Theoretical Study of Raman and Infrared Active Vibrational Modes in Highly Compressed Solid Hydrogen

Abstract

The Raman and infrared active vibrational modes in solid hydrogen at mbar pressures have been studied, using first-principles methods, and assuming hcp lattices of the molecular centers and several patterns of molecular orientations. The midlying phonon mode whose frequency reaches about $1900{\mathrm{cm}}^{-1\mathrm{}}$ at 200 GPa and the frequency difference between the in-phase and out-of-phase vibronic modes are in good agreement with the experiments, which suggests that the lattice of the solid hydrogen is hcp even beyond the phase transition at 150 GPa, and the molecules are canted from the $c$ axis.