Infrared-Absorption Spectra of LiOH and LiOD

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Abstract
Spectra of films of LiOH and LiOD in the region from 450 cm—1 (22 μ) to 4500 cm—1 (2.2 μ) are presented. Polarized spectra of crystals of LiOH, LiOD, and mixed crystals of LiOH and LiOD at room temperature and at 15°K are also presented. The OH— ion stretching fundamental at 3678 cm—1 (2.7 μ) and the OD— ion stretching fundamental at 2713 cm—1 (3.69 μ) are each bracketed by sidebands in a nearly symmetric manner. The sidebands on the low wave‐number side of each fundamental, the difference sidebands, all vanish at 15°K, while the main features of the sidebands on the high wave‐number side of each fundamental, the summation sidebands, are preserved at low temperatures. In addition, the increased resolution resulting from reduced bandwidths at low temperatures reveals several new summation sidebands not observed in the room‐temperature spectrum. A correlation of the LiOH and LiOD summation sidebands is presented and the deuteration effects discussed. These effects are observed to be in very good agreement with the theory which assumes that the sidebands are the result of binary combinations of low‐frequency lattice modes with those modes which involve the hydroxyl ion stretching motions. They are not, however, in agreement with the rotation theory which proposes that the sidebands of LiOH, Mg(OH)2, and Ca(OH)2 are the results of the coupling of localized rotations of the OH— ions with their internal stretching frequency. An analysis of the mixed‐crystal spectra reveals the significant role of hydroxide—hydroxide interactions in producing the observed intensities of the sidebands.