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Notes: We report electroluminescence (EL) degradation studies of thin-film organic light-emitting diodes under ambient conditions. Bilayer organic ITO/TPD/Alq3/Mg/Ag devices were studied via EL and photoluminescence (PL) microscopy. In situ imaging of device luminescing areas and measurement of sample luminance were performed, allowing for a detailed study of black spot formation and luminance reduction under constant voltage stress conditions. Post-stress devices were further characterized using PL microscopy, and it was found that black spots result from delamination of the metal at the Alq3/Mg interface initiated by pinholes on the cathode, caused by substrate defects. © 1996 American Institute of Physics.

Notes: Dissolved paramagnetic ions generally provide an efficient mechanism for the relaxation of nuclear spins in solution, a phenomenon called the nuclear magnetic resonance-paramagnetic relaxation enhancement (NMR-PRE). Metal ions with electron spins S≥1 exhibit rich NMR relaxation phenomena originating in the properties of the zero-field splitting (zfs) interaction, which vanishes for spin-〈fraction SHAPE="CASE"〉12 ions but which is nonzero for S≥1 ions in site symmetry lower than cubic. For S≥1 ions in the vicinity of the zfs-limit, i.e., at magnetic-field strengths low enough that the zfs energy exceeds the Zeeman energy, the NMR-PRE depends strongly on the detailed structure of the electron spin energy levels as well as on the spatial quantization of the spin motion. It is shown theoretically and experimentally that the NMR-PRE produced by integer spins can be influenced strongly by the small intradoublet zero-field splittings, i.e., the splittings between the components of the non-Kramers doublets, which are produced by noncylindrical components of the crystal field potential. These small splittings produce relatively low-frequency oscillations in the dipolar field associated with 〈Sz(circumflex)〉 (the spin component along the molecule-fixed z(circumflex) axis). These motions decouple the nuclear spin from the electron spin, thereby depressing, in some cases very strongly, the NMR-PRE. The presence of a relatively small Zeeman field, comparable in magnitude to the intradoublet spacing but small compared to the larger interdoublet zfs splittings, causes a major change in the spin wave functions which has profound effects on the motions of the electron spin. When the Zeeman energy exceeds the small zfs splitting, the oscillatory motion of 〈Sz(circumflex)〉 damps out, with the result that the electron spin couples more effectively to the nuclear spin, providing a more efficient NMR relaxation pathway. NMR-PRE data are presented for the S=1 complex Ni(II)(o-pda)2Cl2 (o-pda=ortho-phenylenediamine) which confirm the importance of the splitting of the mS=±1 non-Kramers doublet on the NMR relaxation efficiency. The zfs E-parameter was measured from the NMR data to be |E|=0.26 cm−1. The S=2 spin system, Mn(III)-tetraphenylporphyrin sulfonate, exhibits a related phenomenon which arises from the effects of a small zfs splitting, Δε±2, of the mS=±2 non-Kramers doublet that is caused by a fourfold rotational component of the crystal field potential. The splitting Δε±2 was measured from NMR data to be 0.20 cm−1. © 1998 American Institute of Physics.

Notes: Electronic couplings in long-range electron transfer have been calculated using ab initio molecular orbital theory to investigate the effect of the binding energy of the electron on the decay of through-space and through-bond couplings. Through-space and through-bond couplings for anions and cations of the CF3 dimer and of CnH2n and CnF2n chains were calculated by ab initio molecular orbital theory. The anions and cations provide systems for which the electron binding energies, Be, differ by about a factor of 10. Through-space couplings decay exponentially, exp(−βR), with increasing distance, R, between the donor/acceptor carbon atoms. The decay coefficient β varies approximately as B1/2e. In contrast, the decay coefficients for through-bond coupling in CnF2n and CnH2n chains are not affected significantly by the binding energy of the electron. © 1996 American Institute of Physics.

Notes: We report on experiments using the 100 TW laser at LLNL (40J, 400 fs, 5×1019 W/cm2 focal intensity) and planar multilayer targets (Mo/Sn) to study the generation and transport of electrons with MeV energies. Such fast electrons are of prime importance to many proposed applications, e.g., the fast ignitor fusion concept. X-ray emission spectroscopy is used to study the electron transport. Characteristic K α photon emission produced by the fast electrons in the front (Mo) and rear (Sn) layers of the target is measured with a charge-coupled device detector (single photon counting mode) to infer the electron energy deposition. The electron energy spectrum is measured by varying the thickness of the Mo layer to attenuate the electrons by different amounts. Penumbral imaging of the K α emission is used to give information about the angular distribution of the fast electron emission. Details of the measurement techniques and experimental results will be discussed and compared with modeling calculations.© 1997 American Institute of Physics.

