High-pressure Raman scattering and x-ray diffraction studies of the supercritical fluid of hydrogen

The high-pressure properties of the supercritical fluid H2 have been investigated in the Raman scattering and synchrotron radiation x-ray diffraction experiments at room temperature. The pressure dependence of four vibrational modes, i.e., Q1(0), Q1(1), Q1(2), and Q1(3), and four rotational modes, i.e., S0(0), S0(1), S0(2), and S0(3), were precisely obtained, and three rotational constants under pressure, i.e., B0, D0, and H0, were estimated from theoretical formulas. A peculiar change in the pressure dependence of the Raman spectra was observed at 1–2 GPa. Through x-ray experiments, halo patterns were collected within a wide pressure range of 0.1–5 GPa, and the molar volume at each pressure was estimated from the d-value of the halo peak. The obtained pressure–volume relation suggested that the fluid H2 showed a change in compressibility at around 1 GPa and became incompressible above this pressure because the repulsive term of the intermolecular potential became dominant. The dependence followed the relational expression of P ∼ Vm−3.11 above 1 GPa, whereas fluid O2 and N2 of the same homonuclear diatomic molecule followed the relational expression of P ∼ Vm−4.32 above 0.2 GPa. It was found that the fluid H2 behaves differently from fluid O2 and N2 and is more easily compressed than those. The behavior of Vm was significantly correlated with the pressure dependence of the Raman spectra, and the peculiar change of the Raman spectra has been attributed to the enhancement of the intermolecular interaction due to the transfer to the solid-like pressure–volume relation.