Nuclear physics is not my forte, but if spin is to be conserved, wouldn't there need to be a spin-isomer transition: one photon in, two photons out --> spin qua

Nuclear physics is not my forte, but if spin is to be conserved, wouldn't there need to be a spin-isomer transition: one photon in, two photons out --> spin quantum number of nucleus must change to balance? I think one good way to narrow your search would be to identify a single candidate element (or a few) that has such a "forbidden transition" where, despite being forbidden, the time constant is still quite short (so you can get a lot of light out of repeated excitation of a single nucleus) and the energy gap that gives rise to one of the photons is a mere 1.5~3.5 eV (visible). With a good searchable database of nuclear isomer energy levels, it should be possible to identify transitions like these, and if only a few elements have them, you can start including those names in the search and maybe get results that point more in the right direction. Or do you already have the names of some of those oddball nuclei you mentioned? I haven't found a good searchable database, just an ugly scanned PDF of Kocher's "Radioactive Decay Data Tables" (1981). Maybe someone has published a more computer-searchable version of the data since then.