Experimental details for the taking of infrared absorption spectra of thin solid layers at various temperatures are given and a method described for measurement of their thickness. An account of the preparation of suitable samples for reflection spectra is also included.
The spectra of the four ammonium halides at 20°C and −150°C have been obtained and the results interpreted in terms of their crystal structure. The data support the following symmetries for the ammonium ion in the low temperature form: (I) Td in ammonium chloride, (II) Vd in ammonium bromide, (III) Vd in ammonium iodide, and (IV) C3v or C3 in ammonium fluoride.
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