  In this article, we explore the anisotropic electron energy loss spectrum
(EELS) in monolayer phosphorene based on ab-initio time dependent density
functional theory calculations. Similar to black phosphorous, the EELS of
undoped monolayer phosphorene is characterized by anisotropic excitonic peaks
for energies in vicinity of the bandgap, and by interband plasmon peaks for
higher energies. On doping, an additional intraband plasmon peak also appears
for energies within the bandgap. Similar to other two dimensional systems, the
intraband plasmon peak disperses as $\omega_{\rm pl} \propto \sqrt{q}$ in both
the zigzag and armchair directions in the long wavelength limit, and deviates
for larger wavevectors. The anisotropy of the long wavelength plasmon intraband
dispersion is found to be inversely proportional to the square root of the
ratio of the effective masses: $\omega_{\rm pl}(q \hat{y})/\omega_{\rm pl}(q
\hat{x}) = \sqrt{m_x/m_y}$.
