Optical Spectra of M, F 3 + ‐Singlet, and F 3 + ‐Triplet Centres in NaF

X irradiated NaF crystals contain after bleaching the F band mainly M and F   3 +centres. Complete orientation of the M centres and partial orientation of the F   3 +centres can be achieved. The F   3 +centres, identified by their 545.6 nm zero‐phonon line, have broad absorption bands at 512 and 303 nm. Upon strong excitation in the region of the 303 nm band a temporary bleaching of both the bands and the zero‐phonon line is found and ascribed to population of a metastable F   3 + ‐centre triplet state with a lifetime of τ = 1.9 s at 77 °K.