  Low-temperature electronic properties of rare-earth intermetallics are
substantially influenced by the symmetry and magnitude of the crystal electric
field. The direct spectroscopic analysis of crystal field splitting can be
challenging, especially in low-dimensional systems, because it requires both
high spectral resolution and pronounced sensitivity. We demonstrate the
eligibility of electronic Raman spectroscopy for this purpose by the direct
determination of the $4f$ level splitting in ultra-thin ordered CePt$_5$ films
down to 1.7 nm thickness on Pt(111). Crystal field excitations of Ce $4f$
electrons give rise to Raman peaks with energy shifts up to $\approx$ 25 meV.
Three distinct peaks occur which we attribute to inequivalent Ce sites, located
(i) at the interface to the substrate, (ii) next to the Pt-terminated surface,
and (iii) in the CePt$_5$ layers in between. The well-resolved Raman signatures
allow us to identify a reduced crystal field splitting at the interface and an
enhancement at the surface, highlighting its strong dependence on the local
atomic environment.
