  The collective excitation spectrum of two-dimensional (2D) antimonene is
calculated beyond the low energy continuum approximation. The dynamical
polarizability is computed using a 6-orbitals tight-binding model that properly
accounts for the band structure of antimonene in a broad energy range.
Electron-electron interaction is considered within the random phase
approximation. The obtained spectrum is rich, containing the standard
intra-band 2D plasmon and a set of single inter-band modes. We find that
spin-orbit interaction plays a fundamental role in the reconstruction of the
excitation spectrum, with the emergence of novel inter-band branches in the
continuum that interact with the plasmon.
