Infra-red spectra of evaporated MgF2 isolated at liquid helium temperatures in neon, argon, krypton, xenon, nitrogen and carbon monoxide matrices are reported. In contrast to previous studies, the bands at 480 and 740 cm-' are assigned to a dimeric molecular species, (MgF&, on the basis of the equivalent dependence of their intensities on the rate of deposition of MgF2 vapour. No symmetric stretching vibration was observed, which suggests a linear structure for MgF2. Deviations of the bond angle in M E n from 180" calculated from isdopic splittings of the asymmetric stretching mode v3 of MgFz for the different matrices are similar to those obtained from previous studies. The small differences of the calculated angle from linearity could result from an inherent inaccuracy of the isotopic shift method for bond angles near 180".

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Infrared spectra of MgF2 vapour co-deposited with vapours of alkali metal fluorides, LiF, NaF, KF and CsF and alkaline earth difluorides, CaF, and SrF,, in argon matrices at liquid helium temperatures are reported in the range 400-1000 cm-'. New frequencies are identified as belonging to mixed fluorides MMgF3 and M'MgF4. The frequencies are tentatively assigned to these species assuming a planar cyclic structure of C,, symmetry. A correlation with the spectrum of the dimer Mg2F4 of D2h symmetry is presented.

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