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.
L. F. H. BOVEY, "The Infrared Absorption and Reflection Spectra of the Ammonium Halides," J. Opt. Soc. Am. 41, 836-848 (1951)