Two-Photon Spectroscopy of Core Excitons in Barium Fluoride Using Synchrotron Radiation and Laser Light

We have conducted two-photon spectroscopy of core excitons in BaF₂. Synchrotron radiation and laser light were used for $5p$ core-electron excitation and Auger-free luminescence was detected as the signal. Two-photon excitation enables access to f and p orbitals that cannot be reached by one-photon excitation of electrons in p orbitals. It has been found that the spin-orbit splittings of $4f$ and $6p$ states of the Ba ion in BaF₂ are 0.7±0.1 and 1.4±0.1 eV, respectively.