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 $\gamma$ phase with electron configuration $4f^1{\mathrm{}\left(5d6s\right)\mathrm{}}^3$. When it is compressed to about 7 kbar, a $\gamma -\alpha$ phase transition occurs. To understand this phase transition, the energy position of the $f$ level in $\gamma$-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 $K\alpha$ 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 $K\alpha$) measurements on clean Ce films ($\gamma$-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 $\gamma$-Ce is 1.9±0.2 eV relative to the Fermi level. To account for the $\gamma -\alpha$ 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 $\gamma$-Ce. Thus, the present results disfayor these models. However, in the model describing the $\gamma -\alpha$ 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 $\gamma -\alpha$ transition is due to a Mott transition of the $4f$ electron and a subsequent hybridization with the $\mathrm{sd}$ band.