Hydrogen (deuterium) vibron frequency as a pressure comparison gauge at multi-Mbar pressures

The Raman spectra of compressed hydrogen (deuterium) have been measured in the multi-Mbar pressure range in the diamond anvil cell using the position of the stressed first-order Raman diamond edge (DE) to estimate pressure. We find that the Raman spectra are very consistent in repeated experiments for the critical frequencies of the hydrogen (deuterium) bands at the phase transition pressure-temperature (P-T) points. However, measurements of pressure determined from the DE in different experiments vary substantially with virtually identical Raman spectra of H2 (D2), giving in some cases a pressure variation as high as 20 GPa. These variations do not only depend on sample and sample chamber geometry but also on the P-T path taken, thus making the DE pressure determination uncertain. Therefore, we propose in experiments on hydrogen (deuterium) to use the Raman frequency of the intramolecular vibration (vibron) of H2(D2) as a pressure sensor. By comparing the vibron frequencies and considering the geometrical changes of the sample chamber caused by very high pressures, we propose an alternative explanation to the recent claims of a transformation to a metallic liquid state of hydrogen (deuterium).