Insulating surface layer on single crystal K <InlineEquation ID="Equ1"> <EquationSource Format="TEX">$\mathsf{_{3}}\mathsf{C}\mathsf{_{60}}$</EquationSource> </InlineEquation>

Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K<Superscript>3</Superscript>C<Subscript>60</Subscript> is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved. Copyright Springer-Verlag Berlin/Heidelberg 2004