  In this work, we studied the bulk band structure of a topological insulator
(TI) Bi2Se3 and determined the contributions of the Bi and Se orbital states to
the valence bands using standing wave-excited hard x-ray photoemission
spectroscopy (SW-HAXPES). This SW technique can provide the element-resolved
information and extract individual Bi and Se contributions to the Bi2Se3
valence band. Comparisons with density functional theory (DFT) calculations
(LDA and GW) reveal that the Bi 6s, Bi 6p, and Se 4p states are dominant in the
Bi2Se3 HAXPES valence band. These findings pave a way for studying the
element-resolved band structure and orbital contributions of this class of TIs.
