  In-source resonance ionization spectroscopy was used to identify an efficient
and selective three step excitation/ionization scheme of thorium, suitable for
titanium:sapphire (Ti:sa) lasers. The measurements were carried out in
preparation of laser spectroscopic investigations for an identification of the
low-lying Th-229m isomer predicted at 7.6 +- 0.5 eV above the nuclear ground
state. Using a sample of Th-232, a multitude of optical transitions leading to
over 20 previously unknown intermediate states of even parity as well as
numerous high-lying odd parity auto-ionizing states were identified. Level
energies were determined with an accuracy of 0.06 cm-1 for intermediate and
0.15 cm-1 for auto-ionizing states. Using different excitation pathways an
assignment of total angular momenta for several energy levels was possible. One
particularly efficient ionization scheme of thorium, exhibiting saturation in
all three optical transitions, was studied in detail. For all three levels in
this scheme, the isotope shifts of the isotopes Th-228, Th-229, and Th-230
relative to Th-232 were measured. An overall efficiency including ionization,
transport and detection of 0.6 was determined, which was predominantly limited
by the transmission of the mass spectrometer ion optics.
