The nucleus of 229Th with its isomeric state 229mTh at about 8 eV energy is a candidate for an ultra-precise nuclear optical clock, allowing for tests of fundamental principles. Nevertheless, the exact excitation energy of the isomer is not yet known. This experiment aims at the laser excitation of the isomer via the electron shell in 229Th+, 229Th2+ ions and the direct laser excitation of the nucleus in 229Th3+ ions for an accurate determination of its transition frequency.