Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens

Raman measurements performed on dense H_{2}, D_{2} and H_{2}+D_{2} in a wide pressure-temperature range reveal the presence of the ΔJ=0 rotational excitation. In the gas/fluid state this excitation has zero Raman shift, but in the solid, the crystal field drives it away from the zero value, e.g., ∼75  cm^{-1} at around 50 GPa and 10 K for both isotopes and their mixture. In the case of deuterium, the ΔJ=0 mode splits upon entering phase II suggesting a very complex molecular environment of the broken symmetry phase (BSP). In the fluid state and phases I and II the frequencies (energies) of the ΔJ=0 transition for H_{2} and D_{2} scale neither as rotational (by factor of 2) nor vibrational (by sqrt[2]) modes and appear to be completely isotope independent. This independence on mass marks this transition as unique and a fundamentally different type of excitation from the commonly considered harmonic oscillator and quantum rotor.