  We study the microscopic origins of photocurrent generation in the
topological insulator Bi$_2$Se$_3$ via time- and angle-resolved photoemission
spectroscopy. We image the unoccupied band structure as it evolves following a
circularly polarized optical excitation and observe an asymmetric electron
population in momentum space, which is the spectroscopic signature of a
photocurrent. By analyzing the rise times of the population we identify which
occupied and unoccupied electronic states are coupled by the optical
excitation. We conclude that photocurrents can only be excited via resonant
optical transitions coupling to spin-orbital textured states. Our work provides
a microscopic understanding of how to control photocurrents in systems with
spin-orbit coupling and broken inversion symmetry.
