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otations seem to be weaker and the librational modes of methyl groups are shifted to the
energy range of the lattice modes below 15 meV.
From the obtained IINS results we can conclude that due to the presence of methyl
groups in the mesitylene molecules a number of rotational modes is added to the lattice modes
of the solid phases I and II. It seems that phase II can be classified as a “protonic glass”, because
the density of phonon states G(ν) of this phase below 10 meV is softer than in phase I.
The relatively high density of phonon states in phases I and II below 10 meV is promising
from the point of view of using solid mesitylene as a cold moderator. The G(ν) spectra presented
in Figure 9 can be used for calculations of neutron scattering cross-sections needed for
optimization of an advanced cold neutron moderators [9].
ACKNOWLEDGEMENTS
The authors are indebted to Dr. L. Cser for his idea and stimulation us to neutron scattering
investigations of solid mesitylene. We are also grateful to Dr’s. H. Conrad,
R.J. Granada, M. Prager and E.P. Shabalin for their interest and stimulating discussions of our
results, and Ing S.I. Bragin for his technical assistance with experiments on the IBR-2. Support
from the Grant of Polish Plenipotentiary at JINR for technical equipment of the NERA
spectrometer and purchase of deuterated samples is gratefully acknowledged by the authors.
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