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The observation of photon cascade emission in activated under 185 nm vacuum ultraviolet excitation is reported and discussed. In this host lattice, the lowest energy state of the 4f5d excited configuration is located above the state. The excitation of the 4f5d state by vacuum ultraviolet photons results in radiative emission originating from the level state. The absence of emission from the state is attributed to the efficient multiphonon relaxation in this lattice.
Srivastava et al. (Sun,) studied this question.