Resonant photoemission in CeAl, CeSi2, and LaSi2

We report valence-band photoemission at photon energies near the $4d$ absorption resonance for CeAl, a trivalent antiferromagnet with $T_N=10\mathrm{}$ K, and Ce${\mathrm{Si}}_2$, a nonmagnetic valence fluctuation compound with a characteristic (Kondo) energy of order 200 K. The main difference between the two spectra is a factor-of-2 difference in the width of the peak centered 2.1-2.2 eV below the Fermi energy ${\epsilon }_F$. We interpret this peak as the $4f$ emission; it is well fit by a Lorentzian whose halfwidth is $\Gamma =0.45\mathrm{}$ eV in CeAl and $\Gamma =0.8\mathrm{}$ eV in Ce${\mathrm{Si}}_2$. These results suggest that the difference in magnetic behavior of the two compounds reflects different Kondo temperatures resulting from different hybridization widths. The trivalence of both compounds (as opposed to fractional valence) is corroborated by measurements of the fine structure in the constant-initial-state (CIS) and constant-final-state spectra near the resonance. A second feature occurs near ${\epsilon }_F$ in both materials; this is most readily interpreted as $5d$ emission. Consistent with this explanation is an edge shift of the resonance (as measured in CIS) to lower photon energy for the peak near ${\epsilon }_F$ as compared to the peak near 2 eV.