  We performed group theoretical investigation of symmetries of excitations in
topological insulators \ce{Bi2Sb3}, \ce{Bi2Te3}, \ce{Bi2Se3} and \ce{Sb2Te3},
focusing on selection rules for optical processes. Electronic transitions of
bulk states to bulk states, surface states to surface states and bulk states to
surface states are studied over the entire Brillouin zone. A new technique is
used to deal with transitions between surface states and bulk states. Time
reversal symmtry is also included in the analysis. Our results show that only
(\Gamma), (\Lambda) and (M) points of the Brillouin zone would depend on light
polarization for transitions between bulk states and surface states. As an
example of application, electron spin polarizations of photoelectrons are
calculated at (\Gamma) point. A general rule for the direct product between the
representation at an arbitrary point in the Brillouin zone with itself is
presented.
