Raman spectra of proton ordered phase XI of ICE I. Translational vibrations below 350 cm-1

Polarized Raman spectra from single crystals of ice XI (proton ordered phase of ice Ih) were measured and assigned for the modes below 350 cm^-1 in the translational vibration region. In contrast to the proton disordered ice Ih, the spectra in ice XI show clear polarization dependence and several new peaks are observed. Most of the vibrational modes were successfully assigned by the simplified point mass model with the symmetry C_{2v}^{12}(Cmc2₁) and by the depolarization effect. In particular, LO-TO splitting of the mode near 240 cm^-1 was experimentally confirmed for the first time, which indicates that the long range force effect appears distinctly in ice XI.