Phonon Raman Spectra, Molecular Motions, and Phase Transitions of Dlmethylacetylene Crystal*
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چکیده
The phonon Raman spectra of crystals of dimethy~~tylene, pcrdeyrero-cjimetfiylacetylene and their n%ures have been investigated down to 20°K. The high temperature phase shows only one Iine and the Iow temperature phase only two. Neither of these is related to the R, (metbyi) rotation, which seems to remain pmctidiy free. However, the latter motion is found to be restricted in a newly discovered metastable phase, which is reported here fist, displayins 5-6 phonon bands. The well-known A-point transition appears little related to methyl rotations. The internal rotation of d~~:hylacetyiene seems to stay practi&y free down to 20°K in the stable phases.
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