  Charge neutrality and their expected itinerant nature makes excitons
potential transmitters of information. However, exciton mobility remains
inaccessible to traditional optical experiments that only create and detect
excitons with negligible momentum. Here, using angle-resolved photoemission
spectroscopy, we detect dispersing excitons in the quasi-one-dimensional
metallic trichalcogenide, TaSe3. The low density of conduction electrons and
the low dimensionality in TaSe3 combined with a polaronic renormalization of
the conduction band and the poorly screened interaction between these polarons
and photo-induced valence holes leads to various excitonic bound states that we
interpret as intrachain and interchain excitons, and possibly trions. The
thresholds for the formation of a photo-hole together with an exciton appear as
side valence bands with dispersions nearly parallel to the main valence band,
but shifted to lower excitation energies. The energy separation between side
and main valence bands can be controlled by surface doping, enabling the tuning
of certain exciton properties.
