Stimulation von ZnS-Phosphoren mit langwelligem Infrarot

Stimulation experiments with ZnS phosphors, using IR wavelengths from 2 to 15μ, were performed at the temperature of liquid helium. For this purpose a cryostat was constructed which allowed to keep the samples and the screening device at the temperature of liquid helium. Moreover glow curves after different decay times at 6 °K were taken. After excitation of ZnS phosphors a strong release of carriers from relatively deep traps is taking place although the phosphor is kept at the temperature of liquid helium. This emptying of traps is accompanied by a luminous recombination of the released electrons with the activator levels producing an intense afterglow which can be observed over a long time. This phenomenon cannot be explained by thermal release of trapped electrons into the conduction band, followed by recombination with the activator levels because of the depth of the emptied traps. The rate of thermal carrier release was calculated to be about 10^?29 sec^?1 for 0,05 eV deep traps, but the observed rate was of the order of 10^?4 sec^?1. An emptying of traps by IR-stimulation can be excluded because the phosphor was surrounded by a concentric screening device kept at 4.2 °K. The effect can be explained by luminous tunneling of trapped electrons to the activator levels. An estimate of the tunneling rate gives a value agreeing with the experimental results. If such a “tunnel afterglow” does exist a spectral shift to longer wavelengths compared to the usual fluorescence and phosphorescence is to be expected. This could be actually observed. Additional experiments included IR-stimulated emission after various decay times, glow curves taken after such stimulation, and studies of the influence of temperature. The results rule out the possibility that the carriers were released from the traps by IR light. Apparently, IR radiation affects the potential barrier between the traps and the activators so that the rate of recombination by tunneling increases.     

Aqueous chlorination of α-terpineol

Of the multitude of products obtained upon the aqueous chlorination of α-terpineol (1), under both acidic and basic conditions, the major ones are the diequatorial trans, 4, and diaxial trans, 5, Markovnikov adducts of hypochlorous acid to the olefinic center. With the exclusion of small amounts of dichloride formation, the remainder of the products appear to be derived from 4 and 5 by hydrolysis or elimination sequences. The product distribution exhibits an expected pH dependence that is useful in the structural assignments.     

Phonon detection by shallow electronic trap states in luminescent ZnO

High-frequency acoustic phonons are detected in a ZnO crystal by phonon- induced luminescence radiation. Our experiments indicate that this radiation is caused by recombination processes of carriers captured at very shallow traps with an ionization energy of about 4 meV. Propagation of the phonons is studied by a time of flight method.     

Intensitätshysterese der Photoemission von Multialkalikathoden bei 77 °K

In a preceding paper it was reported on the photoemission of multialkali photocathodes with temperatures of about 77 °K, where nonlinearities, discontinuities and hystereses of the emission current were observed when the wave length and the intensity of the light was varied. In the following a detailed account is given on measurements of hystereses with alternating light intensities. Furtherly the absorption of the photocathodes was measured, while the emission current was at a discontinuity with a jump of the current by a factor of two and more. The absorption was independent of the jumps. From this and from a detailed discussion of the intensity hystereses it can be concluded an independence of the effect of: a) the primary interaction of the light with the cathode, b) the conduction mechanism of electrons through the cathode (for then otherwise a band gap of 7.05 eV should follow), c) the potentialstep at the cathode surface. Observed quantum yields of 100% are not due to electron multiplication by the initial energy of the excited photoelectrons but seem to indicate an exciton mechanism and/or field effect, the latter being quite different from the Malter effect. The intensity at which the upward jump occurs increases by a factor of ten when the cathode temperature rises by 0.36 °K. This temperature sensitivity is interesting in application as radiation detector.     

Chiroptical properties of 1-carbapenam and orbital mixing in nonplanar amides

Optical rotatory properties of β-lactam structures are calculated using semiempirical Extended Hückel and CNDO wavefunctions. Possible interpretations of CD and UV absorption data for a recently reported 5-epi-1-carbapenam, (5S)-1-azabicyclo-[3.2.0] heptan-7-one, are proposed. This compound lacks the sulfur of a penicillin nucleus, but still shows a Cotton effect at 231 nm. Analysis is aided by calculations on some monocyclic β-lactams, N-methyl-3-aminoazetidin-2-ones, where the Me carbon is incrementally bent out of the plane of the 4-membered ring. When N is pyramidal, the n and π orbitals of planar amides become mixed into two orbitals which correctly yield opposite signs for the Cotton effects at 231 nm and at just below 200 nm. A correspondence is proposed between the CD bands of 5-epi-1-carbapenam and those observed for another carbocyclic, nonplanar lactam, 4-azatricyclo(4.4.0.0^3,8] decan-5-one. The origin of the spectroscopically relevant orbitals of penams, penam sulfoxides, and other β-lactams is shown in a correlation diagram. In the spectral calculations, shortcomings of the semiempirical methods are encountered and discussed.     

Brilliant Sm, Eu, Tb, and Dy chiral lanthanide complexes with strong circularly polarized luminescence

The synthesis, characterization, and luminescent behavior of trivalent Sm, Eu, Dy, and Tb complexes of two enantiomeric, octadentate, chiral, 2-hydroxyisophthalamide ligands are reported. These complexes are highly luminescent in solution. Functionalization of the achiral parent ligand with a chiral 1-phenylethylamine substituent on the open face of the complex in close proximity to the metal center yields complexes with strong circularly polarized luminescence (CPL) activity. This appears to be the first example of a system utilizing the same ligand architecture to sensitize four different lanthanide cations and display CPL activity. The luminescence dissymmetry factor, g(lum), recorded for the Eu(III) complex is one of the highest values reported, and this is the first time the CPL effect has been demonstrated for a Sm(III) complex with a chiral ligand. The combination of high luminescence intensity with CPL activity should enable new bioanalytical applications of macromolecules in chiral environments.     

The Application of Nafion Metal Catalyst Free Carbon Nanotube Modified Gold Electrode: Voltammetric Zinc Detection in Serum

Metal catalyst free carbon nanotube (MCFCNT) whiskers were first used as an electrode modification material on a gold electrode surface for zinc voltammetric measurements. A composite film of Nafion and MCFCNT whiskers was applied to a gold electrode surface to form a mechanically stable sensor. The sensor was then used for zinc detection in both acetate buffer solution and extracted bovine serum solution. A limit of detection of 53 nM was achieved for a 120 s deposition time. The zinc in bovine serum was extracted via a double extraction procedure using dithizone in chloroform as a zinc chelating ligand. The modified electrode was found to be both reliable and sensitive for zinc measurements in both matrices.     

Die Brauchbarkeit des Geigerschen Spitzenzählers für β-Strahlen verschiedener Geschwindigkeiten und die Zahl der β-Strahlen von RaE und Ra D

Zeitschrift für Physik A Hadrons and nuclei - Der Zweck der Untersuchung war die Beantwortung der Frage, ob beim Radium E, das ein reiner β-Strahler (ohne γ-Strahlemission) ist, die...