 those states that are in the reduced zone scheme one above another (thus the name "direct optical transitions").Another set of selection rules comes from formula_61 (or formula_62) when the photon polarization contained in formula_29 (or formula_64) and symmetries of the initial and final one-electron Bloch states formula_48 and formula_49 are taken into account.Those can lead to the suppression of the photoemission signal in certain parts of the reciprocal space, or can tell about the specific atomic-orbital origin of the initial and final states.