Resonant photoelectron spectroscopy studies have been performed on the NiO valence band at photon energies corresponding to the Ni 2 p, 3p, and O 1s absorption thresholds. Strong resonances are seen in the vicinity of the Ni 2p threshold, which confirm earlier conclusions from the weaker resonances seen at the Ni 3p threshold. No valence-band resonance is observed at the O 1s threshold. The analysis of this data confirms the picture of NiO as a strongly correlated charge-transfer insulator by identifying the highest-lying states as being of mainly 3d 8 L គ final-state character. The existence of localized excited Ni states, as well as the delocalized nature of the O states, are confirmed. Comparisons with the configuration-interaction model and quasiparticle calculations are also made.

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The InAs(111)2~2 and ~n~s (i i i) l x l surfaces have been studied with high resolution core level spectroscopy. For the ~n~s (i i i) l x l surface both the In 4d and the As 3d core levels display strong surface core level shifts, while for the InAs(l11)2x2 surface only the In 4d level shows a detectable surface shift. The results indicate that the ~n A s (i i i) l x l surface is relaxed, with atom layer displacement extending to subsurface layers. Unexpectedly, we find no surface shifted anion core level for the InAs(111)2~2 surface.

Photoelectron spectroscopy data from Bi 2 Sr 2 Ca 12x Y x Cu 2 O 81d single crystals, for x 0, 0.16, and 0.55, are presented. It is shown that there are core level shifts related to the opening of a pseudogap and that similar shifts are observed at the opening of the superconducting gap in optimally doped samples. This result is in agreement with pair formation above T C as suggested by V. J. Emery and S. A. Kivelson [Nature (London) 374, 434 (1995)]. [S0031-9007(97)03557-6]