  We aim to perform direct optical spectroscopy of the Th-229 nuclear isomer to
measure its energy and lifetime, and to demonstrate optical coupling to the
nucleus. To this end, we develop Th-doped CaF2 crystals, which are transparent
at the anticipated isomer wavelength. Such crystals are illuminated by tunable
VUV undulator radiation for direct excitation of the isomer. We scan a 5 sigma
region around the assumed isomer energy of 7.8(5) eV and vary the excitation
time in sequential scans between 30 and 600 seconds. Suffering from an
unforeseen strong photoluminescence of the crystal, the experiment is sensitive
only to radiative isomer lifetimes between 0.2 and 1.1 seconds. For this
parameter range, and assuming radiative decay as the dominant de-excitation
channel, we can exclude an isomer with energy between 7.5 and 10 eV at the 95%
confidence level.
