Ammonia--Spectra

The Raman spectra of ammonia solid I and II were studied
in the lattice and intramolecular region. The study of the internal modes of phase I suggests the presence of
disorder in the ordered phase. The lattice modes of
phase II are, for the first time, carefully reported as
two very broad lines superimposed on an overdamped
feature. The results confirm the structure previously
determined by neutron scattering and NMR that phase II is
a typical disordered phase with space group P6,3/mmc. A
mode assignment was tentatively made and Gruneisen
parameters of the two assumed translational modes
calculated. In terms of the Michel-Naudts theory of
rotational-translational interactions, phase II is in the
slow relaxation regime. A four-band model was used for
the intramolecular stretching region.

The frequencies of three lattice vibrational nodes of solid
ammonia I were measured as functions of temperature and molar
volume, via Raman spectroscopy. Calculated Gruneisen parameters
of 2.6, 2.4, and 0.8, for modes with frequencies near
108 cm^-1 , 140 cm^-1 and 306 cm^-1 , respectively, identify the
first two modes to be translational and the third librational
in accordance with previous assignments. The Gruneisen parameter
of the libration is lower than expected for purely octopolar
electrostatic interactions between molecules. The
change in frequency with temperature of all three modes is
due to their interaction with the mode near 108 cm^-1. The lifetimes
of both translations are determined by their rate of
combination with librations near 306 cm^-1
and 313 cm^-1. The lifetime of the libration near 306 cm^-1 depends on its rate of
combination with the two translations (108 cm^-1 and 140 cm^-1) and a libration near 440 cm^-1.