Notes: We investigate the shielding of superconducting and μ-metal forms in axial and transverse directed background magnetic noise fields. Analytical expressions are obtained for the improvement in signal-to-noise ratio obtained by placing a superconducting disk in the presence of a dipole source and a uniform noise field. Axial and transverse shielding factors are then compared for identical superconducting and μ-metal cylinders. The signal-to-noise ratio is found to be infinite at certain points inside a superconducting cylinder as well as a superconducting cylinder with a central partition. Shielding factors obtained here are relevant to SQUID measurements of small dipole source fields in the presence of large background noise fields such as those encountered in biomagnetism and nondestructive evaluation. © 1999 American Institute of Physics.

Notes: We report on the first plasma experiments with the 100 TW laser at LLNL. We have experimentally investigated the interaction of 1 μm, 40 J, 400 fs, 5×1019 W/cm2 laser pulses with multilayer solid targets. Various diagnostics were used to characterize this interaction. An x-ray pinhole camera monitored laser spot size and plasma location on target. Part of the laser beam was split off to probe the density scale length of the preformed plasma. An optical system for Schlieren photography was designed to record the images of plasma with electron density gradients as low as 1021 cm−4 with characteristic scale length of 10 μm. The range of electron densities was 1018–2020 cm−3. CR-39 plastic track detectors were employed to measure the fast ion blowoff from the plasma. Using a foil-filter technique the proton energy spectrum up to 6 MeV was measured. It was found that the high energy tail usually had a narrow (FWHM ∼10°) cone of expansion. Energy conversion to the fast ions (above 0.5 MeV) was estimated to be ∼0.3%–1% of laser energy. In addition, charge-coupled devices were used for detecting characteristic K α emission caused by fast (〉1 MeV) electrons. (See article by K. Wharton for further discussion of the fast electron measurements.) Estimated fast electron yield ∼3%.© 1997 American Institute of Physics.

Notes: A set of 19 dome-shaped divertor Langmuir probes similar in design to probes used in JET and JT-60 has been used successfully in DIII-D to measure the divertor electron temperature, particle flux, and floating potential. A comparison of the power flux using IR cameras with the particle flux and electron temperature from the probes using collisionless sheath theory has indicated that the particle flux to the divertor surface may be strongly modified by collisions within the magnetic sheath. In order to study this effect, a set of probes have been designed that can be inserted into the divertor plasma using DIMES. Two dome-shaped probes compare fluxes intercepted both above and within the magnetic sheath of the divertor surface. In addition, a third probe oriented normal to the magnetic field is used to verify the projected area of the probe surface. The probe design accommodates parallel power fluxes up to 50 MW/m2 for 30 ms, allowing for the study of beam-heated plasmas in DIII-D. © 1995 American Institute of Physics.

Notes: We have studied the inhibition of superconductivity in high-temperature superconductors oxide films by aluminum ion implantation. Aluminum ions, with doses ranging from 1×1015–1×1016/cm2, were implanted into epitaxial YBa2Cu3O7−x (YBCO) films with an injection energy of 100 keV. Doses of 1×1016/cm2 completely suppressed the diamagnetism of the YBCO film without the need for annealing. Lower doses of 1×1015/cm2 inhibited the superconductivity after low-temperature annealing. The results of the aluminum implantation are compared with previous silicon and boron implantations. © 1995 American Institute of Physics.

Notes: The mode transition from a capacitively coupled mode (E mode) to an inductively coupled mode (H mode) was observed in an inductive Ar plasma source by applying an axially uniform low B field. The applied fundamental rf was 13.56 MHz and many harmonic components were observed. A beat and standing wave patterns of azimuthally symmetric (m=0 mode) first and second harmonic pseudosurfaces and helicon waves were measured at various densities (n∼9.0×1010–2.2×1011 cm−3) and B fields (12–28 G). Wave propagation mode changes, from pseudosurface to helicon waves and from helicon to pseudosurface waves, were observed at critical conditions, ωc/ω〉3.0 and n∼2.2×1011 cm−3. © 1998 American Institute of Physics.