X-ray and high-resolution uv photoemission spectroscopy of the conduction band of Am metal directly reveal that the 5/electrons in Am are localized, in contrast to the 5/electrons in the lighter actinide metals. The observed 5/ final-state multiplet structure arises from a trivalent 5/ 6 Am ground state. The withdrawal of the 5/electrons from the Fermi level is also manifested in the core-hole screening mechanism: The 4/ core-level spectrum shows the poorly screened peak (s,d electrons) to be the most intense one. PACS numbers: 71.70.Ms, 71.25.Pi, 79.60.Cn It has been suggested 1,2 that the 5/electrons for the lighter actinides are itinerant and take part in the chemical bond whereas the 5/ electrons of the heavier actinide metals are localized. The solidstate properties of Am indicate that Am is the first rare-earth-like metal in the actinide series. 3 Theoretical calculations 4 have also supported the localized picture for Am. The ground-state electron configuration is assumed to be 5f 6 (nonmagnetic). Previous valence-band photoemission spectra of U metal 5 "" 9 provided clear evidence for the itinerant character of the 5/ electrons, which were identified to be at the Fermi level. Mainly because of the radiation hazard of the heavier actinide metals, only for Pu metal 10 " 12 have low-resolution x-ray photoemission spectroscopy (XPS) measurements been published supporting the itinerant description of the 5/states. To examine where the 5/ electrons become localized in the actinide series we modified a LeyboldHeraeus LHS-10 photoemission spectrometer, enclosed in glove boxes to enable XPS and, in particular, high-resolution ultraviolet photoemission spectroscopy (UPS) measurements on highly radioactive material. 13 In this Letter, we present photoemission spectra of the 4/core levels (XPS) and of the conduction band (XPS, UPS) of Am metal, which holds a key position in the actinide series.Polycrystalline films of Am metal (5 /xm thick) were condensed by evaporation on W substrates 14 and afterwards transferred to the photoemission spectrometer. The surface Am oxide layer was removed in situ by Ar-ion sputtering. Conductionband data were recorded at room temperature with an energy resolution of 1.30 eV (XPS, Mg Ka line) and 0.12 to 0.17 eV (Hei and Hen lines, respectively) ; the Am 4/ core levels were recorded with an energy resolution of 1.0 eV (XPS, Mg Ka line). The base pressure was in the low 10" 9 Pa range.After repeated sputtering cycles no oxygen surface contamination was detectable in XPS (O Is emission), but was in UPS (O 2p emission); the O 2p signal was slowly growing in time as a result of segregation of oxygen to the surface. To improve statistics several sets of spectra were recorded directly after sputter cleaning. By calculating the difference spectra the weak oxygen contribution was determined and finally subtracted.The conduction-band spectra of Am metal (Fig. 1) show for increasing photon energy that the emission at the Fermi energy E F becomes less important compared to that of the structu...

the stabilization is more pronounced with lower cerim oxide concentration. X-ray absorption spectroscopy measurements done on the L,,, edge sbow the same variation as a function of cerium oxide concentration, but in the bulk the reduction is mu& lower. Cerium in Pt/TiO,-CeO, catalysts exhibits the same behavim.

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