Attempt to optically excite the nuclear isomer in 229Th

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-229-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 short radiative isomer lifetimes. In our data analysis, we assume that radiative decay is the dominant de-excitation channel. Further, we suppose that the luminescence background has a super-Poissonian character and may be represented as a sum of a constant term and several decaying exponents whose time constants do not depend on the wavelength of the excitation. With these assumptions, we can exclude an isomer with lifetime between 0.2 and 1.1 s and energy between 7.5 and 10 eV at the 95% confidence level.