  A point charge near the surface of a topological insulator (TI) with broken
time-reversal symmetry is predicted to generate an image magnetic charge in
addition to an image electric charge. We use scanning tunneling spectroscopy to
study the image potential states (IPS) of the topological semimetal Sb(111)
surface. We observe five IPS with discrete energy levels that are well
described by a one-dimensional model. The spatial variation of the IPS energies
and lifetimes near surface step edges shows the first local signature of
resonant interband scattering between IPS, which suggests that image charges
too may interact. Our work motivates the exploration of the TI surface geometry
necessary to realize and manipulate a magnetic charge.
