Raman Studies of Molecular Motion in Liquid and Solid HCl

Abstract

The Raman scattering technique was used to study molecular motion of HCl in its condensed phases. In the liquid and in cubic phases, the depolarization ratio, the spectral shape, and the spectral width of the H–Cl stretching mode were measured as functions of temperature. It was found that the modes of HCl molecular motion are not significantly changed as the liquid–solid phase transition was traversed. The depolarized linewidth measurements of the intramolecular stretching vibration also indicate that the energy barrier for HCl molecular reorientation is small, suggesting a nearly free tumbling of the HCl molecule in the cubic solid at temperature higher than 120°K. Below 120°K, satellite peaks develop, suggesting that evolution of the lower symmetry structure other than the cubic has already taken place at temperature higher than Tλ. The temperature study of the low-frequency spectrum in the 0–300-cm−1 region suggests that HCl polymeric fluctuations are responsible for the light scattering spectrum. Moreover, the libration time of HCl polymers (formed as a result of hydrogen bonding) is short compared with the mean reorientation time of a single molecule. The spectra of the lattice phonons as well as the internal stretching vibrations of HCl molecules below the first-order phase transition temperature (Tλ = 98.4°K) were also studied.