  Atomically precise armchair graphene nanoribbons of width $N=7$ (7-AGNRs) are
investigated by scanning tunneling spectroscopy (STS) on Au(111). The analysis
of energy-dependent standing wave patterns of finite length ribbons allows, by
Fourier transformation, the direct extraction of the dispersion relation of
frontier electronic states. Aided by density functional theory calculations, we
assign the states to the valence band, the conduction band and the next empty
band of 7-AGNRs, determine effective masses of $0.42\pm 0.08\,m_e$, $0.40\pm
0.18\,m_e$ and $0.20\pm 0.03\,m_e$, respectively, and a band gap of $2.37\pm
0.06$ eV.
