions. The isotopic composition of trapped krypton in the different temperature fractions of the Tell and Scurry chondrites displays smaller fractional changes than xenon, but the isotopic composition of these two gases covary in the manner expected from mass dependent fractionation

A description of the low-lying positive parity levels of the odd mass isotopes of Sb and I is presented on the basis of the intermediate coupling approach of the unified model. Such analysis helps to understand many properties of those levels since a rather satisfactory agreement with experimental data is reached

The stability of a gas in an external gravitational field Φ is investigated for arbitrary initial conditions. It is shown that Schwarzschild's entropy criterion ds/dΦ > 0 is both necessary and sufficient for the spatial quadratic mean of a linear disturbance to be bounded in time. The variational principle usually applied is not always sufficient for stability

Two possible ways are reported of influencing the spectral response of PbS films prepared by chemical deposition. An increase in the ratio of hydroxyl to lead ions in the deposition solution causes a shift of the peak spectral response from 1.3 μ to 2.8 μ. The addition of a reducing agent (hydroxyl amine hydrochloride) also markedly influences the spectral response. The time constant, resistance and thickness of the films have been measured and an attempt is made to explain all the observations on the basis of a modified conduction model

The ionic species N 2 + B, CO + A, CO + B, and O 2 + b have been produced by irradiation with the He I resonance line. Franck-Condon-factors both for excitation and deexcitation have been deduced from the fluorescence of the molecular ions. Most of them agree with calculated values of the Franck-Condon-factors. Measurements at different pressures ranging from 0.005 to 5 Torr lead to the deactivation cross sections for collisions with the parent molecule. N 2 + B also is deactivated in collisions with O 2 . Cross sections vary between 50 Å 2 and 250 Å 2 depending upon the ionic species and its vibrational quantum number. The deexcitation is described as a radiationless transition where electronic energy is transfered into vibrational energy

The optical properties of molecules may he affected by an electric field. With suitable molecules, these effects allow to determine the electric dipole moments and certain components of the polarizability tensors in the ground state and in excited electronic states, the directions of transition moments and certain components of the transition polarizability tensors. The magnitude of the electrooptical effects depends on the effective electric field acting on the molecule. In preceding papers the representation of the effective field was based on the Onsager model. More recent experimental investigations have shown that this approximation is not sufficient when using polar solvents. Here, local fluctuations of the electric field have to be taken into account. Basing on previous theories, an extension including these effects is developed which agrees with the experimental results, as will be shown in the following paper 7 . Also, an approximate expression is derived for the mean square of the effective electric field

The effect of an external electric field on the optical absorption of molecules in solution allows certain components of the polarizability tensors in the ground state and in excited electronic states to be determined. Electrooptical absorption measurements were performed on some aromatic hydrocarbons (anthracene, tetracene, perylene, bianthryl) and some tetraphenylpolyenes at different temperatures and in different solvents. For the aromatic hydrocarbons, the polarizabilities in the first excited singlet state were found to be 20 to 70 p.c. greater than those in the ground state. The increases are either mainly parallel or mainly perpendicular to the direction of the transition moment of the absorption bands. For the tetraphenylpolyenes, the measurements yielded much greater increases of up to 400 p.c. almost totally in the directions of the transition moments, which are approximately in the directions of the chains. Fluctuation effects of the local electric field were observed, they may be interpreted using the extended model 9

Differences in the reflectance spectra of n- and p-doped Ge at the fundamental edge are interpreted in terms of the spatial field-dependence. These changes in the dielectric constants are achieved by a numerical solution of the Poisson equation and a calculation by the Franz-Keldyshtheory. Considering the inhomogeneous perturbation as a multilayer thin film problem the resulting reflectance was found by a recursion relation. Calculations were carried out for several dopings showing the existence of an optimal relation between the doping and the surface field. The ratio of field penetration to wavelength of the incident light accounts for the difference in optical behaviour. These results may well be understood by means of an exponential optical profile replacing the calculated one

