Valence band ofγ-cerium studied by ultraviolet and x-ray photoemission spectroscopy

At room temperature and low pressure, pure cerium is known to be in the γ phase with electron configuration 4f¹(5d6s)³. When it is compressed to about 7 kbar, a γ-α phase transition occurs. To understand this phase transition, the energy position of the f level in γ-cerium is of utmost importance. However, it has not been possible to determine an experimental value of the f-level binding energy using photoemission. In spectra excited by monochromatized Al Kα radiation, the overlapping $4f$ and $5d6s$ emissions are not resolved, and it has not been found possible to draw firm conclusions as to the relative order of the f level and the s-d band from the structure observed. The present paper concerns ultraviolet-photoemission (UPS) (He i and He ii) and XPS (Mg Kα) measurements on clean Ce films (γ-phase). Even though the $4f$-level and $5d6s$-band emissions are not resolved, the $4f$-level energy position can be estimated from comparison of XPS and UPS valence-band spectra. In the spectra from Ce films exposed to oxygen at room temperature the emission from the $5d6s$ band is vanishing, thus allowing for an identification of the $4f$-level emission. The observed binding energy of the $4f$ level in γ-Ce is 1.9±{}0.2 eV relative to the Fermi level. To account for the γ-α phase transition using either the promotion model and its various extensions or the sd-f hybridization model, it is required that the $4f$ level is situated just below the Fermi energy in γ-Ce. Thus, the present results disfayor these models. However, in the model describing the γ-α transition as a Mott transition within the $4f$ shell, such close proximity of the $4f$ level to the Fermi level is not required. Our results therefore indicate that the γ-α transition is due to a Mott transition of the $4f$ electron and a subsequent hybridization with the sd band.