Phonon Raman spectra, molecular motions, and phase transitions of dimethylacetylene crystal

Abstract

The phonon Raman spectra of crystals of dimethylacetylene, perdeutero-dimethylacetylene and their mixtures have been investigated down to 20[deg]K. The high temperature phase shows only one line and the low temperature phase only two. Neither of these is related to the Rz (methyl) rotation, which seems to remain practically free. However, the latter motion is found to be restricted in a newly discovered metastable phase, which is reported here first, displaying 5-6 phonon bands. The well-known [lambda]-point transition appears little related to methyl rotations. The internal rotation of dimethylacetylene seems to stay practically free down to 20[deg]K in the stable phases.