(h) 50% 80 40 0 -40 80 40 0 -40 θ =25° θ =40° 284.15 θ =52° θ =62° θ =0°  binding energy (eV) φ ( ) (I-I 0 )/I 0 photoemission intensity (arb. units) φ ( ) binding energy (eV)$^{(a)}$ (b) (c) (d) (e) (g) θ φ h ν e - φ =0 284.55 283.75 284.15 284.55 283.75 284.15 284.55 283.75 284.15 284.11 (f) θ =0° fits θ =25° FIG. 1: (a)-(e) C 1 s photoemission spectra taken at a photon energy of 400 eV, for fixed polar emission angles θ in each panel but at different azimuthal emission angles φ (see sketch of the experimental geometry at the top of the figure). The spectra are all normalized to the same height and shown as a group plot, such that binding energy variations become evident. The first and last spectrum in a range of azimuthal angles φ is indicated by a thicker line. (f) Comparison of the spectrum taken at θ = 0 ◦ and one taken at 25 $^{◦}$. The lines are the fits through the data points using the lineshape parameters described in the supplementary material. (g) C 1 s binding energy required to obtain a good fit for the curves shown above (markers) as well as for the entire azimuthal range measured (lines). The green horizontal line marks the binding energy at normal emission. The binding energy uncertainty is smaller than 10 meV. Error bars are not shown. (h) Intensity variation of the C 1 s peak as a function of azimuthal angle. The curves are shifted vertically for clarity. 3