Length Scales in Core Level Excitation: C1sX-Ray Absorption ofC60/Al

We present evidence for atomic site-dependent excitonic effects in the x-ray absorption spectrum of an ordered chemisorbed monolayer of ${\mathrm{C}}_{60}$/Al(110). We observe that the broadening in the ${5t}_{1u}$-derived resonance and the corresponding core level resonant photoemission intensity varies with the x-ray incidence angle. This can be understood as due to changes in the strength of the interaction between the x-ray absorption final state and the substrate bands. This study provides the first direct measurement of the length scale for the localization of an excited state by the core hole.