The Infrared Absorption of the FCl − Heteronuclear Molecule Ion in KCl

Single crystals of KCl doped with KF and exposed to high dosages of Co 60 γ‐radiation develop a simple doublet absorption on the high frequency edge of the KCl reststrahl. This 403.7—399.8 cm −1 doublet is assigned to the fundamental internal vibration of the FCl − heteronuclear molecule ion. The doublet separation, intensity ratio, thermal decay, optical bleaching and polarization properties support this assignment. The internal stretching frequency of FCl − in KCl is not markedly different than would be approximated from gas phase data for the neutral species.