Kaiser, W.; Garrett, C. G. B. (1961). "Two-Photon Excitation in CaF2:Eu2+". Physical Review Letters7 (6): 229. doi:10.1103/PhysRevLett.7.229.
Lewis, Gilbert N.; Lipkin, David; Magel, Theodore T. (November 1941). "Reversible Photochemical Processes in Rigid Media. A Study of the Phosphorescent State" (EN). Journal of the American Chemical Society63 (11): 3005–3018. doi:10.1021/ja01856a043.