  Angle-resolved photoemission spectroscopy (ARPES) is typically used to study
only the occupied electronic band structure of a material. Here we use
laser-based ARPES to observe a feature in bismuth-based superconductors that,
in contrast, is related to the unoccupied states. Specifically, we observe a
dispersive suppression of intensity cutting across the valence band, which,
when compared with relativistic one-step calculations, can be traced to two
final-state gaps in the bands 6 eV above the Fermi level. This finding opens up
possibilities to bring the ultra-high momentum resolution of existing
laser-ARPES instruments to the unoccupied electron states. For cases where the
final-state gap is not the object of study, we find that its effects can be
made to vanish under certain experimental conditions.