Die Molekülstruktur des Tetracyanmethans in der Gasphase wurde mit Hilfe der Elektronen beugung bestimmt. Für die Bindungsabstände ergaben sich folgende Werte: r C - C = 1,484 ± 0,005 Å und r C - N = 1,161 ± 0,003 Å. Außerdem wurden die Bastansen-Morino-Shrinkage-Effekte für die ungebundenen interatomaren Abstände experimentell ermittelt

Photoions produced in methanol and formaldehyde by radiation in the spectral region 450 - 1150 Å were analyzed mass spectrometrically and their relative yields were determined as a function of wavelength. First ionisation potentials were determined and the ion yield curves were interpreted in terms of ionisation processes in conjunction with other data. Fragment ions were detected on mass numbers 31, 30, 29, 15, and 14 for methanol, and 29, 2, and 1 for formaldehyde. The associated appearence potentials were determined and were used to calculate heats of formation of the ions CH 2 OH + and HCO + , and the radicals CH 3 , CH 2 , and HCO. The most important result appears to be the heat of formation found for HCO: ΔH B = 0.43 ± 0.07 eV, corresponding to 9.9 ± 1.6 kcal/mole, as well as the associated dissociation energies for HCO and formaldehyde. Previously existing discrepancies concerning these quantities are thereby clarified

Quite a few investigations have been published on the diffusion of modifiers in glassy substances, but only a few studies have been reported on the self diffusion in a glassformer itself 1 . The self diffusion of oxygen in a boron trioxide melt was measured in this study

The absorption spectra of the pure and lead activated β-modification of Zn (SCN) 2 were investigated. For the width of the forbidden band, 4.7 eV as minimal value is obtained. From the experimental data the conclusion is drawn that strongly localized activator states are responsible for the luminescence. The observed absorption bands can be connected with electron transitions to the 3 P 1, 3 P 2 and 1 P 1 term components of the lead ion

Mean amplitudes of vibration, Bastiansen-Morino linear shrinkages and the most important first order Coriolis coupling constants have been calculated for the molecule Methyl Silyl Acetylene. The effect of torsional vibration on the nonbonded mean amplitudes has also been investigated

Hetero-dimer ions are observed in the field ionization mass spectra of aniline-nitrobenzene mixtures, and also in mixtures of some other organic substances which yield charge-transfer complexes in solutions. It is shown that the structure of the hetero-dimer ions is different from the sandwich structure of the corresponding CT-complexes in solutions. Arguments are given for assuming hydrogen bonds between the components of the complexes, with an additional weak CT-bond

The existence of an one-dimensional metallic state in crystallized transition metal complexes with a columnar crystal structure can be identified by their linearly polarized metal-like reflection. Compounds of this type do not show a macroscopically detectable metallic conductivity

The object of this note is to study the motion of a charged particle entering the magnetic field due to a steady current inside a plasma column. Even if the particles of the plasmas stream mutually interact, their distribution across the stream can be such that the interaction forces are balanced by the transverse pressure gradient in the plasma 1 . It is shown here that the test charge entering the plasma stream remains bounded to the stream between two coaxial cylindrical surfaces and further that under suitable conditions it may remain trapped inside a cylindrical box. FISSER and KIPPENHAHN 2 have discussed the general problem of the motion of a charged particle in an axially symmetric magnetic field in configuration space. The following is a particular case of that problem. HERTWECK 3 has investigated the motion of a test charge in the magnetic field due to a line current. His results are applicable here if the test charge remains outside the plasma stream

The relative differences of the rms ground state nuclear charge radii of 15 N - 14 N and 18 O - 16 O have been measured by low energy elastic electron scattering to be (1.3 + 0.7)% and (2.4 + 0.6)%, respectively. Both values are less than those following from an A 1/3 dependence

Durch Reaktionen von Metallcarbonylen mit Sauerstoff-, Stickstoff- und Wasserstoffatomen in der Gasphase bei Totaldrucken von einigen Torr werden elektronisch angeregte Produkte gebildet, die ein charakteristisches Spektrum emittieren