A pair-ion plasma without electrons consisting of $\mathrm{C}_{60}{}^{+}$ and $\mathrm{C}_{60}{}^{-}$ is generated through the processes of electron-impact ionization, electron attachment, and magnetic filtering. Properties of electrostatic modes propagating along magnetic-field lines are experimentally investigated by externally exciting them with two types of electrodes. It is found that four kinds of wave modes exist and a frequency spectrum of phase lag between the density fluctuations of $\mathrm{C}_{60}{}^{+}$ and $\mathrm{C}_{60}{}^{-}$ is unique in comparison with ordinary electron-ion plasmas. One of the modes is an ion acoustic wave which is divided into two branches at around the ion cyclotron frequency in the presence of a backwardlike mode joining them. The phase lag of the ion acoustic wave strongly depends on the frequency, while those for the other ion plasma and intermediate-frequency waves are constant at $\pi$ independent of the frequency.

• Received 25 September 2006

DOI:https://doi.org/10.1103/PhysRevE.75.056403