This paper presents a review of investigations of detailed spatial and temporal structures of high-voltage pulsed nanosecond discharges in the form of fast ionization waves. The most distinctive features of this type of discharge are a high propagation velocity (10 9 -10 10 cm s −1 ), good reproducibility of the discharge parameters at a moderate (tens of hertz) repetition rate and spatial homogeneity over a large gas volume. The discharge was initiated by voltage pulses of negative polarity with an amplitude of 10-15 kV, a duration at half maximum of 25 ns and a rise time of the front of 3-5 ns. The behaviour of the electric field and electron and excited-state concentrations were analysed on the basis of experimental data within the frame of the unified kinetic approach. It was found that the longitudinal component of the electric field has a sharp (2-3 ns) maximum and that the electrons and excited particles are produced preferentially behind the front in relatively weak electric fields. The peak field value was close to or even stronger than the threshold for the generation of runaway electrons in a steady-state uniform electric field. An analysis based on absolute time-resolved measurements of the spectrum of two molecular bands showed that, behind the breakdown front, the EEDF should be substantially overpopulated with high-energy electrons. Energy branching in the discharge was analysed. Possibilities of application of the fast ionization wave as a source of a uniform pulsed plasma were suggested and justified.

We have studied the electronic structure and its changes across the metal-insulator transition in the spineltype compound CuIr 2 S 4 using photoemission and inverse-photoemission spectroscopy. Photoemission spectra near the Fermi level show a gap opening of ϳ20 meV in the insulating phase, consistent with the transport activation energy. Core-level spectra indicate that the Cu ion is monovalent, and hence Ir is in the intermediate valence state of ϩ3.5. Comparison between the spectra and band-structure calculation reveals that the Ir 5d density of states is strongly distorted, probably due to electron correlation in spite of the general belief of weak correlation in 5d-electron systems. ͓S0163-1829͑97͒51924-2͔ RAPID COMMUNICATIONS PHYSICAL REVIEW B CONDENSED MATTER THIRD SERIES, VOLUME 55, NUMBER 24 15 JUNE 1997-II RAPID COMMUNICATIONS Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in the editorial office and in production. A Rapid Communication in Physical Review B may be no longer than four printed pages and must be accompanied by an abstract.

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The electronic structure of LiCu 2 O 2 was studied using x-ray emission ͑Cu L ␣ , O K ␣ ) and photoelectron spectroscopy ͑valence band and core levels͒ as well as band-structure calculations in terms of local spindensity approximation ͑LSDA͒ and LSDAϩU approaches. According to the x-ray-emission and photoelectron spectra the valence states of the Cu atoms are found to be mixed, i.e., 2ϩ and 1ϩ. The LSDA calculations are contradictory to the experimental data and cannot reproduce the band gap and magnetic properties of LiCu 2 O 2 . The LSDAϩU calculations describe the insulator and antiferromagnetic properties much better but the overestimation of the screened Coulomb parameter U leads to a binding-energy shift of the Cu II 3d states and this distorts the proper modeling of the valence-band structure. The magnetic structure of LiCu 2 O 2 is discussed, taking our LSDAϩU band-structure calculations into account. ͓S0163-1829͑98͒04108-3͔ D. A. ZATSEPIN et al.

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