Loss-spectroscopy on sparse arrays of aligned single-wall carbon nanotubes

We performed angle resolved electron energy-loss spectroscopy on vertically aligned single wall carbon nanotubes. We observe two distinct loss-peaks in the energy range from 4.6 eV to 5.1 eV. This doublet of loss-peaks is attributed to the on-axis and crossed plasma oscillations of the π electrons in sp2 carbon. The unique identification and assignment of these two oriented excitations is derived from their dispersion relations. The available momenta are aligned on the nanotube axis and reflect the confinement of a particle on a wire. The observed angle resolved loss-peaks are a gapless extension of polarized optical spectroscopy into finite momenta.