Knowledge Bank

The Raman spectra of $C2H2$ and $C2D2$ have been observed as a function of temperature down to $30\circ K.1$ The spectra in both the intramolecular and lattice vibration regions are affected upon passing through the known solid-solid phase transition temperature. In the low temperature phase the number of components resolvable for the fundamentals of $C2H2$ and $C2D2$ are: 2 and 1 $(\nu 1),2$ and $2(nu2)$, and 3 and $4(\nu 4)$ respectively. The lattice bands shift approximately by the ratio of the inverse square root of the moment of inertia upon isotopic substitution, which substantiates their assignment to modes of primarily librational origin. The assignments will be presented on the basis of suggested crystal structures, and the relative intensities will be discussed in the framework of the oriented gas model.