Neutron scattering studies of methyl derivatives of benzene selected as potential materials for cold neutron moderators

Vibrational spectra of toluene, m- and p-xylenes and mesitylene, measured by inelastic incoherent neutron scattering at 20 K, are compared. Internal barriers for methyl rotation in free molecules of these compounds are quite low. External barriers caused by crystal packing significantly increase the frequencies of methyl librations in solid p-xylene and the low temperature phase III of mesitylene. The vibrational spectra of glassy states of mesitylene in solution with toluene and m-xylene indicate relatively low barriers for methyl librations and, typical for disordered solids, additional density of states at low frequencies, which makes these materials preferable as potential moderators for cold neutron sources. (C) 2004 Elsevier B.V. All rights reserved.