Zero-phonon lines in CaF2and SrF2

A number of prominent zero-phonon lines have been investigated in additively coloured CaF 2 and SrF 2 using uniaxial stress. Stark effect and magneto-optical techniques. It is tentatively suggested that a line at 598.8 nm in CaF 2 may be a transition of a square planar F 4 complex. There is a corresponding centre in SrF 2 . Other lines investigated appear to be associated with impurities. A line at 553.7 nm in CaF 2 is a transition of a centre with cubic symmetry; a corresponding centre occurs in SrF 2 . A line at 742.6 nm in CaF 2 also arises from a centre with cubic symmetry. A transition at 833.8 nm in CaF 2 is associated with a centre with C 2v symmetry. An E to A type transition in a centre with tetragonal symmetry occurs in CaF 2 at 524.7 nm A line at 591.0 nm in CaF 2 is an A-A type transition in a centre with trigonal symmetry. No lines have been found in SrF 2 corresponding to the 742.6, 833.8, 524.7 and 591.0 nm lines in CaF 2 . Vibronic structure associated with the zero- phonon lines is described and problems of distinguishing between intrinsic F-aggregate centres and impurity associated centres are mentioned.