Infrared study of hydrogen up to 310 GPa at room temperature

We observed a strong difference of the pressure dependence of the infrared (IR) active molecular vibron of hydrogen in phase IV in the 200–310 GPa pressure range in comparison with the Raman vibrons. While the Raman vibron strongly splits (∼250 cm−1) at the transition from phases III to IV at 220 GPa, the IR vibron nearly does not change. This small spitting of IR vibron is not described by the graphene-like structure proposed for phase IV. The combined pressure dependence of Raman and IR vibrons provides a sensitive test for further theoretical models of phase IV.